Tag Archives: energy efficiency

Heat Pump retrofit to house with resistive electric heat

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In this post I want to talk about the cost-effectiveness of retrofitting my 3-bedroom house in Pemaquid with mini-split heat pumps. Four years ago, just before I had our first mini-split installed, the house was heated with electric resistive, baseboard heaters. The house was shut down in the winter (water drained, heat turned off) so these heaters were used sparingly in spring and fall. We spent our first winter here four years ago so decided to invest at that time in one mini-split heat pump that served the central living area.

We now have four single-zone, mini-split heat pumps installed, a 15 kBtu/h unit for the main living space and three smaller units (6 kBtu/h, 9 kBtu/h, and 9 kBtu/h) in an office and two bedrooms. Electric baseboard heat remains the only heat in the third bedroom — but my wife likes a cold bedroom so we NEVER use that electric heater. We regularly use a small, 1.5 kW ceramic electric heater in the bathroom — this heater runs about 1 hour a day.

The heat pumps provide summer cooling, but this is not their main function. They were installed to lower our heating cost. Electricity is expensive here in Maine and I was led to believe that heat pumps would use about 1/3 as much energy as the electric resistive heat. Based on my data I find that they are not, on average, that efficient. Instead they use 1/2 the energy (average COP = 2) used by electric heaters.

While I have electric bills from previous years, it would not be useful to compare them with my more recent bills. Everything in the house is electric — hot water heater, stove, clothes dryer, etc. — so with three people now living in this house full time the energy use is too different from what it was in previous years when we only summered here.

Shortly after installing the heat pumps I installed Iammeter electric power monitors that keep track of their energy use. These devices and their associated cloud storage have been very reliable and I strongly recommend them.

The heat pumps have been installed incrementally over four years. The larger unit was installed in October 2020. After rebate the net cost to me was $3,369. The office unit (6 kBtu/h) was installed in October 2021. After rebate my net cost was $2,789. The two bedrooms units (9 kBtu/h each) were installed in September 2024 after I moved permanently to ME. After rebate my net cost for these two units was $7,800. I am still hopeful that I will receive a $2000 federal tax credit (tbd) which would lower my net cost to $5800. Assuming I do get the tax credit, our net investment in these heat pumps is just about $12,000.

This heating season (Oct. 1 – March 20) all four heat pumps have used 3884 kWh. As I did earlier, I will inflate this a bit (multiply by 7/6) to account for electricity I expect to use for the remaining 6 weeks or so of the heating system. This gives a projected use of 4,500 kWh, which, at $0.25/kWh has a cost of $1,233.

Based on my assessment that the average COP for these heat pumps is 2.0, that means that I expect had I not purchased these heat pumps, I would have supplied this same heat with electric heaters at double the energy use, or a projected cost of $2,266. That means the heat pumps saved me half this amount, or $1,233.

If you divide my capital investment by my annual savings (for just this year, of course) you get a simple payback time of about 10 years. That is the expected lifetime for the heat pumps. Keep in mind that I already had electric baseboard heaters installed. There was no capital investment in continuing to use them.

The return on investment is much worse if I consider only the heat pumps I recently added to the two bedrooms. My daughter keeps her bedroom fairly warm and her door is usually closed. For the heating system her heat pump has used 867 kWh which, multiplied by my 7/6 factor is 1000 kWh. Based on earlier analysis this is the savings as compared with the baseboard electric heat that was formerly in her room — a savings of $253. If I divide the $5,800 cost of these bedroom units by two, then this heat pump cost $2,900 to install. The simple payback is 11.5 years. The savings for our guest bedroom are far lower and the ROI for that much lower.

Investing in these two heat pumps was a bad decision. I think I am happy with my investment in the first two heat pumps — it is not a slam dunk case, but overall I am happy with that initial investment. These two heat pumps have a simple payback time of 7 years.

Of course, there is still the air-conditioning benefit. But this is far less than I had imagined. For the last year’s cooling season (June 1 – Oct. 1) the two house heat pumps used a total of 222 kWh. The two cottage heat pumps for this same period used 80 kWh. In both cases, these heat pumps provided welcomed dehumidification and cooling for some very warm summer days and nights. But this relief could have been easily delivered by much cheaper window air-conditioners.

I realize that this is an over-simplification of cost/benefit analysis. I have not included any maintenance cost. With out-of-control inflation I don’t know how to account for the rapidly increasing cost of the heat pumps nor the increasing cost of electric energy (and other fuels). But I think the basic ideas I have presented are sound. Other smart people might make different assumptions that then produce different conclusions. I think a full discussion is needed.

Note that I have used only my net cost in determining my simple payback time. Someone else pays for the subsidies, and I question whether it is good public policy to encourage people to invest in something that, at full price, is not cost-effective. The subsidizing organization is happy with their return on investment (carbon savings per their invested dollars) and the home-owner is happy with his/her investment. Both claim success while, in fact, the overall investment is not cost-effective. This is an important issue that I have not addressed.

