Custom Controller Ups Heat Pump Efficiency

Heat Pumps are an extremely efficient way to maintain climate control in a building. Unlike traditional air conditioners, heat pumps can also effectively work in reverse to warm a home in winter as well as cool it in summer; with up to five times the efficiency of energy use as a traditional electric heater. Even with those tremendous gains in performance, there are still some ways to improve on them as [Martin] shows us with some modifications he made to his heat pump system.

This specific heat pump is being employed not for climate control but for water heating, which sees similar improvements in efficiency over a standard water heater. The problem with [Martin]’s was that even then it was simply running much too often. After sleuthing the energy losses and trying a number of things including a one-way valve on the heating water plumbing to prevent siphoning, he eventually found that the heat pump was ramping up to maximum temperature once per day even if the water tank was already hot. By building a custom master controller for the heat pump which includes some timing relays, the heat pump only runs up to its maximum temperature once per week.

While there are some concerns with Legionnaire’s bacteria if the system is not maintained properly, this modification still meets all of Australia’s stringent building code requirements. His build is more of an investigative journey into a more complex piece of machinery, and his efforts net him a max energy usage of around 1 kWh per day which is 50% more efficient than it was when it was first installed. If you’re looking to investigate more into heat pumps, take a look at this DIY Arduino-controlled mini heat pump.

59 thoughts on “Custom Controller Ups Heat Pump Efficiency

      1. “This specific heat pump is being employed not for climate control but for water heating”

        Either way, they only make sense if you’re utterly adverse to gas/can’t get gas/oil or electricity is really inexpensive. They’re horribly expensive here in the UK.

        In my view, it would be great if such systems were more cost effective and could be used as part of load side power balancing on the grid to soak up inexpensive excess renewable power – thus allowing more room for variable renewable power.

        1. As far as I know it was cheaper to heat with a heat pump compared to oil and gas even before everyone started boycotting Russia.

          Combining a heat pump with traditional oil/gas heating is done as well under the term “hybrid”.

          1. Cost depends on outside temperature. Using my US numbers from a few years ago, Heat pump is cheaper while it’s above ~40-45 degrees (F), 5-8(C) outside. Below that, oil/gas is cheaper.

            Cause: Heatpump efficiency varies based on temperature. Efficiency drops linearly with temperature (within operating range). Once you get too far below freezing, heatpumps stop pumping heat.

        2. They definitely make sense if one does the math, unless one has a very high kWh:therm cost ratio and very low appetite for long-term investment. However, generally speaking the building structure should be made efficient first to minimize the amount of heat pump required; a good design practice regardless of heating/cooling tech, but especially important for heat pumps. Alas, UK homes are notoriously under-insulated.

          In any event, if you’re a staunch advocate of burning methane in your home, when it comes time to replace a boiler, a gas-powered absorption heat pump from Rober may be a good alternative. It still burns methane, but uses the heat to drive a heat pump, and can therefore exceed 100% efficiency/provide more heat than was in the expended methane.

        3. Gas does seem oddly cheap, the data I can find says it was the same price in 2020 at the start of the pandemic as it was back in 2000. If the price were tied to carbon output electricity might be so much cheaper than gas with a green grid. My local one often manages 50g CO2 per kwh vs natural gas at 184g so even a resistive heater would be better for the environment.

          1. If you’re talking about the UK , prices are heavily regulated at the retail level, which explains why so many retail providers are going into administration , as retail prices seem to be disconnected from the real world.

  1. Replace that LED thermometer with an LCD version, to save some more Watts 24/7. Or run the display supply through a momentary switch, so it is only active when you push the button and are actually looking at that thing.

  2. Please could Hackaday writers be more circumspect when it comes to the performance of heat pumps? Upto 5x the efficiency of electric resistance heating – sure, but under what conditions? The greater the temperature difference between the condenser and evaporator thermal interfaces, the efficiency (or coefficient of performance because it goes above 1 and that makes people uncomfortable using the word “efficiency”) drastically reduces. So in ideal marketing conditions, air source heat pumps look really efficient. But on a cold day, when you need to get the external heat exchanger well below your ambient outdoor temperature, they will be poor converters of electricity to heat, relative to the hype around COPs of “upto” 5.

    The choice of heat transfer to the indoor space is important too. Low temperature difference, high surface area systems like underfloor heating are better suited to heat pump systems. Smaller surface area (convecting) radiators need hotter water to heat a room to the same degree, so are a poorer match to heat pumps.

