Hah I've been researching that too. I've got some whacky ideas of my own but there are companies that sell the units and install them - search for "water cooled air conditioning" as I don't want to link to any specific company. They're designed for flats and listed buildings that can't have outdoor units, and they're normally plumbed into the mains water supply. There's a thread on mumsnet where people talk about their experience: https://www.mumsnet.com/talk/property/4193494-Info-on-water-cooled-air-conditioning-Not-evaportaive-cooling-but-ordinary-ac-with-water-cooled-condensers

I thought about calling them up and asking if they can plumb it into radiators, rather than mains water. But in the end I decided against as I think normal ATA mini-splits would be much cheaper. The benefit would be no additional outdoor units (and any planning permission that would require).

One of my whacky ideas involve dehumidifiers rather than portable AC units, mainly because I have a dehumidifier which I don't use and don't mind ruining. Dehumidifiers have the same refrigeration cycle as AC units, but the "hot" side is right next to the "cold" side and the air passes over both. In winter you actually get a mild heating effect as the water gives up some of its heat when it condenses, but if we separate the hot side from the cold side and cool the hot side with (cold) water from the ATW radiator loop, we can dump the heat outside of the building instead.

It's fantastic that the actual professional engineers at Octopus are trying to build units that work with their existing ATW heat pumps. I do hope they make a marketable product out of it before next summer's inevitable round of heatwaves. It also looks simple enough that you could probably DIY install it...

Posted by: @scrchngwsl

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"water cooled air conditioning"

Heat capacity of water is 4180 J/(°C.liter), therefore 1kW = 0.24 °C.liter/s

Assuming inlet water temperature 20°C outlet 65°C deltaT 45°C, 1kW cooling power uses 20 l/h or 7.6c€/h at 4€/m3

Assuming lowish EER 2.5 due to the high outlet temperature, electric consumption 400W or 8c€/h

Total 16c€/h per kW of cooling power.

On the other hand, a decent split aircon will have EER 4-5 so running costs about 4c€/h per kW of cooling power.

When cooling the slab (17°C water, 30°C air) my A2W heat pump casually flexes EER above 8, it consumes 500W and outputs 4kW cooling power lol ; running cost zero (powered from PV) ; obviously the best time to cool a slab is at night when air is at 25°C, EER then goes 🚀 but at night electricity is not free.

Conclusion: by needlessly preventing people from installing AC that actually works, the result is people will either use mobile units (terrible cost efficiency) or water cooled units (also terrible cost efficiency, plus waste of water).

Always compost your greens!

Posted by: @scrchngwsl

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It's fantastic that the actual professional engineers at Octopus are trying to build units that work with their existing ATW heat pumps. I do hope they make a marketable product out of it before next summer's inevitable round of heatwaves. It also looks simple enough that you could probably DIY install it...

Potentially a winner this idea if it can be made cheaply enough, because its also potentially a fancoil radiator, good for 'difficult' rooms where rads end up too big, which are in many cases likely to be the ones needing cooling. 

There are some quite smart monobloc aircon units available that do not need an ODU.  They are intended to be mounted on an outside wall with two vents punched through the wall.  Essentially they are an ASHP combined with a fancoil, in a single indoor package.  Here is one example.  A version of this without the exterior vents and with water inlet/outlet that you connect up to the rad circuit is whats needed.  Not sure exactly how you would run the refrigerant cycle for heating (when the feed water is already at the right temperature) - maybe with little or no compression so the refrigerant is simply used as a heat exchanger

I do the same thing with my heat pump and my underfloor radiant. I did not need it this year since summer wasn't hot, but this is a graph from 2025:

Green line is outside temperature, in the shades.Yellow line is inside temperature. Both temperature sensors are Ruuvi tags.

Water circulated in the floor between 15C and 18C depending on the humidity.

