Tag Archives: sustainability

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.

The Fourth Great American Lie

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There is this standing joke about the three great Amercian lies:  1) “the check is in the mail;” 2) “of course I will respect you in the morning;”, and 3) well … let me skip the last one. I think it is time to add a fourth lie to the list — this green project will lower energy use.

In my last post I mentioned that my home town of Oberlin, OH recently purchased new, automatic loader trash/recycling trucks and spent an extra $300,000 so that three of them included fuel-saving, hydraulic-hybrid technology.  Town leaders claimed these trucks would save fuel and reduce carbon emissions.  Simple cost/benefit calculations using their cost and fuel savings figures showed that this was an awful investment that would never pay for itself (in fuel savings) and that the cost per ton of carbon saved was astronomical.

A few weeks ago I requested from the City fuel consumption data for the first six months of operation of the new trucks.  The City Manager and Public Works Director, instead, asked me to wait until after their July 6 report to City Council on the success of the new recycling program.  They both assured me that fuel usage would be covered in this report.  I was promised access to the data following their presentation.

Last Monday, in his presentation to Council, the Public Works Director highlighted data which showed that for the first six months of operation the City recycled 400 tons — as compared with the 337 tons it had recycled in the comparable period prior to acquisition of the new trucks.  This represents a 19% increase in recycling. Unfortunately there was no mention of fuel usage or savings.

Yesterday I obtained fuel consumption data from the Public Works Director for Oberlin’s new garbage/recycing trucks along with comparitive fuel data from previous years using the old trucks. The new trucks are on track to use 2,000 gallons MORE diesel fuel than were used by the old trucks, annually.  That’s right, not less fuel, but MORE fuel.  This is a 19% increase in fuel usage.  Gee what a surprise!

Soon the spin will begin.  City Adminisrators will point out that fuel usage would be even worse were it not for their $300,000 investment in the hybrid technology.  They will point out that the increased fuel usage is due to the new, automatic loading technology included in these trucks (though they failed to mention any expected increased fuel usage when the project was being sold to the public) — which enabled the use of larger recycling containers and the improvement in recycling.  What they will fail to tell us is that they could have achieved the same increase in recycling using the older style truck without automatic loaders.

This is the second recent City project for which the public has been mislead regarding expected enegy savings. The first was the LEED-certified Fire Station renovation.  This green building was supposed to save energy.  It, of course, is bigger and better than the building it replaced — oh yes, and it uses more energy.  But the increase in energy use wasn’t as much as it might have been because it was a green building.  Now we have the same result for the trash and recycle trucks.

Oberlin College is in the process of constructing a new, green hotel — called the “Gateway Project” as it will usher in a new era of green construction.  But people should understand, this new green hotel will use more energy than the old hotel —  it will be bigger and better, and its energy use won’t be as big as it might have been — and this should make us feel good.

And in the next few months Oberlin residents will be asked to approve additional school taxes to construct new, green, energy-efficient public school facilities.  But don’t be surprised when these new facilities actually use more energy than did the old ones.  Don’t get me wrong — they will be more energy efficient than the old facilities, but they will be bigger, and better and — use more energy.

This is the new lie — that our new stuff will use less energy than our old stuff.  But it isn’t true.  Fundamentally we want bigger and better stuff.  People like Donald Trump just build bigger and better stuff and proudly proclaim it.  But isn’t pallitable for most of us — we feel guilty about wanting bigger and better stuff.  So instead we find a way to convince ourseles that our new stuff will be green, it will lower carbon emission, it will make the world a better place — oh, and yes, it will be bigger and better.

We need our lies to make us feel good about doing what we wanted to do all along.  Don’t get me wrong — sometimes the check is in the mail and sometimes the green project does save energy.  But more often than not these lies are offered for temporary expediency,  And, of course, I really will respect you in the morning.