    1. I see those “up to” claims as “no more than” instead of “typically”. for the US. Not very “cold”
      >At soil depths greater than 30 feet below the surface, the soil temperature is relatively constant, and corresponds roughly to the water temperature measured in groundwater wells 30 to 50 feet deep. This is referred to as the “mean earth temperature.”

      For cold climates, you want to to dig deep below ground depending on your definition of cold.

      Some other website points out:
      >In Manitoba, vertical loops are normally installed in boreholes measuring 50 to 300 feet deep and 10 to 20 feet apart.

      1. Typically assumes normal conditions within manufactures “testing” spec,(which could be ideal conditions). Up to is worse number, it says nothing about any expectation, usually a theoretical number. Based upon every variable being perfect. Never achievable in actual use.

        1. I think the issue is how the COP is measured for Heat Pumps is not realistic.

          I assume they calculate the COP, by filling the tank with cold water, turning on the heat pump then waiting for it to reheat the entire tank. Then simply calculate the heat energy put into the water divided by the electrical energy consumed.

          However this is not typical as this would only occur when you first switch on the hot water service or if you ever fully run out of hot water. 99% of the time the tank will be partly full of warm or hot water and the heat pump will do a partial reheat of the tank. Under this condition the performance of a heat pump is greatly reduced as it may be reheating a large amount of already warm water a much lower efficiency.

          I suspect this is a throwback to calculating the efficiency of an electrical resistance hot water service as a partial reheat does not have any effect on the efficiency of these type of units.

          This is pretty much the problem I found and resolved with this controller.

    2. Even if your typical heat pump is not COP 5 in practice (assuming it was rated as such, that’s 17 HSPF in conventional US ratings) except under very poor system design with adverse use scenarios it is rare for them to not exceed a long term average of COP 2. That’s ~200% efficient “at the tap”, and when you factor in grid efficiency it places them somewhere around 87% total efficiency, worst case. Compare that to a furnace which is most likely 80% but may be up to 96%. Then account for methane losses in the pipe network, and thermal losses in the ductwork if it’s not very well installed/inside the living area, and you get something like 80%/1.05*80%=61% to 84% overall efficiency. A hydronic system, especially one with a condensing boiler will fare better, around 95%/1.05=90%, but the boiler an furnace’s efficiency can never get much better, whereas the heat pump’s total efficiency ill improve as the grid decarbonizes. (You could use a gas-powered absorption heat pump, but then you’re just being obstinate. They are very specialized and outside of Europe have low availability and high cost for residential-scale units like those from Rober)

    3. This isn’t at all how heat pumps for air are sold, at least in the US. HPs are more efficient than running a 10K heater coil, and anyone who’s ever owned one where “Emergency Heat” is kicked on from the thermostat has seen their electric bills skyrocket to know the difference. But they do have a flaw that once you fall to or below 0°C outside, it’s not that they are suddenly inefficient, it’s that they can’t suck any more heat out of the atmosphere–they’re not designed to attain absolute zero and the refrigerant would condense and freeze well before that point anyway. In extreme cold, a heat pump will leave you with a high electric bill since your thermostat will turn it off and use the backup resistive heater coil, assuming you didn’t cheap out and installed it with your air handler. For most of the Southern US, HPs are energy saving solutions over gas any day, and they’re often just as efficient cooling than ACs. Get too far North and it’s a waste of money but arguably so is an AC too. If you’re interested in some of the science measuring HPs, look for SEER, EER, and HSPF for measurements of their efficiency. And if you’re really interested in the secrets between an 18 SEER and 20+ SEER or 95% vs 97% AFUE furnace, it has more to do with the fan moving the air than compressor or heat exchanger.

      1. The fact that heat pumps are used in Norway and other very cold places says that they can work at low temperatures. I suspect the difference is water vapour in the air since heat pumps lose a lot of heating power if they need to de-ice every 5 minutes but below a certain temperature the air gets dry again. It could also be refrigerant choices, maybe US spec machines are intended for 32-100F when some states need -40 to 30F units.

