@mrkfm 

Yes and no. 

There are some issues

Condensation:

Because the (existing) pipework between the heat pump and the emitters is not vapour sealed, condensation will occur if the circulating water is below dew point, which is not good.  Vaillant heat pumps are natively capable of monitoring the dew point and adjusting, thus keeping the FT above, but of course that means that its not so much cooler than the room so has only a limited effect.

Emitters:

With conventional radiators plumbed as we do in the UK (both flow and return at bottom) all the cold water stays at the bottom so the surface area for heat exchange with the room is tiny.  Some parts of continental Europe plumb in at top, out at bottom other end.  This is better for heating and, by reversing the flow in the heating system (easily achieved with some changeover valves) also means you get a greater area of cold for cooling.  But its still limited.

With fancoil radiators this problem doesnt occur and I can confirm from personal experience that you get a cool breeze from them even though the FT is above dewpoint, which does provide some relief.

With UFH I am told that there is a measurable cooling effect, but I do not have personal experience.

None of these 'above dewpoint' approached removes humidity from the air.  Humidity is an important element of what we 'feel'.  You (obviously) need something below dewpoint to do this.  A separate dehumidifier is a possibility but of course pours some heat into the room.

In summary without properly air sealed pipework the effect is limited, but offers some limited relief if you have UFH or a few fancoils

Finally note that the Vaillant heat pump, unlike some others, requires a 'coding dongle' to enable cooling, costing about £400.  Its actaully a 4k resistor and the exact same part (so far as anyone has been able to tell) is also obtainable as a 'coding plug' for their boilers, for about £11, albeit with a different part number on the stick-on label.

Possible futures:

This might be an area where some product development is needed!

Octopus teased this summer that they would bring out an add on unit, basically like one of those (highly inefficient) standalone aircon units but with a water heat exchanger to connect into the CH system instead of the pipe to the outside.  This would use the existing outdoor unit as the means to 'get the heat out' and would allow the circulating water to be above dewpoint whilst delivering full aircon cooling.  The product concept Octopus teased already exists and is on limited sale (not by Octopus) in the Netherlands. Unfortunately I dont have the link to hand. 

There are also units (water loop heat pumps) designed to be plumbed into water circulating at about 20C, which can be used for cooling or heating and which could deliver the full aircon experience again using the existing outdoor unit.  They cost about 3K but thats probably because they are designed for swanky office/apartment blocks.  If someone were minded to 'consumerise' these they would be ideal.

What I am doing

By far the best way to cool houses is to stop them heating in the first place.  The southern Europeans know how to do this and we need to learn!  Opening windows when its cooler outside than in, closing curtains in South facing rooms are a good start.  Exterior shading is very helpful if possible and/or low transmittance glass in glazing units.  I had all of my glazing units replaced because they were beginning to mist up - just the glass not the (1980s era) frames.  This made a massive difference both to heating and cooling!  Reflective film on windows is another trick, I have one particularly difficult one, but a £3 survival blanket from Go Outdoors taped over the window is transformational.

This year I did the above together with a bit of light cooling using the heat pump (I have one fancoil and a large radiator with add on fans).  Apart from about one day the house was tolerable, remaining below 28 even at the height of the summer, and most of the time below 26.  I plan to run with this for a couple of years, possibly fitting another fancoil or two (I so wish I had specified a couple more in the first instance) whilst keeping a close eye on new products.

Hope that helps.

I agree with @jamespa's comments which are a good summary.

I would just add that we have installed Jaga Strada fan-assisted rads and Jaga Canal trench rads both of which are designed for what Jaga describe as "light cooling".  They recommend keeping the FT above 16C and doing that this summer we have had no condensation problems.

The Strada rads can produce a strong flow of cool (but not cold) air which we find makes a significant difference to our indoor temperature on hot days.  In particular we close bedroom curtains on the sunny side and keep the door closed and with cooling on full all day we find evening temps stay around 23C which is bearable.

Our property is a timber framed, black-timber boarded barn, with 50mm of dense polystyrene insulation in walls, floor and ceilings.  The black timber cladding gets too hot to touch after a few hours in the sun.

We have a Mitsi 14kW ASHP which just required a dip-switch in the control box to be flipped to enable cooling.  So a no-cost option.

Other points:

1  When cooling, the ASHP will cycle (5 mins on and 10 mins off) as the cooling capability of the HP is far higher than the rads can collect heat from the house.

2  The Jaga rads are well engineered and correspondingly priced.  You should allow about £800 per room.  There are UK suppliers that will give a discount on list price, which is higher.  You could easily spend £10k on a full set for a 4 bed house.

3  Given that cost, you might want to consider a more traditional mini-split air to air system for cooling, which I believe can be much cheaper.

Posted by: @jamespa

↑

With UFH I am told that there is a measurable cooling effect, but I do not have personal experience.

I've been doing it, 16-17°C water in the slabs.

The effect is subtle, ie it is only noticed when it's off, "hey it's hotter than usual here!..."

Using a fan increases the floor's cooling power.

It does not dehumidify, so if outside air is hot and humid, cooling it without condensing the water will result in uncomfortable moist cool air. If outside air is quite dry, then it's fine.

However it is a great complement to my portable AC units on wheels. These do dehumidify, but they have terrible efficiency. On the other hand, the heat pump working at a highish flow temperature above the dew point has excellent efficiency, especially at night with say 10°C difference between outside air and flow temp.