Posted by: @jamespa

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There are some quite smart monobloc aircon units available that do not need an ODU.  They are intended to be mounted on an outside wall with two vents punched through the wall.  Essentially they are an ASHP combined with a fancoil, in a single indoor package.

These are identical to mobile aircon units (or US style window aircons) you get a compressor and two refrigerant/air heat exchangers in one box.

Drawback is the compressor is in the box, therefore they create more noise indoors compared to US-style window units which put the compressor outside. Installation requires much less bureaucratic pain and permitting than splits though.

Posted by: @jamespa

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A version of this without the exterior vents and with water inlet/outlet that you connect up to the rad circuit is whats needed.  Not sure exactly how you would run the refrigerant cycle for heating (when the feed water is already at the right temperature) - maybe with little or no compression so the refrigerant is simply used as a heat exchanger

A nice application would be the bathroom, where you can never fit a big enough radiator so it ends up forcing a higher flow temperature for the whole house.

Keywords are "water loop heat pump", Daikin and Clivet have some. It's an air to water unit for indoors use. Mostly for big buildings.

In fact a used reversible R290 mobile unit could probably be modified by replacing the outdoors air to refrigerant heat exchanger with an air to water heat exchanger...

Posted by: @bobflux

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In fact a used reversible R290 mobile unit could probably be modified by replacing the outdoors air to refrigerant heat exchanger with an air to water heat exchanger...

That's more or less what octopus posted about on Facebook referred to up thread.

I have been scouring the internet and can't find an existing product that would suit an individual domestic environment, although the Panasonic aquarea loop may come very close.  Its essentially a re-packaging task, but someone has to do it to make it available.  Maybe octopus will.  Electricity price cuts and an integrated heating/cooling offer that doesn't require planning consent and can be retrofitted to a house with radiators could prove a real boost for heat pumps.  I think the noise of the indoor compressor would be tolerated for cooling, less so for heating so a bypass for the refrigerant circuit may be needed.  The unit would operate as a regular fancoil in winter and an Aircon unit in summer, in both cases supplied by the heating system.  The option of either a built in  condensation reservoir (like a dehumidifier) or a drain would complete the package.  

PS - this (Innova WLHP) looks interesting.  It even introduces the concept of a condensate drain injection kit - basically the condensate is disposed of by pumping it into the circulating water, which obviously needs some sort of drain elsewhere.  Not sure what that does for acidity etc but maybe this feature is for open loop systems only where the feed water is a river/canal 

Its not quite right, for heating it expects water in at 10C as opposed to say 45C which would be the case if combined on a single system where some rooms have conventional radiators (which is what you would likely do in a domestic setting), but its getting close and very stylish in appearance.  Its also phenomenally expensive (1.6-2kEUR)

What I really want is an F-gas qualified engineer who's willing to do weird stuff with my house. I tried calling one of the companies who do the "water cooled AC" and they just weren't interested. I got the impression that they were getting so many "normal" orders that they just weren't fussed about my weird request - they just wanted to bang some units in while the demand is high.

Fair enough, there's no way I'm going to get someone to engineer a bespoke Air-to-Water <--> Water-to-Air heat pump set up when they can make the same money doing standard AC during a heat wave in half the time. But man, how neat would it be to use the existing heat pump system we already have and just bolt on some AC units to it? Maybe in 10-20 years when more people have heat pumps there'll be a market for this...

As an aside, I tried opening up an old dehumidifier to see if I could re-site the hot side so that it can be cooled with the radiator. Unfortunately it's pretty tightly engineered so there's no slack/space to shift things around. I'm still not completely over this idea but the easier way is simply to buy a portable air conditioner and stick the hot exhaust pipe into the bottom of the cold radiator, then cover the top up and most of the bottom so that the air is forced through the cold bit of the radiator. Problem is the heatwave has completely eradicated any stock of air conditioners, so they're all 1.5x - 2x the price. One for next summer sadly.