My conclusions rely entirely on my determination that the effective annual heating COP is 2.0, not higher. Many will disagree with this conclusion — but I don’t see any real world data to demonstrate the average COP is higher. Measuring COP in the laboratory under stringent conditions is not the same as evaluating performance in the field with uncontrolled conditions. I have found it very difficult to determine the heat delivered by the heat pumps and have worked hard to nail this down. I have confidence in my measurements. More to come.

Cost-effectiveness of mini-split heat pumps in Maine cottage

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I have to begin by making this disclaimer — there are many issues involving cost that I cannot address. I don’t install or maintain hvac systems. I am the wrong guy to provide cost estimates for many things. What I can do here is compare the fuel costs and some capital investment costs for different heating systems that I have purchased — based on my limited experience.

The single thing I bring to this discussion is that I have, I believe, determined the average heating COP for the Mitsubishi heat pumps used in my house and cottage. This COP = 2 is different from the aspirational numbers that are supplied by those who sell and promote heat pumps. I don’t know of other studies that really nailed this.

Fuel Cost

First, let’s talk about fuel costs. Heating a residence involves heating the building. This heat has to come from some source of energy that you typically have to pay for. Here I will compare my heat pumps with three kinds of alternative heating systems: 1) electric baseboard heat, 2) through the wall propane heater, and 3) a home-heating oil furnace or boiler.

In my earlier post I provided a table with various assumptions regarding the fuel required for each of these systems to deliver 1,000,000 Btu of heat. Let’s now talk about fuel cost. This is very specific to location — my electric cost in Bristol, ME is nearly double what it was in Oberlin, OH. In OH I heated with natural gas. I don’t have access to natural gas here in Bristol, ME — only propane and home heating oil.

I am paying $0.25/kWh for electricity here in ME. This charge is always increasing, and who knows what it will be in a few months owing to the tariff war with Canada. The actual billing formula is complicated, but if you take my monthly bills and divide by the number of kWh I purchased for many months it averages to this.

And I am billed $5.09/gal for propane by my local supplier. This is not representative — I use very little propane and the per gallon price is high because of that. If I used 300+ gallons a year the price from my supplier changes to $4.39/gal. The maine.gov web site lists the statewide average prices (as of March 10) for propane and home heating oil to be $3.53 and $3.76. For my table below I will use $4.39/gal for propane and $3.76/gal for home heating oil.

While natural gas is not an option for me in rural Maine, it clearly is the dominant heating fuel in much of the USA, and is available in Portland and other larger cities here in Maine. The billing formula for natural gas is complicated, but the maine.gov web site lists typical February gas bills for residences in five areas that use 149 therms of gas (1 therm = 100 cu. ft.). If I average these prices I obtain $2.14 per therm or $0.021 per cf of natural gas.

The above numbers are folded into the tables that I posted earlier to yield the table below that shows the relative cost to produce 1,000,000 Btu of heat using different heating systems. The last column shows the cost per million Btu of heat delivered.

Clearly natural gas is the cheapest option, by far, with respect to fuel costs. This option is not available to me here in Bristol, ME. Of the options available to me, home heating oil at $30 (per million Btu) would have the lowest fuel cost, with the heat pump at $37 being next. Higher yet is propane at $60 (or $69 using the price I am actually paying), with electric resistive heat highest at $73 per million Btu.

So based simply on fuel cost, the electric heat pumps are the cheapest heating option available to me — though I have not considered a wood stove.

Capital Investment

I don’t have the expertise to discuss the capital costs for all of the possible hvac systems available. But I can speak to what I actually paid to install heat pumps at my house and cottage, and what I paid to install a through-the-wall propane furnace for my cottage. And, I have installed base board electric heat in my house.

My 1000 sf cottage has a very open architecture and my centrally-located, through-the-wall propane furnace does a wonderful job of heating it in the winter. (See earlier post for more information.) The furnace is a Rinnai EX22 that delivers heat at a maximum rate of 16,560 Btu/hr. The manufacturer claims 80% efficiency. This system cost me (2021) $4,700 to have installed, along with the propane lines and tank that support it. It is difficult to know how long it will last, but it is robust and I see lots of older units still in service. I will estimate the useful life to be 15-20 years.

I also had two Mitsubishi, low-temperature, mini-split heat pumps installed in the cottage, one a 6 kBtu/h size and the other a 18 kBtu/h unit. They both dump their air into the same space (from opposite ends of the cottage). In the heart of winter I run them both, but I have on occasion staged their use, using only the smaller or larger unit depending on the outside temperature. (I figure one unit will run more efficiently when not competing with the other to control temperature.) The invoice for this project was $8,921. Efficiency Maine provided a $1200 rebate. So my net cost was $7,721.

It was not necessary to install both the propane furnace and the heat pumps. I did this mainly so that I could study heat pump performance. I wanted a back up system in case the heat pumps failed in real cold weather and also an alternative if we lost power. Propane alone would have been sufficient.

To find the economic savings provided by my cottage heat pumps I first have to determine the annual energy required to heat the cottage — and that, of course, will vary from year to year. Since September 1, 2024 the cottage has been exclusively heated with the heat pumps with a constant set temperature of 67oF. From Sep. 1 through Mar. 16 my cottage heat pumps have used 2222 kWh of electric energy. At a price of $0.25/kWh this has cost me $556. We still have another 4-6 weeks left in the heating season. To account for the remaining energy (and I will confirm this later this year) I will multiply this number by 7/6. This puts my heat pump heating energy cost for this heating season at about $650.

