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.




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

  1. 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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