Posted by: @scrchngwsl

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What I really want is an F-gas qualified engineer who's willing to do weird stuff with my house. I tried calling one of the companies who do the "water cooled AC" and they just weren't interested. I got the impression that they were getting so many "normal" orders that they just weren't fussed about my weird request - they just wanted to bang some units in while the demand is high.

Fair enough, there's no way I'm going to get someone to engineer a bespoke Air-to-Water <--> Water-to-Air heat pump set up when they can make the same money doing standard AC during a heat wave in half the time. But man, how neat would it be to use the existing heat pump system we already have and just bolt on some AC units to it? Maybe in 10-20 years when more people have heat pumps there'll be a market for this...

As an aside, I tried opening up an old dehumidifier to see if I could re-site the hot side so that it can be cooled with the radiator. Unfortunately it's pretty tightly engineered so there's no slack/space to shift things around. I'm still not completely over this idea but the easier way is simply to buy a portable air conditioner and stick the hot exhaust pipe into the bottom of the cold radiator, then cover the top up and most of the bottom so that the air is forced through the cold bit of the radiator. Problem is the heatwave has completely eradicated any stock of air conditioners, so they're all 1.5x - 2x the price. One for next summer sadly.

There is definitely a market, or possibly 3 markets only 1 of which products cover

1. Personally I would be prepared to pay a premium for the 'right' bit of kit (failry close to but not exactly this ) that replaces a radiator, does proper cooling/dehumidification in summer (noisy acceptable) and does heating in winter (light fan noise only).  I wouldn't replace all my radiators, only 2-3 of them, so it would need to work with weather compensated warm flow water in winter

2. Something like a portable airconditioner that could be bolted onto an existing radiator in summer only is a possible second market.  Whether it could be made to work satisfactorily is another matter, price between option 1 and option 3.

3. The existing portable aircon units is a 3rd market

Currently the 'market' only serves #3

Posted by: @scrchngwsl

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Problem is the heatwave has completely eradicated any stock of air conditioners, so they're all 1.5x - 2x the price. One for next summer sadly.

It's a seasonal fruit, like tomatoes lol

I bought 2x mobile 4kW units used last February for 200€ each.

Posted by: @scrchngwsl

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As an aside, I tried opening up an old dehumidifier to see if I could re-site the hot side so that it can be cooled with the radiator. Unfortunately it's pretty tightly engineered so there's no slack/space to shift things around. I'm still not completely over this idea but the easier way is simply to buy a portable air conditioner and stick the hot exhaust pipe into the bottom of the cold radiator, then cover the top up and most of the bottom so that the air is forced through the cold bit of the radiator. Problem is the heatwave has completely eradicated any stock of air conditioners, so they're all 1.5x - 2x the price. One for next summer sadly.

Dehumidifier is likely to be underpowered.

I have doubts on your refrigerant to air to water heat exchange arrangement, it is likely to be terribly inefficient. Much better would be to hack a R290 unit, which does not require F-gas certification, and replace the lower heat exchanger with an air to water one. However, propane is flammable, so caveat.

Posted by: @bobflux

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Dehumidifier is likely to be underpowered.

I have doubts on your refrigerant to air to water heat exchange arrangement, it is likely to be terribly inefficient. Much better would be to hack a R290 unit, which does not require F-gas certification, and replace the lower heat exchanger with an air to water one. However, propane is flammable, so caveat.

This guy did something similar (with a dehumidifier rather than an AC unit):

He just had the heat exchanger sat in a bucket of water, which probably reinforces your thought that a dehumidifier will be underpowered.

Heat of vaporization of water is 2.25 MJ/kg (0.625 kWh/kg), this is the amount of heat that must be removed to condense 1kg (or 1l) of water. Dehumidifiers are usually rated in liters/day, so 1 liter/day ->  0.625 kWh/day -> 26W continuous power. For a decent aircon, aim for 3-4kW, so in dehumidifier ratings that would be 100-150 l/day.