Using numbers in the above table I can then calculate what it would have cost me to heat with resistive electric or propane. Resistive electric would have cost me double this or $1,300 and propane (at the $4.39 price) would have cost me $1,070. At $5.09/gal, the price I actually pay the cost of propane, the annual cost would have been $1230. (The projected annual use is 242 gallons which does not qualify for the lower price.)

Based on the above numbers, the annual savings for heat pumps as compared to electric resistive heat is $650 and the annual savings as compared with propane is $416 or $582, depending on whether I use the lower or higher propane price. Of course, the heat pumps required more capital investment and are expected to have only a 10 year service life.

I don’t actually know what it would have cost me to install electric baseboard heat in the cottage. At Home Depot an 8-ft long, 2 kW baseboard heater sells for $130. I would probably need to install two of these along with five or so 4-ft. models ($67 ea.). I would need to run appropriate electric circuits and add thermostat control. I did all the electrical work in my cottage and could do the wiring for these — but an apples-to-apples comparison requires I get a price for installation. Let me defer this for now.

The heat pump installation cost me about $3,000 more than did the installation of the propane heater. With a fuel cost savings of $416/year, the simple payback time (for the additional capital cost) is 7.2 years. With the higher propane price the savings is $582/year, the simple payback time is 5.1 years.

And, of course, the heat pumps provide me with cooling during hot humid summer nights. This is not so important in the cottage as it is located on the shore of the Pemaquid River where their is more summer wind. But this is of some importance in our house which is farther inland.

I don’t know what to say about maintanence costs. So far my installer (Dave’s Appliance) has provided service without any additional charges. The manufacturer’s warranty is 7 years and I have not experienced problems with the heat pumps. That, of course, will change. But so far I have not had any maintenance costs.

There are many more issues to discuss, but this post is getting too long. In future posts I will discuss the cost/benefit of the heat pumps in my house, share data on the temperature control (which is significantly more stable with electric baseboard or propane heat), and share data that I used to learn the average COP was equal to 2.

Carbon savings with electric heat pumps in rural Maine

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In my previous post I mentioned that my mini-split heat pumps are clearly lowering carbon emission, as compared with alternate ways I might heat my house. Here is the justification for that conclusion.

First, Maine has a relatively low carbon electric grid. It does not come cheap. Maine has some of the highest electric rates in the country. On average I pay $0.23/kWh for my electricity.

The EPA e-grid web site lists the following characteristics for the electric grid in this region. You really have four different ways of thinking about it — and I will tell you which I believe is the best way. First, organized by state Maine’s electricity has a footprint of 0.311 lbs CO2/kWh of electric energy. Second, we are part of the NEWE e-grid sub-region. That sub-region is listed as having 0.537 lbs Co2/kWh. Third, Maine is contained in the NPCC Nerc region which is listed as having 0.506 lbs CO2/kWh. And finally, our grid is part of the New Brunswick System Operator Balancing Authority which has a carbon footprint of 0.169 lbs CO2/kWh. (The New Brunswick Canada grid is almost entirely powered by hydro.)

It is difficult to know which of these numbers better reflects the carbon footprint of electricity I buy from the grid. Let me use the most conservative number of 0.537 lbs/kWh. Below I will offer yet another figure that I believe is more reflective of the true situation.

Consider propane heat as an alternative. Propane has a heat content of 91,500 Btu/gal. The carbon emission from burning a gallon of propane is 12.7 lbs/gallon. Most propane heating systems are 80% efficient (i.e., lose 20% of the heat up the chimney) although condensing furnaces can be as high as 92% efficient.

Consider the home heating oil alternative. Home heating oil has a heat content of 137,500 Btu/gal. The carbon emission from burning a gallon of home heating oil is 22.5 lbs/gallon. Depending on the age of the unit efficiency can range from 70-92%. Google reported home-heating oil furnaces can be as high as 99% efficient, but I don’t believe that for one minute. That was probably obtained with AI from a middle-school science report posted somewhere on the web. At best I would say 90% efficiency.

Finally, consider natural gas as an alternative. Natural gas is not available in rural Maine, but is available in some cities including Portland, Lewiston, Bangor, and Brunswick. Natural gas is, of course, widely available in many other states — but the comparisons presented here apply only to Maine’s electric grid. Natural gas heat content of 1038 Btu/cf. The carbon emission from burning a cubic foot of natural gas 0.121 lbs/cf. Like for home heating oil natural gas efficiency ranges from 70-92%. Most new natural gas boilers and furnaces are close to 90% efficient.

The final number to required is the conversion between kWh and Btu energy units. That conversion is 1 kWh = 3,416 Btu.

Here I assumed reasonable efficiencies for the various systems. Certainly older heating systems will be even less efficient.

Here we consider five heating systems listed in the table below. The question is how much CO2 do they emit (directly, or indirectly) in producing 1,000,000 Btu of heat. The calculations are easily accomplished with a spread sheet, and are shown in the table below.

