Good thread!

First, the clarification on whether all heat pumps can cool. Technically yes... the refrigeration cycle is inherently reversible and every unit has to reverse it for defrost. But that's not the same as having cooling available as a user function. A significant number of units sold into the UK market have it locked out in firmware. For someone considering this as an option, check the settings and your specific model before assuming it's there.

Now the physics... when you run chilled water through a standard radiator, you don't get the convection effect that makes radiators work so well in heating mode. Hot water heats the metal surface, warm air rises, cooler air moves in to replace it... that's the engine of a heated room.

Cold water in the same radiator can't drive that process in reverse. You get a cold surface and almost no meaningful airflow, just a mild radiant effect if you're standing close. Cooling with standard radiators is pretty pointless.

Then there's the condensation issue and this is where it gets dangerous. For cooling to work, the water needs to be colder than the air in the room. But drop it below the dew point (which in a typical UK summer sits around 18-20C) and moisture in the indoor air starts condensing on every cold surface it touches. Pipes, radiators, valves, anything uninsulated. The radiator itself you can see and deal with. The pipe running through the void under your floorboards or behind the plasterboard? That's building up damp, rotting timber and feeding mould before anyone notices.

To avoid this you have to programme the heat pump to never cool water below the dew point, which then severely limits your cooling capacity, especially in the heatwaves where you actually need it.

This is why fan coil units exist. Unlike a radiator, a fan coil has a built-in fan that blows room air directly across the chilled coils, actively moving warm air over the cold surface and transferring heat out of the room. Fan coils are also built with drip trays and condensate drains for exactly the condensate that's going to form. The pipework also needs to be properly insulated with vapour barrier insulation otherwise you're just relocating the condensation problem from the emitter to the pipe.

Underfloor cooling is the other viable option if the pipework is already there, though the effect is gentler. If you go down this route, I've been told it works best paired with ceiling fans to move the cooler air around the room. Good in a well-insulated new build; less transformative in an older house with significant heat gains.

The cleanest solution for anyone serious about cooling from an A2W system is often a hybrid approach... the heat pump handles heating and hot water, and one or two well-placed A2A units provide targeted summer cooling. A2A is designed for this from the ground up, including active dehumidification, which is why it can run much colder without the hidden damp risk. 

Posted by: @etchedpixels

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Curtains help a tiny amount, external shutters help far more

After solar radiation passes through the window, whether it hits a curtain, wall, floor or any other obstacle indoors the result is the same: heat is released in the room. Then windows act like a greenhouse, keeping the heat in. To prevent this, the best solution is to block solar radiation outside: pergola with vegetation, awning, louvred shutters, etc. 

External shading is best, but a light coloured or reflective curtain is still going to reduce  solar gain by about a third as some of the radiation is reflected back out the window. That’s several degrees over a very sunny day and correlates with peoples experience of doing this. 

Another detail, as I understand it, is that with the usual UK connections to radiators where the water enters and leaves at the bottom cooled water does not distribute well. You need the water to come in at the top and leave at the bottom (so called TBOE top & bottom opposite ends). This also has the advantage that you can get to thermostatic radiator valves much more easily. I would still say fans are needed to shift air over the cool surface. Even so if you keep the system above the dew point you will only get a piddling amount of cooling.

I agree with Mars that adding some A2A is going to be much more effective. Of course that means a second outdoor compressor unit. Aesthetics aside am I right in thinking that would need planning permission?

On solar gain it's an issue for me as my living room and bedroom are west facing.

It did improve noticeably when I had cavity wall insulation fitted.

I'm wondering how to tackle the heating through the windows. They have both venetian blinds and curtains. In a heat wave what I find is that the space behind the curtains gets hot. Very like a car does. I haven't measured the house but cars can easily reach 50C and this feels similar. In which case, although it's keeping some of the energy out it still functions like a radiator, and a pretty big one. I'm pretty sure there's quite a lot of the heated air from behind the curtains too. It's 2.7 square meters. At a very rough estimate that could be pumping around a kilowatt of heat into the room.

I suspect the traditional solution, louvred shutters outside, may well be very effective as all that heated air is now outside. Unfortunately on my terraced house faced in rough Yorkshire stone permanent shutters would look awful. I might be able to make some temporary ones to fit during heat waves though. If I do I'll report back.

That would be more difficult on the first floor. I think an internal shutter which completely blocks the window reveal made from PIR in a wooden frame would be effective. Another rough calculation says even 25mm of PIR could bring that kilowatt down to tens of watts. Of course it would completely block out all sunlight but that's not a problem for a bedroom.

I would naturally take the internal shutter out when the sun goes down and open the windows.

By the way that's very effective. I open them all wide, especially the attic roof light, as soon as the outdoor air temperature is lower than indoors. Even the front door (back too if there's not too much wind), our street is very quiet. Talking to my neighbour in the May 26 heatwave I think my rooms are 3 to 4 degrees cooler - 24C rather than 27C.

