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.
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.
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.
I think it has been two years since I have posted on this blog. This post is the beginning of a new direction for me.
A couple of years ago my wife and I built a small guest cottage on our property in Maine. Mostly we will use this in the summers, but we decided to go ahead and make it a year-round house. I also decided to make it a laboratory for understanding the performance of a couple of heat pump technologies.
The house is outfitted both with a Rinnai direct vent propane furnace as well as two Mitsubishi, low-temperature, mini-spit heat pumps. We were going to use an electric hot water heater but, instead, decided to install a Rheem hybrid heat-pump hot water heater. Heat pump hot water heaters offer the potential for considerable energy savings in the summer, but savings in the winter are less obvious. I am anxious to study this.
For the next year I intend to report on my findings for this guest cotttage. I have some systematic experiments planned to answer questions such as:
what is the efficiency (or heating COP) for these heat pumps at different outside temperatures
how does the cost of heating with propane compare to those using electric heat pumps
how does the carbon footprint compare between propane and heat pumps
do night time setbacks produce energy savings with heat pumps as they do with propane
how does the energy use of my heat pump hot water heater compare with an electric hot water heater
what is the impact of the heat pump hot water heater on my winter heating costs
I hope to yield definitive answers to some of the above questions. No doubt other questions will come up along the way.
I am primarily interested in the winter performance of these systems.
During the summer we will have guests using the cottage and that will make it difficult to control various parameters. During the winter my wife will be using the cottage during the day as her office. In the evenings it will be unoccupied — which leaves me excellent opportunity to control the environment. Both day and night the indoor temperature will be closely controlled and documented.
To record data and to control parameters we are running Home Assistant (HA) on a Rasberry Pi 4. All of the relevant devices in the cottage communicate with HA. This allows me to both read and control the heat pumps, hot water heater, propane heater, various temperature and humidity sensors, lights, dehumidifier, electric heaters, etc. Iammeter energy monitors have been installed on the main electric panel and both heat pumps. These communicate with HA, as well. There is also a weather station about 100 ft away from the cottage connected to Weather Underground.
Our main house also contains two Mitsubishi mini-split heat pumps. These, too, along with iammeter power meters attached to each, are monitored with HA.
While a lot of the instrumentation was being developed and installed this last year, we do have some preliminary results worth sharing. These will be the subject of upcoming posts.
The Pragmatic Steward 