What the table shows is that the electric heat pump with COP = 2 (200% efficiency) has the lowest carbon footprint — again, using a conservative estimate of the carbon footprint for the Maine electric grid. The next lowest carbon footprint would be for a 90% efficient natural gas boiler or furnace. This option is not readily available in rural Maine. Resistive electric heat has double the footprint of the heat pumps, but using Maine electricity, still has lower carbon than propane or a system that uses home heating oil.

Bottom line, given the various heating systems available to me, my heat pumps have the lowest carbon footprint — at least with the assumptions I have used.

I must point out, however, that you could look at the carbon footprint of the electric grid differently. Maine’s electric grid is what it is. My adding the load of a heat pump to the existing grid means that the load goes up. How is that load met? Given the economics of power the grid is always using generators with the lowest marginal cost of operation. So renewables are being maxed out, as is hydro and nuclear. If you need more power at any point it is obtained by ramping up a natural gas peaking plant. So the argument could be made that additional load on the grid has the carbon footprint of a natural gas peaking plant — which is much higher than the carbon footprint.

Again, the basic argument is this. If I have an electric heater and I stop using it — this will lower the demand on the electric grid, and accordingly some plant that is supplying electric to the grid will be ramped down. The plant that is ramped down will be the one with the largest fuel cost (i.e., marginal cost of operation). It won’t be a wind turbine, hydroelectric, or solar because they have no fuel cost. The energy saved will be the natural gas that is not needed in a gas peaking plant. In other words, in thinking about the carbon footprint of an electric load we should not look at the average carbon content of the grid, but rather the carbon saved or used when the load is decreased or increased.

A natural gas peaking plant has an efficiency typically of 30-42%. Let’s just call this 36%. At 36% efficiency a peaking plant would need to burn 9.14 cf of natural gas, which then has a carbon footprint of 1.1 lbs. In other words, no matter how clean the overall electric supply is for Maine or any other region — the marginal change in carbon emission when you increase or decrease the electric demand is 1.1 lbs of CO2. I would argue that this figure of 1.1 lbs/kWh is the relevant metric to use for deciding whether to use electricity or a fossil fuel to produce heat.

How do the above calculations change if this figure is used instead of the one used earlier (0.53 lbs/kWh)? With this carbon footprint for electricity, our Table above changes.

With this view, natural gas remains the best option for lowering carbon emission. The next best is the electric heat pump. Both propane and home heating oil still have higher carbon emission than an electric heat pump. The worst is resistive electric heat.

This view will not be popular — and many intelligent, committed advocates of sustainability will disagree. But I believe this is the correct way to think about this. Note that if the heating COP of the heat pump was higher — say 3 rather than 2 — then the heat pump would have a lower carbon footprint than any of the fossil fuel options — even when using electricity produced from burning natural gas in a peaking plant. And that would be the same in any state. Here in rural ME I don’t have the natural gas options — so even with COP = 2, the mini-split heat pump is my best option for lowering carbon.

I believe in just a few more years we will see new heat pump technology that does achieve a COP of 3 or higher over a wide range of operating temperatures. When that happens I will be a strong advocate of heat pumps — subject of course, to a cost/benefit analysis. This will be the subject of my next post.

Main Conclusions from Heat Pump Study

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After using Mitsubishi mini-split heat pumps in two houses for two winters I am finally prepared to share my conclusions on their operational cost and ability to maintain comfortable temperature. It will take a number of posts to unfold the details and nuances, and to compare heating costs with fossil fuel alternatives, but here are my main conclusions.

First, these heat pumps, using electricity purchased from Maine’s electric grid, have a lower carbon footprint than other forms of residential heat owing to the relatively low carbon footprint of Maine’s electric grid. This is the main reason electric heat pumps are being heavily promoted in New England, and from a public policy perspective, are achieving the important goal of lowering carbon emission.

Second, my Mitsubishi heat pumps were able to maintain reasonably comfortable inside temperatures (65-70oF) with the outside temperature as low as -5oF. During one extremely cold weekend (-15oF) the heat pumps struggled to keep my cottage above 50oF inside — but we did not fear freezing pipes and the extreme temperatures lasted just 1-2 days.

Third, on average, my mini-split heat pumps use about half as much energy as would electric baseboard heaters to accomplish the same task. That means they cost half as much to operate as electric resistive heat. In tech language, averaged over my winter conditions, the heat pumps demonstrated an effective heating COP = 2. These energy savings are meaningful but considerably lower than those advertised for these heat pumps which are said to have heating COP’s as high as 3.5. That may be true under certain conditions, but averaged through my heating season they are far less efficient.

The fourth conclusion is that temperature regulation and comfort delivered by my heat pumps, while acceptable, is inferior to that delivered by either my electric heat or propane through-the-wall heater. Even though the set points on the heat pumps remain constant, the indoor air temperature, measured by independent temperature sensors, show fluctuations in temperature in the range of plus/minus 1.5oF. These fluctuations are significantly larger than those experienced with other heaters and display an irregularity that is difficult to understand.

The fifth conclusion is that the cooling and dehumidification provided by these heat pumps is welcome in the summer. However, while providing a degree of comfort not afforded by other heating systems, it comes at the cost of additional electric use and greenhouse gas emission.