By the way although I'm sure you could get used to that sort of temperature in the UK it's only for a few days 3 or 4 times a summer so we don't get the time to adapt.

Posted by: @springswood

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I agree with Mars that adding some A2A is going to be much more effective. Of course that means a second outdoor compressor unit. Aesthetics aside am I right in thinking that would need planning permission?

On a detached house you can have two odus under permitted development otherwise it's 1 and a second requires pp.

Posted by: @springswood

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I'm wondering how to tackle the heating through the windows. They have both venetian blinds and curtains. In a heat wave what I find is that the space behind the curtains gets very hit

We have low e reflective glass, makes a fair difference both in summer and winter.  The double glazing frames are 1980s uPVC but when the sealed units started to fail we upgraded the glass spec.

It doesn't solve the problem alone but it makes a good contribution.

@springswood 

The most flexible and cost effective way of manage solar gain is via those metallic black out roller blinds on the south facing aspect. Bring them down in the summer to reflect radiation during the day and up when the sun has moved over and open the windows. Then in winter keep them open in the day to catch solar gain and down at night to reflect the heat back into the room. But unless you are really old and sick it never gets dangerously hot in the uk, human physiology means we can easily adapt to it. 

Posted by: @papahuhu

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@springswood 

The most flexible and cost effective way of manage solar gain is via those metallic black out roller blinds on the south facing aspect. Bring them down in the summer to reflect radiation during the day and up when the sun has moved over and open the windows. Then in winter keep them open in the day to catch solar gain and down at night to reflect the heat back into the room. But unless you are really old and sick it never gets dangerously hot in the uk, human physiology means we can easily adapt to it. 

Very sensible 

Unfortunately I fear that's too simple for some of the UK population. 

We need a full automated system with lots of controls and the possibility to do full on Aircon.  Otherwise what would we spend our money on (whilst at the same time complaining about the cost of living,  how awful the government is for requiring planning permission for our excesses and rising temperatures)?

That said, if our less responsible politicians (and their media backers) continue to stoke anger in order to increase their electoral prospects, we will need the shutters to protect ourselves from the thugs that they are encouraging.

Thanks @papahuhu those roller blinds would be a lot easier. Though the venetian blinds I've got are needed for privacy.

@jamespa that's encouraging about the glass as I'm beginning to think my double glazing has had it.

On the two huge (10m² each) south facing windows I installed perforated rolling shutters, galvanized steel, painted white. They don't look as good as oak louvred shutters obviously but they're fine and they do the job.

Besides being quite solid and secure, they're very effective against heat, and that's entirely due to the perforations. They're about 30% holes and 70% metal so they get excellent air cooling, let enough sunlight through so it's pleasant inside, but not enough to heat the room. They also work wonderfully for night time ventilation, no issue getting airflow in the m3/s range which is what's needed to actually cool at night.

With a fan, it's possible to ventilate at night without getting the mosquito army inside. Mosquitoes find their targets mostly by smell, so they will enter through the window where the air exits, but they can't smell prey through the window where the air enters. So I put a fan in the corridor and direct the airflow, and.. no more mosquitoes...

Roll-out external awnings prevent the sun’heat getting in.

 This is one example 

https://www.primrose-awnings.co.uk/?wickedsource=google&wickedid=EAIaIQobChMI8vHc5dDwlAMVi51QBh08Mh8bEAAYASAAEgIAlvD_BwE&w_adid=&w_campaignid=17532035178&gad_source=1&gad_campaignid=23412347011&gbraid=0AAAAAD_aSCCJs4r_PbqV9FgcFIFrt4ney&gclid=EAIaIQobChMI8vHc5dDwlAMVi51QBh08Mh8bEAAYASAAEgIAlvD_BwE

Ours are manual roll out ones with a metal winding poll. We have 3 one per south facing patio door or picture window. They drop the kitchen temperature by 6C immediately. For upstairs cord windings from the ground work well too.

@judith 

Agree, sometimes the simplest solutions just work! 

Posted by: @papahuhu

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From what I understand, all ASHPs can cool, it’s just that if they cool the rads they will drip condensation and rust. I was wondering if you could add a diverter valve that fed the cool supply to an air handling unit in the loft that fed cool air through vents in the bedroom ceilings. In theory, that cooled air should sink downstairs to knock the edge off the heat there too. I guess there would need to be a condensate line on the air handling unit to minimise any mould growth and maybe some sort of bag or panel filtration too. Doesn’t seem massively complex.

Its really a non starter unless designed that way to start with. Condensation will be an issue on any part of the system not fully insulated. You need fan coils with drainage although a mild cooling can be gained through UFH (but mustn't condense!) but you'll never get any worthwhile cooling.