Air Conditioning versus Air Source Heat Pump

Click for a larger version. Similarities and differences in how an air source heat pump (ASHP) or an air conditioning (AC) system warms a home. All the components inside the dotted green line are contained in the external units shown. A key design difference is whether or not the working fluid is completely contained in the external unit. See text for more details.

Regular readers will probably be aware that – having reduced the heating demand in my house – my plan is to switch away from gas heating and install an electrically-powered air source heat pump to heat the house and provide domestic hot water.

But next week I am also installing air conditioning, something which is traditionally not thought of as very ‘green’. What’s going on?

Why Air Conditioning?

I have two reasons.

My first reason is that, as you may have heard, the whole world is warming up! Last year it reached 38 °C in Teddington and was unbearably hot for a week. I never want to experience that again.

During the summer the air conditioning will provide cooling. But assuming the heating comes with good weather, the air conditioning will be totally solar powered, and so it will not give rise to any CO2 emissions to make matters worse!

My second reason is that in the right circumstances, air conditioning is a very efficient way to heat a house. That’s what this article is about.

Heat Pumps

Air Conditioners (AC) and Air Source Heat Pumps (ASHP) are both types of heat pumps.

In scientific parlance, a heat pump is any machine that moves heat from colder temperatures to higher temperatures at the expense of mechanical work.

Note: to distinguish between the general scientific idea of a heat pump, and the practical implementation in an air source heat pump, I will use abbreviation ASHP when talking about the practical device.

The general idea of a heat pump is illustrated in the conceptual schematic below.

As shown, the pump uses 1 unit of mechanical energy to extract two units of heat energy from air at (say) 5 °C and expel all 3 units of energy (1 mechanical and 2 thermal) as heat into hot water at (say) 55 °C.

Click for a larger version. Traditional representation of the operation of heat pump.

Heat pumps can seem miraculous, but like all good miracles, they are really just applied science and engineering.

A heat pump is characterised using two parameters: COP and ΔT.

  • A heat pump which delivers 3 units of heat for 1 unit of work is said to have a coefficient of performance (COP) of 3.
  • The temperature difference between the hot and cold ends of the heat pump is usually called ‘Delta T’ or ΔT.

Obviously engineers would like to build heat pumps with high COPs, and big ΔTs and they have used all kinds of ingenious techniques to achieve this.

But it turns out that heat pumps only operate with high COPs when the ΔT is small and when the heating power is low. There are two reasons.

  • Firstly, the laws of thermodynamic set some absolute limits on the COP achievable for a given ΔT.
    • Most practical heat pumps don’t come close to this thermodynamic limit for a variety of mundane reasons.
    • The maximum COP for moving heat from 5 °C to 55 °C is 6.6.
    • The maximum COP for moving heat from 5 °C to 20 °C is 19.5.
  • Secondly, in order to heat a room to (say) 20 °C, the hot end of the heat pump needs to be hotter than 20 °C.
    • Typically the hot end of the heat pump must be 5 °C to 10 °C warmer than the room in order that heat will flow out of the heat pump.
    • Additionally the cold end of the heat pump must be 5 °C to 10 °C colder than the external air in order that heat will flow into the heat pump.
    • The interfaces between the ends of the pump and the environment are called heat exchangers and designing ‘good’ heat exchangers is tricky.
    • A ‘good’ heat exchanger is one that allows high heat flows for small temperature differences.

So now we have seen how heat pumps are characterised, let’s see how heat pumps are used domestically.

Air Source Heat Pump (ASHP) versus Air Conditioner (AC)

The schematic diagrams  below show how a house is heated by an ASHP and an AC system. Both systems operate using a working fluid such as butane, which is ingeniously compressed and expanded. The details of this process are not the topic of this article so here I am glossing over the fascinating details of the device’s operation. Sorry.

Click for a larger version. How an air source heat pump (ASHP) warms a home. All the components inside the dotted green line are contained in the external unit shown. A key design feature is that the working fluid is completely contained in the external unit and heat is transferred to the central heating water by a heat exchanger.

Click for a larger version. How an air conditioner (AC) warms a home. All the components inside the dotted green lines are contained in either the external unit or the fan coil unit shown. A key feature is that the working fluid itself flows into the fan coil unit and heats the air directly.