      2. “below 0°C outside, it’s not that they are suddenly inefficient, it’s that they can’t suck any more heat out of the atmosphere”

        This is simply not true for modern heat pumps, which are rated to work at full efficiency down to -13C or lower, and reduced efficiency at even lower temps than that. This guy just put out a very interesting video which reviewed a full year of temperatures in his home town on Chicago, a place where conventional wisdom says that winters are too cold for a heat pump, and found that resistive heat was more efficient than a heat pump on just 17 days all year. In fact on many of those days it was only that cold for part of the day and in reality there were less that 6 days in a full year where other heat sources were more efficient than a heat pump.

        1. Yes, but….. even as he said in the video, that doesn’t take into account the price difference between nat gas and electric. Anything below about 35 deg my electric bill increases more than my nat gas bill decreases. Saving $5 on gas costs me about $7 on electric. yes, its 2.66 times as efficient, but due to the cost of electricity, ends up costing me more. You get around 96,000 BTUs of heat in a Therm that costs around $1.20, but 100% efficient resistive heat you need 28 kwh to offset those 96k BTUs. Unless you are paying 4.3 cents a KWh, its costing more. I’m paying around 13 cents, so even at 2.66 times as efficient, elec would need to be 11.5 cents/kwh to break even. That sounds like grasping straws, but months I need 130 Therms to heat the house, I would need 1,375 Kwh offset that, which is $156 vs $178, but thats only at 30 deg average. Below that, the spread increases.

      3. Heatpumps work just fine below 0c, when designed for it

        They do loose quite a bit of efficiency though, but still operate above COP 2 most of the time and some might be above COP 3, even in -5 and -10. In my area we rarely see below -10, but resistive heaters in relation to heatpumps are only used to de-ice the unit, as all units sold will still deliver their rated heat-output down to -20c with a COP around 2.
        In even colder climates where the temperatures go down to -30 and -40c, the COP approaches 1 and the heat-output falls considerably, so a traditional furnace (Gas, wood, or even resistive electrical, though uncommon because of cost) becomes a necessity

        1. Over the years I have experimented with several heat pump configurations. The basic system is a 2 ton SEER 10 scroll compressor with 1500W heat strips. We get night time temps as low as teens or single digits in Jan or Feb. By using the thermostat to vary how long the HP vs. the Aux runs and mix the two I was able to strike a balance between peak electric bill and system run time.

          1. Aux heat alone turns on below 25F – $220/month
          2. Heat Pump with 1500W Aux concurrently – $180/month
          3. Heat pump with 750W Aux concurrently – $120/month

          This used a thermostat with a 2nd stage timer of 5 minutes after starting the heatpump and not reaching the set point. I was able to easily unplug half of the strip heater to reduce it to 750w. I would imagine reducing the size of the strip heater to 375 watts would be an even larger saving. If there were an improved controller that could dynamically sense when to add strip heat it might even further improve things.

    4. “high surface area systems like underfloor heating are better suited to heat pump systems. Smaller surface area (convecting) radiators need hotter water to heat a room to the same degree, so are a poorer match to heat pumps.”

      just a little nitpick…whether it requires a high temperature water or not is independent of whether it’s underfloor or radiator. when i installed hydronic heat in my house, i speced out both, and i found that underfloor heating was on the outside edge of possible…i’d have to run two pipes down each joist gap and even so it would be hard to push enough watts through a hardwood floor (with 180F water) to achieve satisfactory performance in indiana winters. but otoh i was easily able to get convective radiators of a reasonable size, one per room, to deliver the number of BTUs i need. and with only 140F water (though in practice my water heater runs a little hotter than that and peaks out at a 2/3rds duty cycle on the coldest nights)

      but your point is right on, no matter what you will have to examine the BTU vs water temperature performance of your radiators and that will determine the other characteristics of the system, especially what technology you use to heat the water

      1. That is awesome for Germany and Austria, but I think smerrett79’s point is that you can’t just spit out a number like 5 when there are so many other factors at play. For instance, broadly speaking, the average winter low temperature in Germany is warmer than the average high in the Upper Midwest of the USA. Similarly in the summer Germany is a good chunk cooler. In fact, Germany is very mild compared to many other parts of the world, so it is much closer to ‘ideal’ than ‘average’ when talking about heating/cooling efficiencies.

          1. In Phoneix, AX: Last summer was the hottest on record, setting marks for the most 95-degree days (172), 100-degree days (145), 105-degree days (102), 110-degree days (53) and 115-degree days (14) in a year while also recording the hottest July and August in history.
            What days to you suggest using “an open window and ventilation fan”? Christmas?

