Is a reversible air-air heat pump a proper heating system?

I have pondered this many times, and indeed, a district council once asked me this very question. They ended up getting me along to analyse why some of their tenants, who had this type of split reversible air-conditioning, had warm houses, low bills and loved them, whist others hated them, complained of draughts, noise and high running costs.

The tenants who experienced success seemed to be mostly one-person occupants who arranged their rooms to avoid the draught from the ‘blower’, but also who left the unit enabled fairly constantly, and who opened/closed interior doors appropriately so the warmth spread accordingly.

Those that hated them had got to that conclusion early-on, so didn’t give them a chance. however, I could see how they disliked them….. a draught blowing over the settee, and with no easy change of settee position, the owner then switched the unit on and off frequently.  i.e., waiting until they were quite cold before switching on, then it goes to full-power, and full-draught.  With guidance and confidence, the user could have improved matters, but there was seemingly no way to win them over.

So, that little exercise didn’t really answer my initial question, only to confirm that this type of ASHP can work surprisingly well in some situation, but not in all.

This actually prompted me to temporarily fit a small unit in our kitchen/lounge, so as to live with it briefly.  I concluded that I would not choose it as a heating system.  The gentle draught and the slight hum were unwelcome.     I removed it within a few days and installed it in the log cabin it was intended for (they loved it, and still do)

I was reminded of the first heat pump (air-air) that I ever built. It was in my parent’s kitchen (in 1979).  When first switched on, the air actually felt a little cold, but then oddly, after ½ hr the kitchen was actually getting warm.  By slowing down the air flow, I improved the perception a little, and fitted a time clock so it could come on before we got up. I have no idea what the COP was, but it certainly made a comfortable kitchen, albeit with a sluggish start.  I certainly had no intention of making it bigger since it did cycle on and off a lot on milder days, and in that respect, it could have done with being smaller.  No variable inverters in those days!

In hindsight, I wish I had given more thought with respect to the sizes of all the units I had observed.  Size, that is, compared to actual heat requirements (heat loss) e.g., the watts/sqm.

Blown-air heating is not very common in mid/northern Europe, but very popular in warmer countries.  I have always believed this to be because air movement is great for people cooling, but generally undesirable for heating.  If your winter is short-lived, you can put up with a bit of a draught from your heater, but if your winter is long and sometimes deep, then surface (floor or radiator) heating seems better.

This issue of using blown air for heating reared its head recently with respect to a small community room about to be built.  This is a well-insulated new-build, so total heat demand would be low.  A rudimentary heat loss calc indicated around 1.5kW of heat was needed, which equated to about 42w/sqm, which seemed the sort of size I was expecting.   Radiators or underfloor with a normal (to UK) ASHP seemed too expensive to install and even the smallest unit would be far too big.

I suggested that quotes for installing a reversible air-air heat pump be sought, and back came a recommendation for a 5kW unit.  I was a bit aghast at this.  It felt to me a bit like putting a 3-litre engine in a small city car.

I had initially thought that a 2kW unit might potentially too big and might cause a draught issue! My worry here is that anything remotely as large as 5kW must have a fairly large draught. You cannot dissipate 5kW without a reasonable breeze.   I realise all units modulate down (inverter), but my 1.5kW estimate was for sub-zero conditions, and much of the year, the heat need would be a small fraction of 5kW.

My experience with ducted air systems is limited, but the most successful one I recall was a Danish-design house that had tiny air flow because the house was highly insulated.  These systems gently blow in a small amount of warm air, and the house is never allowed to cool down much.  However, the only reversible air-conditioners that I have seen were relatively powerful.

I contacted a friend, Phil Jennings, who has fitted this type of system in his house, and has more experience than I. He shared some of his research with me (Thanks Phil).  None of the suppliers/installers that he had approached take into account the heat loss calculations. Most seem to purely go on the rule-of-thumb sizing of between 135 and 150w/sqm.   Yes, you read that correctly…. no matter how well insulated… it’s the same size unit!  This seems to be far from a ‘proper’ way to design a system.  The installers reasoning here, and possible their wisdom, might be that you cannot easily calculate a cooling load, so a one-fits-all approach would be easy, and should always provide enough ‘cool’ or heat.

So, would it be possible to fit a small air-air unit in a large insulated room, and let it simply ‘tick over’ providing a constant low level of heat?  to never expect it to be used as a fast-response, heat-on-demand system.    I doubt if any installers have tried this…. Why would they?  They may not be aware that well-insulated rooms don’t have cold corners, so don’t need much in the way of air ‘throw’.   I would have thought that a small system like this would be very energy-efficient.