In the next few months I will be posting details to justify the above conclusions. In addition, I will discuss the capital investment required to install these heat pumps, and look at the cost-effectiveness of the electric savings delivered. An electric heat pump can be a cost effective way to both heat and cool a residential space. But it is not always the cost-effective solution, particularly if cooling is not required.

Daily Energy Cost Graph for Cottage

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Last week I posted a graph of the Cottage daily electric use. This graph is updated every day. While the graph is very useful, it does not include any propane used. While there are many days that the cottage does not use propane, I do expect to use propane for heat this winter. And there were some days in November when propane was used to heat the cottage.

As I suggested earlier, propane and electric use can be included on a single graph of daily energy cost. Electric energy costs me $0.30/kWh in Maine and propane costs $4.29/gal. (I should point out that propane cost would be significantly lower if I used much more propane. My local supplier is currently charging $3.30 per gallon delivered for customers with 1000 gal annual use. So far my use is at the 100-200 gal/year level.)

This month (December) I am using propane heat much more in order to actually compare my propane flow numbers with the amount of propane that is delivered. I received a propane delivery about a week ago, so my goal is to use a significant amount of propane in the next few weeks so that I can use the next propane delivery to calibrate my propane flow meter.

I have performed a preliminary calibration of the propane flow meter — and that calibration is being used to produce the propane costs in the above graph. But I do expect to adjust these figures once I receive the next propane delivery.

Recall from my earlier post that, under conditions that my heat pump had a heating COP = 3.3, propane heat cost was about 2.2X the cost of heat from my heat pumps. So it is with great reluctance that I turn off the heat pumps and heat with propane. Such is the cost of scientific experiments.

Heat Pump Energy Strongly Depends on Fan Speed Setting

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A few weeks ago I noticed that the Mitsubishi mini-split heat pumps in my house were consuming considerably more electric energy than those in my guest cottage. This applied especially to the large heat pump (15 kBtu/h) in my house living room which carries the most load.

Since then I have looked at this more closely and concluded that the excessive energy use is assocated with the fan speed settings on the heat pumps. The fans on both house heat pumps were set to Auto while the ones in the cottage were set to High (setting 4 of 5 possible manual fan speed settings). When I switched the fan mode of the house heat pumps from Auto to High their energy consumption decreased by 30-50%. That is, they supplied more heat while using less electric energy. The change was substantial.

The data that support this conclusion are convincing. The first measurements I made were to determine the heating COP for the living room heat pump. I did this 3-4 times on different days with different outside temperatures. I obtained COP numbers like 1.95, 1.15, and 1.14. This concerned me so I reached out to my installer, Dave’s Appliance. They, in turn, told me they passed my information along to Mitsubishi. In the intervening two weeks I have not heard back from either.

I don’t know why but at some point I wondered if the fan setting might be involved. Both guest cottage and house were to be unoccupied for a few days so I set the heat pumps in both to maintain an interior temperature of 60oF, 24-hours-a-day. In addition I wrote a Home Assistant automation to change the fan speed setting at midnight so that I could observe the impact of this change on energy use. I performed this experiment with the 15 kBtu/h house living room heat pump and also with the 18 kBtu/h cottage heat pump. For both heat pumps I observed similar results. The heat pumps used 40-50% less power when the fan mode was set to High as compared to when the mode was set to Auto. I also determined that the heat pump used excess power when the fan mode was set to Medium (3rd of five manual settings). I did not try any of the slower fan settings. I have confirmed similar energy savings when the fan is set to Very High (5th of five manual settings). I believe that these conclusions apply to all four of my heat pumps, though the level of savings may vary.

After I changed the living room heat pump fan speed setting to High I again measured its heating COP. This time I obtained a value of 3.7 when the outside temperature was 30oF.

I should mention that each of my four heat pumps make use of the wireless remote temperature sensor sold by Mitsubishi. It can be purchased on Amazon. In principle, this should seemlessly integrate with my heat pumps.

This seems to me to be an important result. If I were not metering (and paying attention to) my heat pump energy I would not know they are not operating efficiently. They are producing heat and the room is comfortable. Heat pump energy use is not monitored for most installations.

But energy (and associated cost and carbon) savings is the only reason to invest in heat pumps rather than inexpensive electric baseboard heaters. After all, electric base board heaters provide more stable and quieter heat and are cheaper to install. If heat pump operation does not deliver the promised savings the heating costs and carbon footprint will not meet expectations. It is quite possible that thousands of heat pump installations in New England alone are using 50% more energy than necessary because their fans are set to Auto mode. It seems to me that Mitsubishi should care.

I don’t know if this problem is related to my use of remote wireless temperature sensors, or if it also would apply even using the internal temperature sensors. (I will not be able to disconnect my remote sensors in order to test this until I return to Maine after Christmas.) It is possible that Mitsubishi software that controls the fan may be written only for the internal tempearture sensors and is not approriate when connected to a wireless temperature sensor. I have seen nothing on the Mitsubishi web site that suggests this.