          2. “Turn the AC off. Open a window and turn on a ventilation fan.”

            Clearly you don’t experience high temps AND 90%+ humidity where you live.

          3. My A/C unit uses less energy to leave on 24/7 through the hot months than using 3 or 4 fans. It also uses less energy (Actually it might be money rather than energy, but it probably correlates to carbon emissions*) to leave it on set to about 23C constant, than to try turning it off and on through cool spells and “it won’t be that warm” spells…. mainly because it takes 3 days running flat out to “catch up” when the forecast was wrong and you’re 3 days into a “pleasant weekend but then cooling” but ending up 30s rather than 20s, that’s suddenly reforecast as a 2 week heatwave.

            *meaning that if it takes up cheap overnight power to “store cold” in the fabric of my house, then that’s likely to be low emission sources, that help it ride out a super hot day where electricity is twice the price at peak times and demand probably means that fossil fuel is burning somewhere in the grid to satisfy demand. Whereas if you started at 27C and it was running to bring all the thermal mass of your house under control at $$$ rates, it would be running more frequently in hottest, most high electricity demand = more expensive, more carbony part of the day.

  3. The fascination with air source heat pump water heaters installed indoors has always mystified me. I can see it in warm climates, but for much of the year in northern climates, they are removing heat from interior spaces. This heat has to come from somewhere else, most likely the building climate control system.

  4. If heat pumps aren’t efficient heaters at lower outside temperatures, they always make good air conditioners. And when they are connected to an off-peak service, the electricity cost (here) is just slightly over half the normal price. That means air conditioning costs will be about half of what they would otherwise be.

  5. Good that he could figure out and find a solution to this problem. But (and I may have missed it), but there didn’t seem to be a lot of discussion as to the choice of a heat pump to heat the domestic hot water. What about solar thermal (evacuated tube type) and a small solar powered circulator. How much sun does he get? How much hot water does he use? What is the outdoor ambient temperature(s) in summer/winter. What is the potable water inlet temperature? There are many questions that would be asked when analyzing the setup he had. But, he fixed his problem and that is good.

    1. I left a lot of details out of the video or it would have been 2 hours long.

      I did a spreadsheet analysis of electrical resistance, gas instant, solar and heat pumps, based on the manufactures specifications, our cost of energy as well as our special requirements. I selected the Heat Pump and it was the correct choice for our requirements. This was the start of the problem as my spreadsheet estimated electrically usage of the Heat Pump and when it was installed, it did not match actual power used.

      We get lots of sun and we have since installed a 6.2kW PV system, so the heat pump is really running off solar.

      We are a 2 person household and use an average of 100 litres of hot water per day.

      We have a pretty mild climate in Sydney, Australia and we are also 1km from Ocean, which makes it even milder. Outdoor average ambient temperatures, Summer (Low 20C, High 26C), Winter (Low 9C, High 17C). It never gets below about 6C and normal maximum is around 38C here.

      I measured the potable water inlet temperature of 12C in winter and 18C in summer. The water supply enters the property underground, so it picks up the ground temperature. During our renovation we had a temporary black plastic 50m above ground water supply, which dropped the temp down to 6C in winter and in summer as high as 30C.

      Yes, there is a huge number of questions to answer when analysing this system, but I wanted to keep the video to a limited length.

      No one has asked the critical question. Exactly how did you measure the COP vs the Heat Pump Inlet water temperature? That would normally take a temperature controlled lab with a lot of precise instruments.

        1. Great question, that’s an entire hack in solving that problem!

          I can measure the COP of an entire heating cycle, by first turning off the heat pump and running out all the hot water. Then switching on the unit and monitoring the power consumption, while making sure not to use any hot water. Knowing the size of the tank, temp of the cold and hot water, the total power consumed you can calculate the COP. There are lots of errors as my measurements are not lab grade, the temperature of the hot water in the tank is not uniform and other errors. But it close enough.

          To measure the COP vs inlet water temperature you need to measure the COP in real time. You need the instant power consumption, which I have via the power monitor. You need the temperature rise, which the Heat Pump debug interface has, output temperature – input temperature. You just need the mass flow rate of water going through the heat pump. I was thinking of getting a flow meter, when I realised the debug interface shows the pump speed. The flow rate will be approximately linear with the pump speed. However I did not know the flow rate vs the RPM of the pump. I needed a calibration cycle to do that.