As I angle back to consider the title of this blog…  ‘Proper heating system’. Is this type of heat pump ever ‘properly’ specified as a heater only?   One thing to note is that many units seem to have relatively high minimum heating set-point. E.g. 16°C.  I’m guessing that at this sort of temperature (and below) draughts could be uncomfortable. However, if I am out all day, or all week, I want to set it at say 12°C, and I feel the inability to be able to do this is environmentally a drawback.   I can see the problem, you enter a 12°C room, turn the heat on, and it feels draughty. However, I would have thought it easy-enough for the manufacturer to be able to limit the fan speed so that cold-feeling draughts are avoided.  Maybe these units are simply not optimised for energy-efficient heating.

I guess for some, the description ‘proper’ would mean a system that can keep the whole house at required temperatures, and with that respect, this might not tick that box unless that house is say open-plan and fairly well insulated.   Furthermore, the ‘blower’ is in one fixed position, and now this, in part, dictate a room layout.   Yes, multiple ‘blowers’ are possible, but I’m not confident this solves all problems

All-in-all it seems that some adapting and general cooperation is required by the occupant.

The sizing thing

We all like and expect heat-on-demand, and this is certainly the easiest way to operate things; you should not be dissatisfied if you have plenty of heat.  However, for most of the winter, the steady-state requirements are a fraction of the plenty-of-umph sizing that seems common.  For more energy-efficiency heating on the average milder winter’s days, a smaller unit running steadily at mid-speed is likely to be better.  Personally, I would be happy to forgo the fast-response if I knew it was more energy efficient.  However, I can see why manufacturers and installers would dislike what they would describe as an ‘undersized’ system.  They are worried about people misunderstanding them and being disappointed.

I would be interested to see some smaller systems on test to see how comfortable and energy-efficient they are.  Given the upcoming challenge for heating our buildings in a low carbon way, we need to try things out on all fronts so we learn what works and what doesn’t, so maybe we need to be trying out far more of them.


These units seem unlikely to tick an ‘easy to repair’ box.  They are built to a price… most things are.  They are difficult to take apart and mend, and would need very specific parts if anything failed.. like a solenoid coil or an electronic board.  That said, they mostly seem to keep on going… like many things.  How long will they last?  Hard to know, but they can start to look shabby after say 12 years or so. The outside part is out in all weathers which will not help longevity, and the indoor part is very compact and hard to get in to.   Refrigerants is another thing (see my YouTube ). It will need installing by an F-Gas installer, and ensuring that pipe connections are 100% is vital.

We need manufacturers to seriously embrace the possible shortcomings with controls etc. i.e. so it’s not an air-conditioner with bolt-on heating.  My apologies if some already have.

They do of course operate in cooling mode.   Whilst many will say this a good thing, I tend to think that their use in ‘cooling’ would very often be environmental laziness.  i.e. before resorting to switching on this refrigerated cooling, we should be following good housekeeping first –  shading the sun, outside shutters, inside blinds, ceiling fans etc.  If that fails…. Well, OK, turn the AC on.

The other issue here is the lack of hot water (DHW) from air-air systems.  However, the losses resulting from cylinders and pipe runs can be very considerable, and I feel that instant point-of-use electric water heating ranks better than we generally think.   I have another blog brewing on that topic.. watch this space.

So, is it a proper heating system?   It’s not a system to ‘bang in’ in every situation, but with a bit of thought, with the right controls and an informed user, it could be very proper for limited applications.




18 thoughts on “Is a reversible air-air heat pump a proper heating system?”

  1. Hi John, this is an excellent blog post and some even more interesting comments. Given the significant number of positive reviews from your readers, would you consider revising your opinion on A2A Heat Pumps?
    Full disclosure – I am considering a split unit for the coldest room in my house – an east facing 25m2 office, but with the option to pump it into another couple of rooms via the side passage.
    Note – First I am fitting internal wall insulation (it is solid brick) and triple glazing!

