Air conditioner or Heat Pump

14 Mar 2016
Reaction score
United Kingdom
I was looking to get an air conditioner now that we have well insulated two rooms in our house, and then I realised that it would be helpful to make it heat aswell - since then the boiler can be off in the rest of the house. However when researching air conditioners I think I concluded they are very inefficient at heating (e.g. 100% efficient rather than 350+% of a heat pump), so should instead look for one of those.

Im looking for a single outdoor unit with two indoor ones (probably 2.5kW+2.5kW or there abouts). Ive installed several air conditioning units in my time, but it seems like heat pumps are more complex - though is that just because of the higher warming potential of the gas and hence F-gas requirement?

Am I missing something here given many air conditioners are described as heat pumps? Is that different to an air-air heatpump?
Why is a twin head split air con unit ~£1000-1500, but a heat pump is more like £5k? Especially if that air con unit is doing *some* pumping.

Sponsored Links
Am I missing something here given many air conditioners are described as heat pumps?
All air conditioners are heat pumps. They move heat from one place to another.
In the case of cooling, it's moving heat from inside to outside. When operated in reverse they move heat from outside to inside to heat the building instead of cooling it.
Used in commercial premises everywhere for decades.
These are air to air units.

Heat pumps as sold for domestic heating are usually air to water units, which are more expensive as they contain additional components for transferring the heat to water which is then circulated to radiators or underfloor heating.
They are also relatively 'new' to the market and so are currently rather overpriced for what's in them.

Air to air units are entirely suitable for domestic use, but due to the obsession of UK homes having wet heating systems, they are often overlooked as a solution.

I think I concluded they are very inefficient at heating (e.g. 100% efficient rather than 350+% of a heat pump)
Any air conditioner should be significantly better than 100% at heating, as it's capturing heat from outside and moving it inside.
As with any system, the exact figure depends on several factors such as outside temperature, humidity, the particular design of the system, controls and so on.
A heat pump operating in heating mode will have a COP greater by 1.0 than when operating in cooling mode (under the same condenser/evaporator conditions) because the heat of compression is used in heating whereas it must be rejected when cooling. Practically speaking, one could probably expect the COPs to be similiar as you may have to use conditions of -3C/30C when in heating mode and ~ say 30C/10C when cooling. So even though "1" is lost in cooling, the compressor is doing less work so COPs should ~ be similiar in both modes, IMO.
I have been looking at A2A heat pumps (air conditioners). Have an online quote from Daikin for top of the range single unit for our ground floor @ £1.8K installed. They say as a heat pump has a COP up to 5. Obviously less efficient heating in colder weather, but the efficiency seems the same as A2W. Noyt saying that's a good quote - just "fishing" ATM to see what's about
Sponsored Links
Thanks for all your responses! Interesting reading. I had read that many air conditioning systems cannot reverse the heat pump direction and hence have low (100%) efficiency. Perhaps that isn't always the case...

I'm aware that an air conditioner is a heat pump of sorts, but how is it different from an air to air heat pump, that is sold by Daikin and others? Is it a different coolant?
Does air-air actually go via water anyway (after the coolant which is in the external unit only) so it uses the same external units irrespective of whether you put a fan based unit inside or a radiator or water tank...?
A split cooling system will have a compressor, one outside unit called a condenser, and either one or more inside units called evaporators. The liquid refrigerant will absorb heat from the hot air which is circulated via a fan in the evaportor which cools the air and evaporates the liquid refrigerant into a gas which then returns to the compressor where is is compressed and condensed back into a liquid in the condenser and onward again to the evaporators.
If lts not a reversible heat pump then when/if heat is requited then will have to be shutdown or maybe the evaporators have external heating elements where the evaporators fan only operates, hence 100% efficiency but wouldn't think there are many/any units like this?
If it is reversible or just a stand alone Air/Air unit then the outside unit becomes the Evaporator and extracts heat from the (outside) air which evaporates the liquid refrigerant into a gas, it is then compressed a nd passes through the inside units which are now condensers and in giving up heat will condense into a iiquid and back to the evaporator and onward again to the condensers via the compressor.