We can compare the operation of the two systems in the table below.

Air Conditioner Air Source Heat Pump
Air at (say) 5 °C is blown over a heat exchanger and evaporates the working fluid.


The same.
The working fluid is then compressed – that’s the bit where the work is done – and liquefies, releasing the captured heat.


The same.
The hot working fluid – now at ~30 °C then flows through a pipe to an indoor heat exchanger (fan coil unit) where air is blown over the pipe and heated to 20 °C. The hot working fluid – now at ~60 °C then flows through a heat exchanger and transfers the heat to water in my central heating system at ~55 °C
No corresponding step  

The 55 °C water then flows through a radiator in my room, heating the room by radiation and by convective heat transfer to air at ~20 °C.

Looking closely at the figures and table above, one can see that the operation of the ASHP and the AC system are broadly similar.

However the ASHP has to operate with a bigger ΔT (~55 °C versus ~25 °C) than the AC system, and also has to transfer heat through an extra heat exchanger.

Both these factors degrade the achievable COP and so for my application, the specified COP for an ASHP is just over 3, but for the AC system, it is just over 5.

In my well-insulated house, when the external temperature is 5 °C, I require typically 36 kWh per day of heating, equivalent to 1.7 kW continuous heating. I can achieve this in several ways:

  • Using gas I must burn ~40 kWh of gas at 90% efficiency costing 40 x 3p (£1.20) and emitting 40 x 200 g = 8 kgCO2
  • Using an ASHP with a COP of 3, I must use ~36 kWh/3 = 12 kWh of electricity costing 12 x 25p (£3.00) and emitting 12 x 200 g = 2.4 kgCO2
  • Using an AC system with a COP of 5, I must use ~36 kWh/5 = 7.2 kWh of electricity costing 7.2 x 25p (£1.80) and emitting 7.2 x 200 g = 1.4 kgCO2
  • Using a domestic battery and buying the electricity at night for 8p/kWh, I can reduce the cost of using an ASHP or AC system by a factor of 3 to £1.00/day or £0.60/day respectively.

[Note: In these calculations I have assumed that the carbon dioxide emissions per kWh are same for both gas and UK electricity (200 gCO2/kWh) which is roughly correct for 2021]

So using an AC system I should be able to achieve domestic heating with lower carbon dioxide emissions than an ASHP.

My plan

In my case I need to heat water for my home to 55 °C for use in showers and basins. So I need an ASHP for that. And since I already have radiators in every room, hooking up the ASHP to the radiator circuits is smart double use.

The AC system I am having installed will have 1 external unit and 2 internal ‘fan coil units’. One unit will be in my bedroom (a sheer indulgence) and the other will be high up on the stairs, allowing air to be either blown down to the ground floor where I hope it will circulate, or blown towards the bedrooms.

My hope is that, when used together, the AC system (COP~5) will reduce the heating output required from the radiators so that I can reduce the flow temperature of the water from 55 °C to perhaps 40 °C. This reduces their heat output, but increase the COP of the ASHP from 3 to perhaps 4.

The main difficulty that I foresee is the extent to which the AC heating will actually permeate through the house and so reduce the amount of heating required by the ASHP.

So I am not sure how much heating will be required by the ASHP acting through the radiators, and whether the radiators will work at low flow temperatures. It is possible I might need to replace a few radiators with ones which work better at low temperatures.

It is not at all obvious that this plan will actually work at all – but I think it is worth a try.


The air conditioning I am having installed is a Daikin 2MXM40 multi-split outdoor unit with two FTXM25 indoor air units. (Brochure)

The model of heat pump I will have installed is a Vaillant Arotherm plus 5 kW. It can supply up 5 kW of heating at 55 °C with a COP of 3  – i.e. it will use just 1.6 kW of electrical power to do that – and heat water to 55 °C. Water storage will be a 200 litre Unistor cylinder. A brochure with technical details can be found here, and a dramatic video showing the kit is linked at the end of this article.

When I have come to terms with how much money I am spending on this, I will share that information. But at the moment it hurts to think about it!

Anyway: the adventure begins next week!


Tags: , ,

14 Responses to “Air Conditioning versus Air Source Heat Pump”


    Simply can’t thank you enough. Not only is it great to use some of the thermodynamics you spent time and patience teaching me but also Anne and I are planning to follow in your footsteps when we move. Thank you for breaking the ground but please break the costs gently to us!

