Arduino Powered Heat Pump Controller Helps Warm Your Toes

Heat pump heating technology is starting to pop up more and more lately, as the technology becomes cheaper and public awareness and acceptance improves. Touted as a greener residential heating system, they are rapidly gaining popularity, at least in part due to various government green policies and tax breaks.

[Gonzho] has been busy the last few years working on his own Arduino Powered Open Source heat pump controller, and the project logs show some nice details of what it takes to start experimenting with heat pumps in general, if that’s your game. Or you could use this to give an old system a new lease of life with an Arduino brain transplant.

In essence they are very simple devices; some kind of refrigerant is passed through a source of heat, absorbing some of it, it then flows elsewhere, and is compressed, which increases its temperature, before that increased heat is lost where the increase in temperature is desired.

This heat source could be a river, a mass of pipes buried in the ground, or simply the air around you. The source and quality of the heat source as well as the desired system operating temperature dictate the overall efficiency, and with ground-source systems it’s even possible to dump excess heat directly into the ground and store it for when required later. This could be the result of a residential cooling system, or even directly sourced from a solar heated setup.

This heat pumping process is reversible, so it is possible to swap the hot and cold ends, just by flipping some valves, and turn your space heater into a space cooler. This whole process can trace its roots back to the super talented Scottish professor, William Cullen who in 1748 was the first person on record to demonstrate artificial refrigeration.

The power needed to run the compressor pump and control gear is usually electrically derived, at least in non-vehicular applications, but the total power required is significantly less than the effective heating (or cooling) power that results.

We’ve covered a few heat pump hacks before, like this guy who’s been heating his house geothermally for years, but not so many platforms designed for experimentation from the ground up.

The associated GitHub project provides the gerber files as well as the Arduino code, so you’ve got a great starting point for your own heat pumping builds.

39 thoughts on “Arduino Powered Heat Pump Controller Helps Warm Your Toes

  1. “but the total power required is significantly less than the effective heating (or cooling) power that results.”

    No, it’s not, but they can have pretty good electrical to thermal efficiency.

    1. Are you trying to troll, or are you just ignorant?
      Not trying to be rude, legitimate question.

      The whole idea behind a heat pump is that it uses energy to *move* energy. Like an AC in reverse. You spend lets say 100 watts to move 50 Watts (random numbers i pulled from my head) and you’re heat source (the ground, a lake, whatever) looses 50W, and your house is now 150W warmer, effectively giving you a 150% Effeciency.

      You haven’t broken any laws of thermodynamics, as long as you consider that you cooled the other end of the loop.

      1. Only under certain circumstances and with proper design can electrical to thermal efficiency higher than 100% be achieved with heat pumps of this type.

        He has four extra losses in this setup (two heat ex-changers, two electric pumps), and the only way 100% or higher thermal transfer could occur is when the heat source (outside, most likely) is of higher temperature and has the thermal capacity to source the energy needed to heat up the floor (inside), which must (initially) be at a lower temperature.

          1. I agree with that, I have seen test data for example for the Mitsubishi Ecodan that achievers 420% heating. However this is still not enough, you need around 800% to make commercial sense in the UK (i.e. replace natural gas with an air source heat pump). e.g. see
            Certification Number: 037-0033-20-03

            I do agree, however, that the efficiency drops markedly in cold conditions, e.g. below -15C you get around 200% efficiency, just when you need it most, as the heat losses from the house increase.

        1. Uhh…. what?
          Proper design =>
          Yes, but this doesn’t mean heat pumps don’t work. Case in point, ground source heat pumps are quite a bit more effective than an air source system, which have been criticized several times below.
          Note that the system in the article apears to be ground source.

          Only >100% when outside is warmer than indoors =>
          When outside is warmer, heating is kinda easy. Like without *any* energy, you can heat your house.

          Also, the ‘losses’ that you mention aren’t truly losses. Any mechanical or electrical losses in the system will mostly function as turning energy into *heat*, which is what you wanted anyways. Whatever energy is left over after the inefficiencys of the transfer system is then able to bump the total system energy transfer to well above 100%.

          As mentioned by a few others below, values of 300% or even as high as 400% are possible.
          Here are some sources for reference:

        2. You do not understand the physics of a heat pump. It takes advantage of the huge energy transfer when there is a liquid to gas phase change. The machine only need expend enough energy to create the proper temp and pressure so that the ambient air is able to do the work of changing the phase state, e.g. condensation. A crude analogy is how you can expend very little energy moving a huge mass laterally (like a piano on rollers). A practical heat pump would not use 100W to move 50W, more like 100W to move 300W of thermal energy. So even AFTER the system loss, i.e. the heat you add to the system due to resistive loss, the thermal efficiency is still 200% to 300%, after all losses.

      2. Looks more like pedantry to me.
        The system has two power inputs, electrical, which powers the compressor, and thermal, which heats the evaporator.

        The total system output energy equals the total system input energy, as per the conservation of energy law.

        1. *That* sounds like pedantry. Yes of course the energy balance is even, that’s not the point. Yes it could have been worded to be more technically correct, but humans are colloquial in communication.
          This is referring to the Coefficient of Performance, of which heat pumps are capable of being higher than 1. Resistive heating elements are at very best able to approach a COP of 1, but never reach it. Heat pumps are often upwards of 3.5 COP, meaning they can move 3.5 units of heat energy from source to sink with only 1 unit of energy supplied to the mechanism doing that work.

        2. You fail to take into account he is using a free source of Heat energy ….. He pays nothing for the heat energy he draws from whether it is outside air or underground water …. He pays nothing to create that heat energy, it’s completely free of charge

      3. As Brian mentioned (in his own way), in real world applications you never experience more than 100% electrical to thermal efficiency.

        I have a heat pump system in my home, and we find it is better than natural gas for heating in my part of the country (electricity is cheaper then gas where I am), but for cooling, we only see a small improvement in electricity used compared to the traditional central air conditioning we had before this, since the heat pump still has a compressor and blower, with the only improvement being the “condenser” (when in cooling mode) is underground, so it stays cooler, the heat is easier to move out of the house.

        1. In order to work out your electrical to thermal efficiency % you would have to have measured how many kwhr of heat output you are getting from your heat pump system. How are you measuring that?

      4. Hello, I am an HVAC/R contractor and will interject my humble opine here. With a heat pump one of the best ways to think of it is as an air conditioner. One end puts out hot air and the other puts out cold. In the summer the cold side can put out sub zero degree air while the hot side can reach temps of 150F. now comes winter and you want to heat your house. You take the air conditioner out of the window and turn it 180 degrees and re install it. Now you have the hot side blowing in and the cold side blowing out. Here is where it gets interesting. Lets assume an outdoor temp of 32F. The refrigerant has to extract heat from the cold to deliver to the conditioned space. The refrigerant is not as efficient at removing heat from cold as it is close to its saturation point. This causes the system to work significantly harder (more power consumption) to achieve the same effect. Now add in the inefficiencies in transfer from one medium to another. In ground water loop in pex tubing (not a great thermal conductor) through copper or stainless (not a great thermal conductor) to refrigerant back to copper and then to air(not a great thermal conductor). I hope I am beginning to show how much loss is actually in the system. You also have to weigh the costs of the fuel being used. In the winter my home is heated with natural gas. My power costs go way down. In summer air conditioning drives my power costs way up. Electricity is far more expensive in the usa than natural gas. Of course this not an apples to apples comparison as i could get into the cost per therm or BTU however the fact is that a heat pump is inefficient. The manufacturers are improving product over time. I live in illinois and air to air heat pumps stop working when it gets cold enough so you still need to augment the system with NG or propane again raising operating costs. Sorry to get long winded.

        1. While you’re right in regards to the heat pumps not working well in freezing temperatures, you’re ignoring the fact the the thermal energy transferred from gas rarely hits efficiency above 80% without a lot of trickery (actually science) related to heat staging and blower motor advances (multistage, variable speed, etc.). The difference is clearly moving the heat from outside to inside vs extracting the heat from a fuel source you’re burning.

          In the North, it simply gets too cold for heat pumps to function, even when you include a lot of the newer tech that can work well below freezing. Wrong application of the tech. Same for installing 95%+ furnaces in Florida, it doesn’t get cold enough to justify the expense and that includes even heat pumps down there. The common installation is an AC with an air handler and maybe a heater coil for those rare 50° F nights. Places where heat pumps are effective: pretty much anywhere below the Missouri southern border and above the Florida panhandle, you know, the fastest growing places in America.