          During a full reheat all 315L of water will be pumped through the heat pump, so that could give me a base for the calibration. The heat pump has a specification COP of 5.0 which I could also use that as a double check. I made a spreadsheet with all the numbers and had to go into an iterative process to adjust the flow rate vs RPM parameter until the numbers balanced.

          Once I had that calibration I could calculate the COP in real time, while also seeing the input temperature. There are lots of errors in this solution, but it’s a hack and it was good enough to fix the problem.

  6. Have two heat pumps. These air to water heat pumps heat the house and hot water. COP generally about 4. Down to 3.3 on cold winter days. Heat pump controller takes How Water Cylinder to 65°C Once every 15 heating cycles to prevent legionella.

    On sunny days, a water solar panel provides all hot water. The solar panel takes precedence over heat pump. The heat pump in turn takes precedence over electric resistance elements.

    A very efficient system. Set up so no chance of reverse flow thermosyphoning, due to good engineering. But thermosyphoning takes care of Solar Panel flow. No pump in this system.

  7. Hello, I find that air source heat pumps produce heat even when it is below 30. Just measure the temperature rise in the air handler. However as the temp drops outside they need the judicious addition of a little resistance heat to make them bearable and not run constantly. I wish there was a controller out there built in to a thermostat that controlled this. Maybe the high end systems have this built in. My SEER 10 system surely doesn’t.

  8. In the comments I’m seeing a hell of a lot of “truisms” about HP not working in the cold (typically “given” has freezing, 0 C). Bullshit.

    You might want to check out the air-to-air heat pumps that are DESIGNED for use in cold climates before guessing on truisms. You can start with the Mitsubishi models. They have add-ons for cold climate installation, or you can get the models with integrated components for cold climate use.

    A bit south of me, a friend is using a Mitsubishi air-sourced heat-pump to heat his house, with winter nights down to -32 C this year, and with most of the family at home most of most days, it’s heating to cozy comfort 24/7. The unit replaced a central furnace, and it has a built-in resistant heat coil for really cold nights, but the breaker for that coil has remained shut off since the whole system was installed – hasn’t been needed yet (lowest was -35 or -36 C, which I believe the coil would heat for less power used than using the HP at that point, or in conjunction with – but the HP does the job). They’re in use throughout the region, with models configurable for public buildings and residential, of various size. My friend has a single outside unit. The library down the road has three. Etc..

    1. Yeah, you see that kind of comment every time this topic comes up. They seem to invariably come from people who either A) formed their opinion about heat pumps 20+ years ago and don’t realize where today’s technology is or B) live in a mild climate where their cheap heat pump struggles to keep up a couple of nights a year.

      Meanwhile there are people all over North America and Europe who are using properly-specified ASHPs in really demanding climates.

    2. meh. i see a lot of this sort of claim, “modern heat pumps can work in the cold”. first off, it’s less efficient when it’s cold, and if it approaches the cost of resistive electric heat on the coldest nights of the year…you will not be happy with your electric bill!

      but the other is just that reality exists. the only reason you don’t want a heat pump in a cold climate is that 9 out of 10 HVAC contractors in your cold weather city don’t consider them suitable and will be on a voyage of discovery as they deal with the issues that come up. “the only reason” is the same as “the reason.” that’s a reason. it’s a legitimate one. if you call the HVAC guy and he says “meh, it’s possible, but”, that’s a solid reason not to go with it.

      i mean, you run into the same problem with hydronic heat or tankless water heaters. it’s a great technology, and if you go your own way you can make it work. but most contractors in the US (at least where i live) have barely heard of either of those and you will not get quality work out of them. reality exists.

      you can make it work but odds are very good you’ll be dissatisfied or you’ll be doing all the work yourself. “it’s sorta possible now” does not refute the old advice.

    1. Its a scroll type inverter compressor. I assume most of the oil ends up in the sump of the compressor. There does not appear to be an oil sump heater shown on the wiring diagram and I could not see one when I opened up the unit. So I was not worried about having the unit off for a long period of time due to oil temperature. The compressor sump is also mounted below the evaporator and condenser, so I assume any oil would run back down into the compressor. Whats your concern with having oil return to the compressor?