    • protonsforbreakfast Says:


      Thank you for you kind words. Moving to a new dwelling offers many possibilities. Obviously it’s stressful but it can offer opportunities to change a place in a way the existing owners never could (too busy with all space filled) before you yourself colonise the place in your own style. And some costs can be partially absorbed in the ‘virtual cash’ that seems to drive property exchanges. Think of it as borrowing cash from the vacuum 🙂

      In any case: best wishes to you both


  2. 171indianroad Says:

    Very nice.

    My knee jerk thought was “why not just have a simpler heat pump and fan coil that does the heating and cooling?” These rigs are readily available and quite inexpensive. I thought dumping heat into the house from the fan coil would be ‘fine.’

    BTW – these heat pump/AC units with the fan coil are a zillion tome (roughly) better than the typical North American central AC/Furnace arrangement. This is because the fan speed and the compressor are continually variable and output to match demand. The typical central AC air handler is a one-speed on/off fan with maybe 2 speed settings via a switch. Hideous.

    BUT on further reading and thinking – you want domestic hot water and to use your existing radiators. A complicating issue. I had been thinking of this in recent months looking for an economical ready-made solution for air source heat pump running in-floor radiant heating. I found one firm in Winnipeg offering a product. Reportedly the heat pump works to -40C.

    A useful graph is the COP versus ambient temperature. The heat pump COP at my current address falls away at about -20C and is supplemented with a 3-stage electric resistance element.

    I find it interesting that even on the coldest winter days (-35C) this 20 year old house has a passive solar heat gain adequate for heating.

    • protonsforbreakfast Says:

      Hi. Yes COP will fall with the Delta T – either due to lower external temperature or higher internal temperature. The working fluids used do get very cold in their expansion phase and so they are suitable for collecting heat even with very low external temperatures.

      But you really must have an exceptionally well insulated house to be able to get sufficient direct heating when it’s – 35 °C outside! Anyway – keep warm, or cool – while the weather charms us with its variability. M

  3. Ed Says:

    Really interesting write up, thanks. I’m researching a similar set up with aircon in the short term and heat pump in the medium term. I’ve been wondering whether the compressors for both can be combined given their similar function and appearance. My thinking was that this would save money and outdoor space. Given your explanation it seems that the functions of the compressors are different so two would be needed. Are you aware of any possible combined compressors?

    • 171indianroad Says:

      Air source heat pumps that heat and provide air conditioning are very common.

      • protonsforbreakfast Says:

        That’s just what I said – but I said it in a very long-winded way! Cheers. M

    • protonsforbreakfast Says:

      Ed, Good Evening. Yes, several ASHP systems can also do cooling. It isn’t actually very complicated. I think the Vaillant Arotherm plus series (that I have) can, but I think several others can too.

      All heat pumps include a de-icing mechanism that runs in the winter when ice forms on the outside heat exchanger. This works by temporarily running the heat pump backwards to heat up the outdoor heat exchanger. During this process, the circulation through the radiators is halted.

      When heat pumps do cooling they are just run ‘backwards’ and heat the outside air and send chilled water through a circuit. Typically people use a second circuit rather (than the radiator circuit) which circulates the cooled water to fan-coil units positioned wherever you want them. These are just like AC units except that the circulating fluid is water rather than refrigerant.

      The arrangement can be very sweet if you can have a clean install. But for me everything seemed enormously complicated and I could barely comprehend what was happening – so I avoided this extra complicated and installed the AC separately.

      There is a difficulty with this kind of system. Sending cooled water in pipes around your house can attract condensation and so one needs to guard against this. The same is true for the AC installation, but for AC installations the pipes are narrower and easier to insulate, and there are well-established mechanisms for removing condensation.

      Anyway: good luck with your installation!


  4. David Says:

    Is the heating water the problem here , I’d like to follow your lead but have solar over running a hot water tank via a divertor which is fine march to nov but not in depths of a uk winter

    • protonsforbreakfast Says:

      Good Morning David.

      Everyone I know who has a solar diverter delights in it. And the reason is not hard to see. In the summer, solar PV without a battery is not well-matched to household demand and is generally substantially in excess of requirements. SO a diverter catches some of that excess. To put numbers on it: My 12 panel system produces about 15 kWh a day *on average* with peaks at 25 kWh/day on sunny days, and household demand is ~10 kWh.

