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Cooling with air to water heat pumps

Papahuhu · 2026-06-01T09:41:02Z

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.

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Air Source Heat Pumps (ASHPs)

Last Post by Mars 2 hours ago

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Mars

(@editor)

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02/06/2026 4:49 pm

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.

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