I should point out that when one uses either Kumo Cloud or the remote control to change settings on the heat pumps this often changes many other heat pump parameters. For instance, if you point the remote at the heat pump and raise the temperature, the remote also sends other parameters it has saved for fan speeds, direction, etc. Someone might set the heat pump fan speed to High using the Kumo Cloud phone app, then later adjust the set temperature with the remote and inadvertently change the fan (and other) settings to those saved on the remote.

One final comment. I did find that the temperature fluctuations were smaller when the fan was in Auto mode than when the fan was set to High or Very High. So there does seem to be a tradeoff between energy savings and temperature control. Experiments continue to better quantify this.

Excessive Heat Pump Energy – Update

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A few weeks ago in my post I described how one of my four Mitsubishi mini-split heat pumps was using excessive energy. Today’s post provides additional information about that. Apparently the excessive energy is by design! For background please revisit my August 12 post.

Just a quick recap — In the last three years I have had four, low-temperature, mini-split heat pumps installed on my property in Maine. The oldest of these is a 15 kBtu/h unit that is installed in my house living room. The model number for its outdoor unit is MUZ-FH15NA. The other three units were installed over the next two years. Their outdoor units have model numbers: MUZ-FS06NA, MUZ-FS18NA, and MUZ-FS06NA. (Apparently the “FS” models are improved over the “FH” models.) All four compressors use R410A refrigerant.

These units have seen minimal use since the beginning of May. On rare occasions we have used them for a bit of cooling or heating. They have simply remained in standby mode for nearly 120 days. Three of these use 3-4 W of continuous standby power but the oldest, the 15 kBtu/h unit, particularly during the night, experiences 70W power spikes every two hours or so that last for about 10 minutes. This causes this unit to use about 0.2 kWh per day more energy than the other three. For three months I have been seeking to understand what is going on.

Back in June I emailed my installer, Dave’s Appliance, questions about this performance including graphs and other details. I have always found Dave’s to be extremely helpful. They could not explain what was going on so they passed the information along to their Mitsubishi support team. A couple of months went by with no answer.

I pestered them some more. Finally, in mid-August, two technicians from Dave’s drove the 50 miles from Winthrop to my house to make measurements on the compressor while on the phone with their Portland Mitsubishi tech support. With the travel time, these guys spent a half day addressing my issue. The only measurements they made were to confirm that a certain thermistor had the correct value.

One of the techs who came to my house was Ean Laflin, the heat pump service manager with Dave’s Appliance. After he was done troubleshooting and speaking with Mitsubishi he explained that there was a 70 W heater in the compressor, and that the control board turned it on whenever the ambient temperature was below 68F. Presumably after the heater ran for 10 minutes the temperature of the thermistor rose above the set point causing the heater to turn off. (It is my impression that there is oil in this compressor, and this heater is intended to keep the viscosity of this oil low so the compressor will start easily when called upon.)

But this begs the question, why would this heater be activated when the ambient temperature ranges from 60F to 68F? I could see the need to heat the oil during the winter. But in my part of Maine from May – October the ambient temperature is usually above 68F for much of the day and usually drops below 68F late at night. For nearly four months I have not needed this heat pump yet the heater keeps using energy, night after night. The only way I can avoid this is to shut off the circuit breaker. This is obsurd!

So why doesn’t this same thing happen with my other three heat pumps? Ean tells me that the control board on these slightly newer models is shipped with a jumper that can be set so as to disable this feature — apparently this is the default setting. He can change the jumpers on the other three heat pumps so that all four of my heat pumps run this heater and waste energy. But there is no jumper to change on my Living Room heat pump to reduce its standby power to 3W like the other three heat pumps.

I conclude from this that Mitsubishi, after shipping thousands or perhaps millions of heat pumps with this control strategy determined it was not necessary and “improved” the next generation of control boards. The only way to “improve” my heat pump would be to install a new control board. I recognize this is not a cost effective way to save the $15/year wasted by this heater.

Which leads me to my last point. Each one of my four heat pumps is connected to the internet and can be controlled using the Kumo Cloud App. Why can’t Mitsubishi download updated firmware over the internet to fix this bug? Hundreds of millions of devices (phones, etc.) that only cost a few hundred dollars can receive updated firmware over the internet. Why can’t Mitsubishi figure this out for heat pumps that cost many thousands of dollars? The technology really needs to be updated.

Initial Assessment of Rheem Hybrid heat pump hot water heater

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In my Maine Guest Cottage I have installed a 50 gallon, Rheem hybrid heat pump hot water heater. I purchased this unit at Home Depot about 16 months ago. The price was roughly $1600 with a large $800 rebate from Efficiency Maine. My cost then was only about $800. My other option was an electric hot water heater which would have cost me about $600 and did not come with a rebate.

There were cheaper heat pump hot water heaters available at the time but I was enamored by the WiFi interface that came with the Rheem unit. This, along with the Rheem Econet smart phone App, would allow me to remotely change the unit’s settings and also monitor its daily (even hourly) electric use.

The energy guide for this unit suggested that it would cost $104 per year to operate, based on an estimated annual electric us of 866 kWh and an electric price of $0.12/kWh. Here in Maine I pay about $0.30 per kWh for electricity.