      And that boost will continue through autumn and winter because solar is not well matched to domestic electric demand. Of course in the two darkest months, generation (for me) falls to ~ 2 kWh/day and there is little excess.

      In my system, in summer the ASHP begins its DHW cycle in the afternoon when the battery is generally already full – so it too uses excess solar. But it makes 3 kWh of hot water for 1 kWh of electricity. In winter, we time the ASHP to run in the night on cheap rate electricity where it produces perhaps 2.5 kWh of hot water for 1 kWh of electricity at the same time as the battery is being charged.

      I’m only a year into having the full system assembled and so I still have to see how it runs over a couple of winters.

      In any case: good luck with endeavours. Michael

  5. Gordon Rowlands Says:

    Best article I have read on AC vs ASHP!

    Given the high price of electricity relative to gas, our annual electricity bill is not far off our gas bill for heating and hot water, so we decided to invest in PV ahead of AC or ASHP. Because we have a system boiler we can use all our surplus PV to save on gas, which in the summer months is only required for heating water. It’s not the best use of PV surplus, but better than exporting until we are able to invest in AC / ASHP.

    Our next investment will likely be in AC, with 2 internal units, one to control the first floor temperature, the other to control the temperature of the living room / dining room area downstairs. This is a very straightfroward installation for our house layout, with short pipe runs and the external unit mounted at the front of the house screened by mature shrubs in the front garden.

    We we will then monitor our gas consumption to heat the remainder of the house (kitchen, utility, and study) and hot water as required, and make a decision on what next in around 18 months time, taking into account the options available then and what we have learnt in the interim. So we’ll have a PV/AC/gas hybrid for heating and powering our house for a while.

    Big unknowns for us at the moment are what off-peak rates will become available (as this affects any battery decision), what basic rates will be available to us when our current 2 year contract ends in September this year, and what export rates will be offered in future.

    • protonsforbreakfast Says:

      Gordon, Good Evening.

      Thank you for kind words. Based on your comments I think you are making very rational choices.

      1. Assuming you have a reasonable roof situation, doing PV first is very sensible, especially capturing excess PV as DHW. My suggestion would be to consider installing a so-called ‘hybrid’ inverter. This has the possibility to add a battery at a later stage – you can typically add battery storage as you need it and you don’t have to add any batteries at first. A hybrid inverter costs around £500 more than an equivalent ‘string’ inverter, but if you plan to later add batteries you will need to replace the inverter with a hybrid inverter in any case.

      2. Good Luck with the AC. We used a twin-split system with one fan-coil unit in the bedroom and one on the landing. I did wonder if we might need them as back-up heaters but in fact we use them solely for cooling.

      3. Monitoring the gas consumption is very smart.

      These articles may help you

      4. Yes, these factors are impossible to know. But maximising the self use of solar PV helps whatever happens. And new time of use tariffs will likely become available which not just offer cheap rates at night, but also have super-expensive rates at peak hours (4 p.m. to 7 p.m.) – or even you paying you to use no electricity at certain periods. Having a battery should allow you to benefit from this kind of tariffs.

      In any case, best wishes with your endeavours!


      • Gordon Rowlands Says:

        Many thanks again for your helpful and encouraging points. We did go for a hybrid invertor as although we weren’t persuaded a battery investment was justified right now, for technical reasons it did enable us to have 2 extra panels on our roof to maximise the use of our South facing roof. So we are battery-ready and in the meantime use our hot water tank as a water battery. It’s 170 litres and I reckon it takes around 10 kWh to fully heat.

        I was interested to hear that you use AC solely for cooling. I have Govee bluetooth max-min thermometers throughout the house including the loft – these also monitor humidity. Our house is well insulated and the minimum loft tempertaure typically tracks the outside minimum temperature closely – during December this was 2.0 to 2.5 degC above the outside, which went down to a low of -6 degC. For that evening the minimum temperature across the house was 15-16 degC.

        I will look at all the links you’ve suggested.

        Best wishes,

  6. protonsforbreakfast Says:

    Gordon: I see you are already ahead of me! Good Luck and best wishes. M

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: