Cooling with an A2W ASHP and radiators

Hi all, I’m pondering cooling with my air-to-water heat pump system (10 wet radiators, 2 bedrooms, 2 storey house, designed for a flow temp of 48C).
As I understand it, the air source heat pump unit itself has enough capability to do a useful amount of cooling, but it won’t be able to put that cooling power through the radiators for two reasons.

  1. You cannot have a low flow temperature running through the system as water will condense on the pipes, causing mould and damp. Options here include switching to underfloor heating so that a certain flow temperature has a more powerful cooling effect, getting a dehumidifier (but this has a heating effect), or getting MVHR to get rid of water as much as possible. None of these are really attractive to me.

  2. The other problem is that my radiators are connected at the bottom on each side, so cold water would just flow across the bottom of each radiator. As it just flows along the bottom it won’t mix with the hotter water above, reducing the heat transfer and preventing the cooling power. So my thought is this: if I add a baffle / upwards flow diverter inside each radiator, would that improve the cooling power of the radiator by forcing the cold water to flow upwards and mix with the hotter water in the radiator? Would that provide better cooling? Would that have a detrimental effect on the pump that has to move water around the system? Would that have an effect on the ability of the radiator to heat normally during winter? Has anyone tried this?

I understand that this won’t provide all the cooling power you could want, but I’m just wondering if adding flow diverters would make it better.


You are correct that you will get condensation on your pipes. You will also get it on the radiators. As an absolute minimum you will need to invest in drip trays for your radiators and to find some way of emptying them.

Thanks for the warning! Would I still get condensation even if I stayed a few degrees above the dew point?

Logic says you shouldn’t get condensation but I’m not sure that the long term results would be particularly good. Remember that the maximum of the target range for humidity is 60%, so if you exceed that you can still get mould and your timber will be more vulnerable to insect attack.

I think the guys at @Heatgeek did a bunch of experiments during the hot days earlier this year. They will probably not recommend doing this but you may as well tap their knowledge.

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Ok that’s helpful to know too. We are sometimes above 60% so maybe I should keep more an eye on this.

I recently came across this RightMove page about heat pumps, and it is good to hear a positive story from an independent source. I would be interested to hear what this forum thinks about it however, particularly their supposed interviewee Chris’s statement on the subject of this forum topic, when he says, under the heading ’ What’s it really like to have a heat pump?’ (fifth paragraph there) "Likewise, in the summer, the passive cooling setting is very popular in our house. A bit like being in a car on a hot day when you wind the windows up and switch the air con on, we’re able to do the same with the heat pump.”

NO NO NO NO etc.

It you reduce the temperature of your wet system relative to the air it will cause condensation to form on the plumbing. Whilst drip trays can be fitted under radiators there is no way of capturing the condensation forming on the pipes. Whilst well insulated pipes shouldn’t have the problem any gaps in the insulation would be vulnerable. Hidden damp in the structure of your home is a very bad idea.

Incidentally, any heat pump capable of cooling will not qualify for a BUS grant.

Thanks, Tim. As I suspected. Rightmove have made a wrong move there.

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I thought I had put to bed the idea of cooling with a heat pump-to-water system. Then while investigating heat pump installation more generally I came across this video from Heat Geek Adam Chapman, and its associated comments from others. It doesn’t look as if Adam is anything like as confident about this whole business as he is about other aspects of heating engineering and heat pumps. But his colleague Richard on this video, who comes ‘from an energy background’, seems to have some experience of cooling with radiators, obviously thinks it can work, and thinks it is feasible at least for new-build.

‘Enry’ fails to explain several things well, so Adam has to chip in on various occasions, which is helpful. Richard assumes, it seems, that we are familiar with the fact that Vaillant have a cooling option in the form of a chip (costing an unnecessary amount, judging by some of the comments), on some of their heat pumps, which switches on the cooling option, and with the general rule of thumb that an underfloor system can generally be taken to provide around 2kW of cooling, if I have understood correctly (which seems rather general - is that regardless of area of floor?). They do both recognise the need for dealing with condensate, which they seem to think can be done, and to use fan convector radiators and a 14 degree minimum temperature, which need an electrical outlet nearby for each radiator. But what do we think of all this in general, I wonder, @Tim_Gilbert and others?

In theory, if you fine manage avoiding the dew point, you could cool with a wet system. Hot air rises, so radiators on the ceiling would be ideal but not very usual.

Underfloor heating is a special case as often the summer sun generates hot spots on the floor, which then emit their energy into the room. Pulling blinds/curtains or a brise soleil would avoid that but if you insist on allowing your floor to heat then underfloor cooling would help to mitigate the problem. Ideally the track of the hot spot across the floor could be mapped in advance and a separate zone in the UFH built in so that other parts of the floor don’t get cooled too much and by increasing Δt, the COP of the heat pump could be maximised.

Passive avoidance is the best plan of action and has no running/energy cost.

Thanks @Tim_Gilbert. Correction to my post: Not ‘Enry’ (second paragraph, first word) but Richard. (I hadn’t caught his name at first, so I started by referring to him as Enry in my post, then corrected it elsewhere once I’d looked back at the beginning of the video. I failed to correct it there however. Apologies for the confusion.) Can we edit a post of ours such as this I wonder? I can’t see any obvious way to do so.

You can edit your own posts by clicking on the pencil icon, which with my browser (Safari) is bottom right.

Instead of radiators, you could also try something like this: Water Fan Coils– Cool Energy Shop
(a water driven fan coil unit called Reverso High Wall). Possibly should be on a separate circuit to the radiator heating circuit. Condensation is still going to be a big issue though.

The OP was asking about radiators, presumably thinking to use the existing system unaltered, but as the subject of system modifications has been brought up I will add that cooling coils can be added to MVHR or other ducted ventilation. These are in a duct section that replaces a slice of existing ducting. They have the advantage of an inbuilt condensate tray and drain, which needs connection to your waste water plumbing or to a free flowing overflow pipe.

Edit: the cooling coils (of either type) can be used to boost heating too. Potentially useful where the system was originally designed for a boiler and now has a heat pump. I note that the OPs system was designed for lowish temperature (48°C) but particularly with a heat pump lower is better.