  2. I have some questions about estimating the running costs of an air to water heat pump.
    I live in an old, uninsulated solid-walled house, and at present we have a pellet boiler fitted for DHW and heating. I have been thinking of replacing it with a heat pump (Reasons: cost of pellets, maintenance hassles, and the niggling feeling that the wood pellet industry is essentially dependent on the soon to be abolished RHI).
    I have been keeping a spreadsheet of the daily energy usage, expressed as kwhr per kg of pellets, and outside air temperatures. I am taking the daily (net) energy consumption as being the total kwhr multiplied by the efficiency of the boiler.
    Would it be correct to estimate the energy consumption of a future heat pump as being the present net energy usage usage divided by heat pump efficiency and then divided by the COP? It gives an instant comparison eg, £6.40 a day in electricity versus £12 in pellets for a heating water temperature of 55 deg C and a COP of 3.63, but it seems a little simplistic. (COP is for a Vaillant HP).
    At present the nominal water temperature is 74 deg C, although once the house has heated up in the morning, the water temperature rarely gets that high before the thermostat shuts down the boiler.
    We have had a quote from for a heat pump installation, and the installer believes that the existing radiators and water cylinder will be adequate, which sounds a little optimistic. He also says that we will be able to claim the new Clean Heat Grant to claim £5000 towards the cost of the installation, which I thought was only eligible to be used for the conversion from oil/gas to renewable heating.
    Anyway, any advice would be highly welcome!

    1. Well done for keeping a good log of your energy use. I think your assessment should be about correct (weight of fuel, and allowing for boiler efficiency). Is it possible that you have other losses from hot pipes and buffer etc, that you may not have with a heat pump?…I don’t know. For the heat pump you would only allow the COP (it is the heat pump efficiency). e.g. if you think you need 10,000 kWh of heat, and your COP is expected to be 3.3, then you should use 3,000 kwh of electricity.
      As you say, the old system ‘blasts’ in heat at 74 degrees, and by running a heat pump for lower and longer can give you very good results. Its hard to know if your existing rads will be ‘adequate’, and there is no easy line to draw… its a sliding scale. If you are frugal, Your rads may be able to operate at low energy-efficient temperatures, but if you like a warm house, bigger rads might help. To my mind, most HP installs are too big (like a 3 litre car), and therefore more difficult to operate at lower temperatures, but I tend to be on the frugal side, so may be biased. Although big heat pumps should ‘modulate’ down in theory, many seem too eager to rev-up. I am however talking fine-tuning here, and most people seem happy with good-enough COP without spending too much time optimising the system

      1. Thank you for you reply. The system does indeed blast the heat in, but it still takes it some 2 1/2 hours to bring the house up to 18 from 16.5 (night time thermostat setting) degrees, so I am not sure that 55 degrees will cut it with the existing radiators.
        As a matter of interest, I carried out the Energy Saving Trust boiler sizing survey, and taking the wall U value as being 1.3 w/k, (600mm red sandstone with a 50mm air gap between it and the lathe and plaster), it gave a boiler size of 14.5 KW. The existing boiler modulates between 6 and 26 KW and the proposed HP is 12 kw.

  3. I’m interested in Air-Air HPs for PH grade buildings. I mostly work on vent systems for PH buildings and sometimes do PH modelling.

    A vent client is looking at AAHPs for heating/cooling, 1 unit per floor. It is a compact retrofit, the FF unit will be on the landing and the GF is mainly open plan so it will be either in there or a bulkhead unit supplying through the wall from the utility room. The peak heating demand for the whole building is under 2KW.

    From my point of view to prevent discomfort from drafts and noise the units should run for extended periods at a low level, but the installers will also have a view.

    Thinking about how to assess the installation how practical is it to use openenergymonitor to calculate real-time COP with AAHPs?

    Many thanks

    1. Its very difficult using any monitoring of air systems to know the quantity of heat. Power input and temperatures… no problem. I tend to agree, low gentle steady heat, but I find most installers don’t think that way. They think warm-up times and wanting a draught for cooling. I don’t have enough actual experience, but would be very interested in the results of a tiny unit here. It may in part depend on the occupants… their level of frugality or otherwise

      1. Finding open minded installers seems to be at least half of the battle.

        Hopefully we can get some useful data. I’ll let you know how it goes.

  4. We have 4 Mitsubishi LN air 2 air units around the bungalow and leave them on 24/7 at 23 degrees. We also have a mvhr system fitted.
    A couple of points firstly no drought is felt on you at any point these using have a thermal imaging unit in them and so the air can be set to never blow on you or vice Vera a eg just blow on you. Secondly on running costs I have been monitoring ours and can honestly say they cost significantly less to run as described than using the biomass pellet boiler and rads.
    Note temp is set to 23 as wife needs a stable temp for her illness to remain stable.

    We went for 4 separate systems over a multi-split system primarily for economics they have significantly better cop values 5.23 on the 2.5kw unit.

    For dhw we have solar thermal and a PV diverter for excess solar electric. Very rarely need to heat tank not on free power

    I would 100% recommend them

  5. Installing an Air-to-Air heat pump i.e. AC unit felt odd – a topic not usually discussed amongst people trying to reduce carbon dioxide emissions. But so far it has been a very rewarding experience.

    Context: Semi-Detached house: Just blown my retirement lump sum on Triple Glazing & External Wall Insulation to reduce heat demand by 50%. The solar panels and a battery – we have been more or less off-grid since March. And finally an ASHP (Vaillant Arotherm 7 kW) for space and water heating. We also installed a Daikin ‘split’ unit with one inverter and two 2.5 kW (thermal) indoor wall-mounted units.

    The purpose of the AC was very specific – to cool my bedroom and the hall of the house during heatwaves which I think have become more common: I find them unbearable. During the hot weather in June 2021 the 2.5 kW unit in the bedroom worked fantastically. Very quiet and the bedroom an oasis of cool. I had hoped the similar unit on the wall of the landing would send “coolth” down to the hall, and this does happen but to a limited extent. Happily hot weather is generally sunny weather and so the AC operates entirely from the solar/battery combination.

    The ASHP can be configured for cooling as well, but the prospect seemed too complex and even more disruptive than the basic installation. Adding these purpose-built units was much more straightforward – as Anne Miller commented . I am hoping to experiment with them in the winter to see if I can usefully combine them with the ASHP which heat via radiators – perhaps lowering the flow temperature of the ASHP to increase its COP.

    A winter of heating experiments awaits – but for now the ability to keep cool when it is over 30 °C outside is a blessing.

  6. We’ve also installed air to air heat pumps throughout our Victorian end terrace – initially for air conditioning in the bedroom (I know, I know, boo hiss) as we live on a main road and can’t have the windows open at night – but we soon realised that they work great for heating as well. Having already completed one season using them, we’ll be ditching the gas boiler ASAP.

    Regarding the cold air – this must be a nuance of the Hitachi heat pumps – our Mitsubishi’s do go through a defrost cycle now and again but it closes the flaps internally when doing it. The only thing we notice is an initial cold puff of air when they reopen before it’s back up to temperature.

    Agree on the programming of them – that seems to be an issue on most of them, but they seem happier anyway when we just set the temp and leave them be.

    Anyway, great bits of kit. Super simple to install (by a proper engineer), little to no servicing, they just purr away.

    1. Very interesting. Many thanks. Of course these units will be optimised as time goes on. Yours seems particularly optimised for heating.

    2. ”Set the temp and leave them be.” – No! You need a Sensibo Sky instead!

      Sensibo Sky uses the IR remote control commands to control a dumb heat pump unit. Controllable remotely via an App.

      My air/air heat pump used to be on continuously in the Swedish winters. Now it bounces between set temps/set schedules or just reacts (with climate react) to whatever environmental conditions I want.
      It’s a cheap option for ultimate control of an older less programmable heat pump! Try it.

  7. I absolutely agree with many of the comments in your blog about why people might hate air to air heat pumps, particularly if they’re inappropriately specified, or the home owner doesn’t understand how to use it. However, we’re pleased with ours, and will be doing more monitoring of it this coming winter.

    We installed a small Hitachi split type 1-RAK-25PSEW RAC-25WSE air-air heat pump in the large kitchen/ living room at the back of our solid walled Edwardian terrace in Cambridge in February 2020. Our is (or certainly was) a very rare model in the UK: Refrigerant R32, SCOP 5:1 (in heating mode for the climate of the east and north of the UK) Declared heating capacity 2.7kW. It seems a good quality model.

    Having been involved in developing heat pumps earlier in my career, I was keen on having an air-air heat pump (rather than air-water) because of the inherent efficiency advantage of producing warm air at say 20C instead of trying to heat water in a central heating system to 40 C or more. Although installation needs an F Gas installer, it’s a simple job.

    We are well aware of the dangers of oversizing, so specified the smallest size available.

    As there’s no RHI for air-air heat pumps, we were free to specify what we wanted, with the help of our local installer. We weren’t bothered about the absence of RHI, because installing an air-air heat pump was way cheaper and much less hassle than air to water: (It cost just £2250 installed and was installed in a morning!) And in any case, we use so little energy the RHI payments would have been pretty low.

    Assuming a 20C difference between internal and external temperatures, I estimated the peak steady state heat demand for our 8.7m x 3 kitchen at about 1.4kW (53W/m2). We’re in the process of installing solid wall insulation and new high performance doors and window to the back of the house and after this is finished I expect the kitchen heat demand to reduce to about 500W (19W/m2). A 2.7kW output felt about right, allowing for some extra capacity so it could heat the room from cold reasonably quickly, or in case of unusually cold external conditions

    After the eco-upgrade is finished, we’re expecting to keep the kitchen door open more, to let heat into the rest of the house. We also have a wood stove (in another room) and gas central heating (which we assume we’ll get rid of at some stage), and some background heading from our neighbours (who keep their home considerably warmer than we do!)

    On its lowest air flow setting, the internal unit is completely silent and just gently warms the room. At the highest air flow setting, although its more audible, it provides a nice flow of warm air. Normally we have it set on one of the 2 lowest airflow rates.

    The lowest set temperature is 16C but it also has a variety of lower temperature modes. For example there’s a “leave home” mode in which the temperature can be set to 10-16C (we haven’t actually used this yet)

    We’ve been doing what we can to monitor performance. Although the heat pump increases our electricity use, it reduces our gas consumption by the same amount, so last year our annual energy use and carbon emissions were more or less unchanged, even though we think we were keeping the room at a higher and more comfortable temperature. We’ll have a clearer view when I compile the 2021 figures, but hopefully continuing grid decarbonisation will increase the decarbonisation benefits of having a heat pump.

    There are a few annoyances:

    There’s a rather industrial hum from the external evaporator unit, particularly when it’s working hard. (This is a potential problem with any ASHP of course)

    It has a programmable timer, but as with so many central heating programmers its tricky to use because the instruction book is poor, and in places the instructions are simply wrong.

    We were puzzled by why it sometimes seemed to stop working and just blew out cold air for 10 minutes or so, before realising that this was when it was in a defrost cycle. This happens at intervals when it’s high humidity and between about 0-5C outside. The evaporator starts to ice up and so the heat pump switches mode briefly into an air conditioner mode to melt off the ice. Its more noticeable in with an air-air heat pump than an air-water heat pump because you don’t have the buffer of a central heating system between the heat pump and the occupant. This cold airflow could be quite unpleasant if it was blowing directly at the sofa! We recently realised that the icing/defrosting frequency was probably being made worse because our neighbour’s gutter was spewing water over our evaporator, so have now fixed this for them. And before next winter, I’m planning to make a rain hat for our evaporator to help keep it drier!

    Overall, my feeling is that a small air-air unit could be a great solution for decarbonising many small homes.. it would have been a good solution for the 2 bedroom Victorian terrace that was my first home for example. But installers need to make sure that the internal unit is sited somewhere where a cold air flow during the defrost cycle won’t matter, and (as with any ASHP) to site the external unit somewhere where the hum won’t be too annoying.

    An air-air heatpump doesn’t do domestic hot water of course, so I’ll be interested in your forthcoming blog article on electric hotwater heating.

    If anyone is interested in knowing more, we’ll be giving a free online tour on 30 Sept for Register here for the “Edwardian Terrace phased Retrofit” tour.

    1. Many thanks Anne, that is extremely interesting. Well done for getting the online Eco-homes tour organised.

  8. Hi John,
    Very informative article, thank you. What do you think about a system of a reversible air-to-water heat pump linked to a chiller coil in the supply ducts of an MVHR system? It wouldn’t offer AC levels of cooling, but might help take the edge of warm days? As the MVHR system/ducting would already be in situ and providing ventilation at trickle rates, it wouldn’t have the issue of draughts or noise.

    The downside as I see it would be that you couldn’t “localise” the cooling, as the MVHR would vent it across the whole house, but the overheating zone might be a first floor south-facing room with lots of glazing.

    Best wishes,

    1. Hi Patrick. Are you thinking normal ASHP? Already heating floor or rads? A coil in the MVHR duct should be OK, but you might struggle dissipating the ‘cold’. It might cycle. As you say, it wont be focussed, so may be wastefully cool un-used rooms. Would it be better to cool the floor to a limited extent? 17°C? Anyhow, adding a bit of heat to vent air whilst also floor heating is a good thing. Down side is probably the complexity for limited advantage.

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