          Source: Worked for an international HVAC manufacturer for almost a decade.

          1. I installed an HP in NJ. Very unscientific, but I’ve measured heating costs vs NG and about 40 F, the HP is cheaper to run then my NG baseboard water heating system. Below 40 it’s more expensive, though it still works fine even into the low teens. The lower the temp, the lower the COP and the higher my bill. Obviously, a lot depends on NG vs electric prices, but HP’s COP drops to near resistance heat below 40 F, which is of course a terrible way to heat your house. IMO, my HP (works at 100% heat output to negative 15 F) is really only a 3 season device. You need a NG heater for the winter if you’re in a cold climate.

      1. The argument boils down to different people using common words in different definition-senses. It’s not quite as pedantic as it sounds. Rather, it’s a really good example of “First, define your terms.”

        The one side is arguing that you cannot get over 100%, because they’re talking only about the heat *added* to the system by the heat pump itself. In other words, if you consume 100 watts of electricity, it turns into 100 watts of heat *somewhere* (or else, light, sound… details :)

        They are analyzing the system in isolation and ignoring everything outside of the heat pump itself. In effect, they’re treating the heat pump as if it were just a motor and a friction load acting as a resistance heater, and ignoring the results of its pumping as “irrelevant for the purpose of analyzing the system”.

        The other side is instead focusing on the problem the heatpump is used to solve, which is to move some quantity of heat from an unwanted location to a wanted location. It doesn’t matter if you’re building a HVAC or a refridgerator, both of them can be described this way.

        In this case, the power consumption of the heat pump is only a minor matter of efficiency. The actual heat is coming from an external source and is being dumped to an external sink. If you can transfer kilowatts of actual heat with only hundreds of watts of power used for pumping, you’re making kilowatts worth of difference in solving the problem. Using even less power to move even more heat is of course welcome, but the task is generally to keep some external item hot (or cold), at some setpoint temperature. The details of how that’s achieved are just engineering.

        So, since they’re focusing on different things and talking about different things, they use the same words in different ways… and of course this means they cannot come to a consensus.

        In this particular case, I think the second group is behaving more appropriate as engineers. The first group is doing the equivalent of analyzing only the thermal results of a rocket motor static test firing while totally ignoring that the PURPOSE of the rocket is to create thrust (and heat is just a side effect). But I’m biased toward practical engineering rather than ideal engineering.

    2. What they meant is that compared to a resistive heating element the amount of heating unit created by a heat pump will be more up to 4 time the amount of heat unit generated by a resistive element. In the old days heating unit were called BTU, dont know the new name now if it changed. So X Watt of electrical power create Y BTU unit. With a heat pump X Watt of power will create 4Y BTU. So they say that a heat pump is 4 time more efficient then simple electrical resistive heating.

      Yes this was a BIG mistake of simplifying the information so that it can be consumed easily by the mass. But who are we to point the finger, after all we are here to hack our way into project for the fun of it.

      Now can we go back to the original design of the post and have some fun leaking some refrigerent stuff into the air so that we will need more effort to lower the heat from our home? ;-)

  2. OK this stuff is so very interesting to me as I have been involved in geothermal, solar electric and heating of air and water. Along with off and on grid wind power systems. I am currently living in a large older Motorhome which I would like to implement a heat pump system into. I currently have 400 watts of solar connected to the battery bank plus shore power, there are 2 24k BTU LPG furnaces installed in the RV. I live in western S.D. and want to cut back dramatically on fuel usage. I used small space heaters last winter for a large part of my heating until I succeeded in getting both furnaces repaired and functioning. After that my electric went WAY down but my fuel bill went WAY up. So now I want to work on as much heating as possible from more efficient sources such as this. I was thinking about constructing a heat storage tank to warm up fluid during the day and hold for night usage to work with the heat pump which would be pumped into a piping system installed under the floor and insulated and making the system so it could be connected to the AC system already installed on the roof and functional. I have nearly everything needed already as far as I can see including the Arduino Nano all the way to the vacuum pump and the AC gauges since I used to work in and at an auto repair facility, I last worked in my own shop which I was forced to shut down on account of diabetes real bad. So far it looks very detailed ! I’m going to have to bone up on this for sure. Being a diabetic I really need my toes warmed up in the winter for sure. Last winter I wore multiple layers of socks even with heavy wool foot warmers and still suffered when it was -20 or lower out there. The year before, I didn’t know I was a diabetic and was driving a tow truck frequently and pulling people out of ditches and wearing insulated boots and sheepskin lined water proof gloves and really suffering. Right now of all my running vehicles(4)(one, the Trans guit 2 winters ago, that one is the only one that has a working heater) not a one of them has a working heater in it right now, so as soon as I start getting my SS income, you can figure out what is a priority this winter. Working on ones own vehicles only costs money and doesn’t earn money. Pretty tough to make a living on your own stuff.