I installed the Rheem unit in my unheated cottage crawl space. Most of our hot water use will be in the summer when we actually have guests staying there. Heat pump operation during that period should help dehumidify the crawl space. In the winter months when we do not use much hot water I will likely have to switch the unit to resistive electric mode as the crawl space will be cold and any heat removed from the crawl space air would have to be made up by some other heating system.

This coming winter I intend to make detailed measurements to understand the energy performance of this unit. But already some information has emerged.

The first issue is the inaccuracy of the energy use reported by the Rheem Econet App. For my first six months of operation this was my only measure of energy use for the hot water heater. According to the App the unit was averaging 2.0 kWh/day. For most of this time no one was living in the cottage so that hot water use was minimal. This daily energy corresponds to an annual use of 730 kWh per year — using essentially no hot water use! (That is, this is the energy loss from the 50 gallon tank.) Imagine what the energy use would be if a family of four was using hot water for showers, laundry, and such. I suspected the energy consumption data were inaccurate.

So, I connected up a single-phase iammeter energy monitor to this 220VAC circuit to measure energy use of the Rheem unit. I found that the Rheem unit used significantly less energy than its app reported. The graph below compares the daily electric use as measured by the Rheem Econet App to that independently measured with my iammeter energy monitor for a three month period.

The Rheem Econnet App for this three month period reports an average energy use of 1.88 kWh/day while the iammeter measured 0.62 kWh/day. The Rheem’s own measurements while highly inaccurate, are correlated to the actual energy use. The graph below demonstrates this correlation. The R-square for this fit is 88%.

I do not understand why Rheem has not provided accurate power measurement on their $1600 hot water heater when numerous companies are selling smart plugs with accurate power measurement for $10.

The second problem I have with the Rheem App is that it seems to have a mind of its own. There were times this last year when I wanted to compare the energy use in electric resistive heat mode to that used in heat pump mode. Accordingly I would use the App to set the mode to resistive heat. A few days later, without warning, I would discover that the unit had returned to heat pump mode though I had not made this change. This is something I will investigate further this coming year.

The third problem I found was that the temperature of the water provided appears to be significantly lower than that displayed on the app or on the unit’s LCD display.

Before our first guests arrived this summer I thought I should make sure we had adequate hot water. The Rheem water temperature was set to 120F and it was in heat pump mode. I took a shower. With the shower mixing valve set to maximum hot the shower was comfortable for me. Admittedly, I like hot showers. This caused me to worry that the hot water would not be sufficient for 2-3 consecutive showers. Accordingly I raised the Rheem set temperature to 130F. But I began to doubt that the water leaving the Rheem tank was at the specified temperature.

So I decided to install a temperature sensor in the hot water supply line from the Rheem hot water tank. This was accomplished with a DS18B20 sensor inserted into a 3/4 in. “T” fitting that I installed on the output water line from the hot water heater. I programmed an ESPHome D1mini board to monitor this sensor and connected it to my Home Assistant system. Initial measurements suggest the actual temperature of the water is 10-15F lower than the 130F set temperature.

The last thing I will mention is the instrumentation that we have added in preparation for our full year of study coming up. In addition to the aforementioned temperature sensor, I have installed two water meters with pulse output, one on the water supply line to the guest cottage and the other on the HW supply line leaving the Rheem HW heater. These meters will allow me to monitor CW and HW use in 1 gallon increments.

The other feature that I installed was a “dump line” from the hot water heater. To study the performance of the hot water heater I will need to regularly use hot water. But no one is living in the cottage through the winter. To address this issue I installed a motorized valve downstream from the HW meter that will allow me to remotely, and under program control, “dump” hot water down the drain. This will, of course, result in wasted energy — but this will be necessary to study the performance of the Rheem under conditions that replicate 2-4 people living in the cottage.

Excessive Energy Use by one of my Mitsubishi Mini-Split Heat Pumps

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In the last three years I have had four Mitsubishi low-temperature mini-split heat pumps installed in Maine. Two of these are smaller, 6 kBtu/h units, one is an 18 kBtu/h unit, and the oldest of these is a 15 kBtu/h unit, located in my living room.

I have been monitoring the electric use of the two cottage heat pumps for more than a year. In March 2023 I began monitoring the energy use for the two older heat pumps in the house.

This post looks at the standby power of the four units. One of the units displays, what I would characterize as strange behavior. I have reached out both to Mitsubishi and to Dave’s Appliance who installed my units and no one has explained the behavior. Perhaps someone reading this post will be able to shed light on this.

These days all four of my heat pumps have power but are turned OFF so that they provide neither cooling nor heating. They are essentially in stand-by mode. As mentioned, three of these heat pumps use about 4W of standby power. A graph of P(t) for the last day or so for my 18 kBtu/h unit in the cottage is shown below.

In contrast to the 4W standby power of the above heat pump, the 15 kBtu/h heat pump in my house living room displays the behavior shown below.

The above heat pump has standby power of about 5W during the day, then starting at midnight, has periodic spikes of close to 70W. These bursts last for only about 10-12 minutes, then re-appear about 2 house later. These bursts stop sometime the next morning, then the pattern repeats the following night.