It is possible to "store cold" very cheaply during the night and enjoy it during the day. This only works if the house has very high thermal mass and good insulation, ie you need to be crazy enough to stick 20cm thick Rockwool exterior insulation on a stone house with walls 50cm thick just like I did.

So it is a nice way to suck heat out of the house silently and efficiently, but it is not a panacea.

To cool below the dew point, you need fan coils and to insulate the pipes against condensation. If you install a new heat pump, AND you repipe the primary, AND there is a place in the middle of the house where a fan coil would fit and cool most of the surface without having to put FCUs in every room and new pipes everywhere... which happens to be my case... then yeah. Otherwise, as a retrofit, too much hassle. 

As James said, #1 priority is to prevent heat ingress, especially solar radiation through windows. Without that, you can actually get more free heating from the sun shining through windows in the summer than what your rads provide in the winter...

@jamespa thanks.  im not used to being on a forum where so many people give genuinely detailed, informed responses to questions! 

i really don't want to be installing and paying out to maintain to separate systems for heating and cooling but im not seeing a way round this but i did also hear about what Octopus are doing which makes me more tempted to buy one of there heat pumps.

Posted by: @mrkfm

↑

i really don't want to be installing and paying out to maintain to separate systems for heating and cooling

I agree, which is why I am currently resisting buying an A2A system to supplement my A2W.  I really don't want a lot of surface mounted pipework for the refrigerant circuit either. 

There are some self contained A2A units with two vents on the back, designed to be mounted inside on an external wall.  These are also worth considering.  With a small and/or very open plan well insulated house then A2A throughout is a viable option.  However for the typical UK house with lost of rooms and an existing system of pipework and radiators/UFH A2W is the obvious choice for heating and it can surely only be a matter of time before an affordable cooling add on, designed to be used in a couple of key rooms only, is produced.  Well that's what I am hoping anyway - all the technology exists, its just a packaging problem.

Octopus seems to get good reviews here.  They do things a bit differently and wont touch some houses (which I think is part of their quality control - limit the complexity), but we don't get reports of poor installation.  There was briefly a bit of a complaint about noise from the Cosy 12, but an OTA firmware upgrade appears largely to have sorted that.

Posted by: @mrkfm

↑

@jamespa thanks.  im not used to being on a forum where so many people give genuinely detailed, informed responses to questions! 

Thanks on behalf of me and others.  This is not Facebook and long may it remain that way.

If you want even more in depth technical discussion on something very specific openenergymonitor is another genuinely helpful forum.  That can get very intense, but its likewise very polite. This forum remains the best I have found for both pre- and post- installation general advice however.

@jamespa n:" there are some self contained A2A units with two vents on the back."  do you mean something like the Panasonic RAC Solo?  I'm curious about this but there is little info/reviews so far and I'm assuming its more expensive to run..

Posted by: @mrkfm

↑

@jamespa n:" there are some self contained A2A units with two vents on the back."  do you mean something like the Panasonic RAC Solo?  I'm curious about this but there is little info/reviews so far and I'm assuming its more expensive to run..

Yes the RAC solo is an example of self contained A2A.

I would expect these to be significantly more efficient, and thus cheaper to run, than the portable standalone units with a hose that you put out the window. 

The latter continually draw warm, moist air into the building (to balance whats going out through the hose), which has to be cooled, whereas the ones with two vents don't.  That's going to make a massive difference to efficiency.

The issue with A2A monoblocs is the airflow needs to be quite high, and pushing it through 150-200mm wide ducts requires a powerful fan that makes noise. The compressor is also in the room.

American-style window units do not have these issues, it's a better form factor.

From experience this summer, a single-split A2A unit on each floor makes a massive difference in a 4-bed house that is not open plan. Warm air rises, so putting an indoor unit in the main bedroom, enables that to be cooled down to a suitable temperature for comfortable sleeping while also dumping a lot of heat out of the house. A further unit downstairs, for example in the living room, is easily enough to get the whole house down to 24C when it is persistently 34C+ outside.

Daikin single-split A2A costs around £1600 fully installed for a 2.5kW unit, a little more for 3.5kW. 

The average efficiency that our system achieved on cooling over the various heat waves in 2026 was over 900%. On the hottest day of the year with many hours over 34C efficiency was still 770%, dumping 36kWh of heat out of the house. Over the winter season, average efficiency on heating was 520%, mainly running off cheap rate overnight electricity. Efficiency is that high because the temperature difference required is much smaller than with A2W systems. Installation is very simple as the system is self-contained, no interfacing to existing pipework is required. The trunking on the outdoor pipe runs is no more obtrusive than an extra fall pipe. The indoor units have a whisper quiet mode that runs at about 20dB, which is imperceptible if you are doing anything other than listening intently for it. 

We try to keep as much heat out of the house as possible on hot days via blackout blinds. Replacement double glazed units that have a g-value of 0.49 compared to the usual 0.7 helps a lot too. However, when it comes to getting the house cool enough to sleep comfortable every night, and living in a consistent 23-24C environment when it is extremely hot outside nothing beats a professionally installed A2A system. Importantly, the humidity remains comfortably low, with the A2A system dumping litres of water outside that it has pulled out of the indoor air.

We had three different sets of friends to stay at different times over summer. It says something that all three are currently in the process of getting A2A systems installed themselves.