    1. what you need is an insulated concrete slab under your RV which has heating coils in it. Or an insulated pile of rocks under your RV which you can heat during the day and let them radiate up to the underside of your RV over night. You’d want to put up insulating curtains around the gap between the earth and the lower edge of your RV on both sides and front and back.

  3. Nice, I have been optimizing my Heat Pump hot water services over the last few years. I have managed to greatly reduce its power consumption, by overriding its control schedule and cycle. This open source controler could be a basis for more advanced optimizations. I cant use this directly as my heat pump has variable speed compressor, fan and pump. But it could be a good start.

  4. I like this. We need ways to use solar panels without hiking then into the grid, or even our house wiring.

    Dedicated panels to just increase comfort or take loads off other systems.

    Pool pumps also come to mind.

    1. That doesn’t make sense. While the sun might be constant for certain hours of the day, your loads are not. They cycle on and off. Dedicated panels means you’ll festoon the countryside with panels for one electrical item and when it’s not running the panel just sits there wasting space. Hooking them into a grid, even if your house, allows for the energy to be used by all systems that need it so you’re solar panel footprint is much smaller. Also add in the face a common 250W panel is roughly 12V at 20A then invert it to 120V at 2A…. you’re not going to power many things with 2 amperes of current. That doesn’t even include inverter losses. You’d need a lot of panels for individual motors or… make one grid and feed just the loads that need them at that time.

      There’s a reason the actual grid is real time and closely watched to maintain balances and is scaled up and down depending on the load.

      1. My thought is to augment current hvac so the panel is constantly used.
        Or augment pumping to clean water in a pool to reduce the time the main pump runs.

        Yes, a single panel can’t run a big compressor/pump. But it could run a different system which is what would be fun to hack/create.

  5. Personally I prefer the “forced air” variant over the “passive radiation” variant for several reasons

    1. Air filtration so your home is less dusty and if you have allergies pollen, pet dander, etc. can be removed
    2. You get a more even heating/cooling
    3. You have some air movement
    4. You add add humidification in the winter and it naturally dehumidifies in the summer, You get neither with passive radiation
    5 . You can use “setback” thermostatic control to gain even more overall efficiency
    6. In existing homes you don’t have to tear up the floors or walls to add pipes but can leverage the (usually) already existing duct system

  6. Thats great and intetesting project. I am going to install this autumn Gorenje Terragor earth heat pump on/off with 3F Coppelant compresor in my house. The heat pump has 6.5kw output energy with 1.5kw input powet (COP4.3) if temp of primar loop is 0°C. But I expect the COP higher because I will return the heating energy during sun days back to the earth colector with 2kW solar collector. So the temperature of primar will be higher.
    It really worth because in comparison with air heat pump. The earth pump can work with constant COP in monovalent state all 365days. But air hp not. I used electric heater(boiler) last years and my house needed for heating (floor 30°C and hot water 55°C) in average 9MWhours.
    With the heat pump I should pay only for 2MWh. If you take into the account FVE panels 3kWp you can save much more many for electricity.
    I know cases with water-water system heat, pumps where the input T is constantly 10°C from a well and the COP is 7.

  7. Hi from France;
    Nice project !
    But….you didn’t think about the 4 ways valve.
    In my country this 4wv is mandatory to defrost heatpump external unit during winter (hot gaz technique)
    It would be a good idea to add this option.
    Best regards.

  8. Could I use this to remove the old brains from my geothermal heat pump? The main board of my factory/dealer-installed unit keeps losing the ability to control the pumps and heat exchanger.

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