The excessive energy use is not much on a day-to-day basis. My three heat pumps use about 0.11 kWh per day per unit in standby mode. The one with bursts of power uses about 0.23 kWh per day in standby mode. The excessive use is about 44 kWh per year which, at $0.30/kWh, costs about $13 per year. Again, the excessive energy is not that much — but what is its origin? What is it about the control of this heat pump that is different from the other three?

These graphs just illustrate that a lot is going on in these heat pumps that you would not notice if you don’t measure their power use. They all seem to be operating normally, otherwise.

I would welcome any input if anyone can explain the behavior.

For reference, the model numbers for the 15 kBtu/h units that show anomalous behavior are MUZ-FH15NA for the outdoor unit and MSZ–FH15NA for the indoor unit.

All of the heat pumps are 220VAC units. I monitor their electric use with iammeter single-phase units. The data from these units are regularly logged with Home Assistant.

Dehumidifier Energy

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The town of Oberlin, OH was essentially built on a swamp. Oberlin basements commonly experience water problems. When we first purchased our Oberlin home in 1988 there were a few storm events that left water puddles in our basement. Various measures eliminated this problem, but high summer humidity remains an issue. For years we have employed a basement dehumidifier to prevent mold and mildew.

Our house in Maine also has humidity issues. We are on the Atlantic coast where the relative humidity is always above 70%. Both our house and guest cottage are built on granite ledge sloping down to the water. I wouldn’t have thought so, but it turns out the granite ledge is relatively porous to water flow. After every rain storm, water flowing down the hill towards the river creates water issues in our crawl spaces. Dehumidifiers have proved to be important to prevent condensation on our water pipes and mildew on the wood.

I have read that you need to keep the relative humidity level in your crawl space below 60% to prevent problems. The usual way to accomplish this is to 1) prevent water from entering the crawl space using water and vapor barriers, and 2) use a dehumidifier to remove what water does enter.

I have three dehumidifiers located in 1) our guest cottage crawl space, 2) house basement/crawl space, and 3) our Oberlin house basement.  All three are plugged into smart plugs that record their energy use. The two Maine units are commercial units from AlorAir and the Oberlin one is a Honeywell residential unit.

In Oberlin I have a heated basement, so the dehumidifier never has to remove water from cold air.  The Honeywell unit can handle that.  I do not run it during the winter. In contrast, the Maine crawl spaces are not heated and get quite cold.  The AlorAir commercial dehumidifiers can remove water even when the air temperature is in the 40’s.  I ran both of them last winter — not sure if I need to and will try to figure that out.

The graph below shows they day-to-day energy use of our guest cottage. As the graph shows, the dehumidifier uses 4-5 kWh of energy daily.

The AlorAir unit uses 600 W when the compressor is running. I normally leave its set point at 60% relative humidity and the compressor cycles on and off. A typical graph of its power vs time is shown below.

When I lower the set point the duty cycle increases (i.e., time that the compressor is on is longer) and the daily energy increases — all as expected.

I am a bit worried about the frequent cycling of the above dehumidifier. I don’t understand why the control doesn’t include a larger “deadband” so that the compressor does not switch on and off so frequently. I attempted to reach out to AlorAir to learn more but I found their technical support to be unresponsive.

The first graph above shows that the dehumidifier used much more energy on July 17. The reason for this is that I decided to try cooling the guest cottage air by circulating it through the crawl space. This exposed a continuous source of humid, warm air to the dehumidifier and it ran nearly continuously. After one day I concluded this was not the optimum way to cool the cottage.

The average energy used by the dehumidifier in my Maine house for July has been 3.4 kWh/day. The area of the house crawl space is larger than that of the cottage, but it is better sealed. Unlike the cottage the crawl space floor in the house is fully sealed with concrete.

The average energy used by the dehumidifier in in Oberlin for July is about 12 kWh per day. Our house there is over 100 years old and all the walls lack modern vapor barriers. The graph below shows the power used by this dehumidifier. This unit removes about 60 pints of water daily and runs almost all the time during the summer.

This dehumidifier is about a year old. My experience with these “basement units” is that they work well for a few years. After a few years they continue to use lots of energy but don’t remove much water. When I bought this Honeywell unit last fall, the dehumidifier it replaced was 4 years old. I ran the two of them side by side for a few days and found the new unit removed water at a rate 5X that of the old one (even though they had the similar specs and were using similar energy). It is hard to throw the old one away because it still removes water — just not good at it.

What I don’t know is the importance of keeping the relative humidity of my Maine crawl spaces low in the winter. My instinct is to believe that mildew and mold won’t grow during the winter in my cold crawl spaces even if the relative humidity is high. But when the temperature warms in the spring I need to keep the humidity level below 60%. But this is just a hypothesis. So far I am erring on the side of caution and running the dehumidifiers year round. They just use less energy in the winter.

I would like to reduce the energy and carbon emission associated with dehumidification. That would be accomplished by raising the relative humidity set point on the dehumidifier. But under no circumstances can I tolerate mold or mildew. I would be interested in learning what people have to say about this issue for cold and warm weather. Is RH% a relevant metric when the air temperature is low as is the case in the winter? Does it make sense to use lots of energy to achieve 60% relative humidity in a crawl space whose temperature is below 45F? I don’t know the answer to these questions and would value informed input.