      1. Compressors discharge oil along with hot gas as they run. Systems are typically designed so that refrigerant velocity is high enough to entrain the oil and bring it back to the compressor. Variable speed systems often dip below this velocity as they unload. I don’t know enough about your system to say anything for certain, but just because the evaporator and condenser are above the compressor isn’t a guarantee that everything is pitched to allow a gravity return. Most variable speed systems I’ve seen have an oil return cycle where the compressor runs at full speed for a period of time. It’s either initiated at every start up, after a period of run time, or by an oil level sensor.

        1. Thanks Bill, I did not know that about variable speed compressors. I have not seen this compressor run through such a cycle.

          Over 90% of the time the compressor runs around 41Hz (2460 RPM), never slower. The fastest I have ever seen was 52 Hz (3120 RPM). I would expect it would run faster under much colder ambient, but it never gets below 6C here. The compressor speed control loop is quite slow around 15 minutes. I expect this is because the unit can modulate its output by controlling the variable water pump speed (1000 to 5900 RPM) rather than the compressor. It modulates the pump speed every second and if the speed approaches 1000 RPM it will increase the compressor speed. Decreasing the compressor speed as the pump reaches 5900 RPM.

          It also does a very soft start of the compressor during its startup sequence. Taking about 60 seconds from start to get to the 41 Hz speed. But on shutdown it does a hard stop. The manufacture says that it fine to run the system under control load (OFF Peak), so they don’t expect it to be powered all the time. But maybe being shutdown for 6 days may be pushing it, especially in a cold climate.

          1. Thinking, it may not be an issue with hot water services. They should be designed to heat the tank as fast and efficient as possible. They don’t have much need for unloading as they simply shutdown. So maybe they never dip below the minumum oil velocity. That would be more of an issue for air handing systems as they may have too deeply cycle.

  9. I had a heat pump installed a few months ago due to a $1000 rebate that almost put it on par with a regular central air. I live by Lake Superior, and since I work from home, AC is starting to become necessary. My watt meter shows the heat pump using 2000 to 3000 watts depending on temp. It’s a Bosch 3 ton. At 45 degrees it works great. At 30, it pretty much runs all the time and takes a whole day to increase the temp 5 deg. At 20 degrees, it will run constantly and the house will slowly get colder over the course of the day. I pay about 13 cents a kwh, and $1.20 for nat gas. My 97% furnace costs about 70 cents an hour to run, and the heat pump about 25 to 36 cents an hour to run…. but….

    At 30 degrees, the furnace will run for 6.5 hours costing about $4.50, the heat pump will run 24 hours and cost $6-7 to run. At lower Temps, it will get even worse – unless you get a low rate on on an interruptable power meter (5 cents a kwh), bringing it down to $2.40 to $3/day. Resistance heating is extremely costly, might as well just mine crypto, and least you get some cash back

    1. How many square ft is your house and how tight is it?
      Double or single pain windows?
      How much and what kind of insulation in the ceiling and walls?
      Is the ductwork in the conditioned space?
      If not, what is the R value or type and thickness of the ductwork?
      That all males a big difference!

      1. 1180 sqft. Built in 1971. R7 fiberglass in walls. Maybe a foot in the ceiling. Double pane windows installed 14 years ago. Ductwork at ceiling in semi heated basement (stays at 55 deg in winter). My brother has a similar house in the same area, less than 10 years old, and mine costs about 50% more to heat with gas using a similar efficiency furnace. Improving the insulation would likely save $250 over a typical heating season, but payback would take 10 to 15 years.

  10. Air source heat pumps will lose efficiency when ambient temps drop below 5c due to moisture freezing on the outside heat exchanger. Ground source units prevent this, by using a secondary fluid pumped underground to keep the evaporating temp above freezing. Both types are capable of being 3-4 times as energy efficient as an electric resistance heater or gas or wood. Inverter type heat pumps can improve efficiency even further by speeding up/slowing down the compressor.
    Example – electric boiler for radiant underfloor heating 8kw – 250sqm house. Was costing approx $400p/month in direct electrical cost. We replaced that heater with a heat pump – cost dropped to approx $140per month, temp was set higher – customer was generally happier. Cost of installing this unit gave a payback time of around 8years. It lasted 13yrs before failing and needing replacement to a newer/more efficient model. I don’t have a cost if customer had just replaced his electric heater elent after 13yrs of use, but it would not be more efficient than any other electrical resistance heater, but it would be cheaper than a new heat pump.

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