In large cooling systems, water may be used which is chilled down to ~8C and used as the secondary cooling liquid circulating through the indoor units but these wouldn't be termed air/air units.

Similiarly the heating units. if Air/water will heat the water and this is then distributed around the house either through under floor heating or the existing rads or through fan heat emitters. The other "advantage" of air/water vs air/air is that you can have DHW without the need for a auxilliary source, the HW might be supplied at ~ 40C which is boosted up to 60C once per week for legionella protection.
Thanks very much for the response. Ok I think its starting to make sense.

So most 'heat pumps' as they are called in common usage will create a water based interface, and use either ground source or air source. That interface can then be used in a radiator / cylinder / air convector. These can be really efficient (say 5x heat being moved compared to input), but are typically much more expensive, and potentially may struggle to offer much cooling unless they use air convectors (as otherwise there is some risk of condensation I guess).

An air-air heatpump is basically a reversible air conditioning unit (which is most of them), and will operate with a refrigerant based interface (between external and internal units). These can still move 3-4x the unit energy, but typically cheaper and can only operate with the 'air convector' style unit inside.

Im not totally sure how an air conditioner (or air-air heatpump) can have the evaporator inside, and still do heating, as evaporation will almost always absorb heat (i.e. cooling only)? Hence I assumed air conditioners just have a heating element inside like a hair dryer, and their heating effect doesnt use the coolant loop at all....

Is that a fair summary?
Last edited:
By and large, yes, a good summary.
It's only a HP used in cooling mode that can/has the evaporators inside, as the liquid refrigerant passes through a expansion valve to reduce its pressure, the liquid refrigerant then "boils" and extracts heat, it gives up its latent heat and becomes a gas.
In its heating mode, the evaporator is external but due to the refrigerants properties can still extract heat from air at very low temperatures after the expansion valve and expanding through the evaporator, air being blown over the coils at say -5C can fall to -10/-15C depending on the volume of air being forced through the outside of the evaporator coils.
The expansion valve is one of the most important items in a Heat Pump.
Last edited:
Ok so in heating mode there is a second evaporator which can be chosen with valves being opened/closed, and in this case the evaporator is outside. Though does that not also mean the condenser also needs to be be moved inside, to actually extract the heat out of the refrigerant that was absorbed outside....
NO, they just reverse roles in a reversible HP. In cooling mode, the condenser is the outside heat exchanger, after it condenses the returning refrigerant into a liquid, a expansion valve then expands (lowers the pressure) the liquid in the inside heat exchanger, the evaporator, which extracts heat from the room or whatever.
In heating mode a different (in most cases) expansion valve expands the liquid in the outside heat exchanger which now becomes the evaporator which extracts heat from the air which causes the liquid refrigerant to evaporate into a gas, it is then compressed to a pressure sufficiently high enough to condense (and give up its heat) in the inside Hx, which is now the condenser, to become liquid once more and is then expanded in this exp.valve back through the evaporator (outside unit).
Ok got it, thanks so much. I found this diagram which helps explain in a similar way

I kind of understand how the coil can function on both sides, but I dont understand how the expansion valve can work in both directions as I always assumed it would need different size pipes on each side of it... Will do some more reading!

Thanks again! Sounds like the reversible air conditioner is probably good enough for me until I finish some more insulating!
I havent decided really, but the ones on my shortlist were:
* Cheapest but unknown brand -
* Have read good reviews but dont know for sure -
* Ive also heard good things about LG being quiet -
* Spare option 1 -
* Spare option 2 -

I was interested in Daikin too, but found too many options and couldnt differentiate between their tons of models!

Ideally I'd find one that is semi smart so I could control it from WiFi/Phone.
Last edited:
@Johntheo5 Do you have any recommendations, or think any of them would be a bad choice?
Thanks again for all your help!

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links