Posted by: @jamespa

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Im told cooling the slab with A2W/UFH also works well.  Unfortunately I don't have UFH so cant try it!

Cooling a slab works fine, I tried back in the day with the old ground source heat pump. I did it passively, without running the heat pump, since the ground source is around 14°C.

When I say "works fine" I mean the slab is cooled. However, the effect on comfort is quite disappointing. The reason is that in summer, hot outside air may have reasonably low relative humidity, but it has quite high absolute humidity. When cooling this air above the dew point, there is no condensation, humidity is not removed from the air, thus absolute humidity remains the same but relative humidity of the cooled air rises substantially.

At warm temperatures, comfort strongly depends on how fast sweat evaporates. In reasonably warm and dry air, with a fan, sweat evaporates very quickly and cools the person, flow of warm air from the fan is pleasant, and it is comfortable. However, cooling the air makes it both cool and moist (high relative humidity) thus sweat evaporates slowly, and since the air is cool, a fan is less pleasant. Comfort isn't great, it feels a bit like a cool swamp. Do not recommend.

Basically, a large part of the comfort provided by air conditioning comes from dehumidifying. Actually cooling the air is nice, but secondary. Therefore "air conditioning systems" that don't dry the air are pretty useless at providing comfort.

This means if you want air conditioning for summer comfort, you need fan coils (for water) or air to air heat pump with indoor units that can handle the condensation.

Air conditioning with water is extremely annoying because you need to vapor tight insulate every bit of pipe that gets lower than the dew point, otherwise it'll be covered in condensation, drip, and rot the house. This is not possible to do fully, so stuff like valve handles and the like will get wet and corrode. You also need diverter valves to direct the flow to the appropriate destination (hot water cylinder, fan coils, UFH, rads, etc) depending on what mode the heat pump is running and water temperature. This to avoid insulating pipework that doesn't need to be. Also fan coils are enormous and difficult to fit pretty much anywhere that isn't a supermarket ceiling plenum. More precisely, the units I bought are 1.30m wide, 30cm high, 90cm deep, and they only cool about 2.5kW. But I got them "almost new" for 5% of the price, so yay. Pretty silent though.

On the other hand, the air to air heat pump is subject to the egghead hazard. In the future, some day, soon, a bureaucratic egghead will decide the refrigerant in your pipes is unclean and you will probably be more or less screwed, less being retrofitting everything with R290, and more being forced to repipe everything for CO2 or give up. Also, you are not allowed to touch the pipes, reroute them, install indoor units, etc, etc, so it is very DIY unfriendly unlike water.

If the house is large, with many rooms, the above restriction makes it difficult to stick an indoor unit in every room. You'd need a variable refrigerant flow system like in office buildings. Thus there is the ceiling unit which blows hot or cool air via hoses into rooms, but that runs into the issue that thermal capacity of air is very low, so unless the house is modern and super insulated, to move a meaningful amount of heat, you need to move a truly humongous amount of air. This requires enormous ducts, which are only possible to fit in the house if the whole house was built around it.

In other words, no perfect solution, everything is a compromise that depends on system requirements.

@bobflux thanks.  Perhaps light (above dewpoint) cooling plus a dehumidifier.  We are currently only talking a few days per year after all, so it's not worth going overboard!

If it's only a few days a year, I'd recommend a second hand mobile air conditioning unit, those with caster wheels, can be had for 150€ for 3kW cooling power. Maybe cheaper if it doesn't work because the exchangers are clogged with dust ; five minutes vacuuming the inside equals 75% rebate lol.

The trick is to mount it as a window AC unit like the Americans do, with the condenser air inlet/outlet handling outside air, and the evaporator inside the house. Of course it has the wrong form factor for that, because the evaporator is on top and the condenser on the bottom. The Americans build their window AC units in the proper way, with one exchanger behind the other, not on top of the other.

I did that on a sliding window, by opening the window, then mounting a hollow polycarbonate panel in the open window... cut a hole in the panel corresponding to the vents at the back/bottom side of the AC unit... this about doubles the efficiency since it no longer blows all the cold dry air outside and sucks hot moist air back in the room. It cools the living room to 19°C with 40°C outside, but it is so damn loud.

There's another technology option, sat between A2A and A2W which is a hybrid of both which looks quite attractive if it takes off and becomes more widely available - Panasonic have a system called Aquaria Loop, intended for apartments/large buildings, which uses in-room W2A units (powered with their own integral heat pump) that can provide both heating and cooling, which are connected to the building's wet pipework system with an outdoor unit, which maintains the loop at something around room temperature, above dew point to prevent condensation.

When the in-room units are heating they pull heat from the loop, when they're providing cooling they reject the heat into the loop. The outdoor unit then either pushes heat into, or pulls heat from the loop to maintain the temperatures.

What I thought was really cool was that it allows a single wet pipework system to be used for both the heating and the cooling, without the need to either completely replace it with fully insulated and sealed pipes.

What would be even better for a household installation, following a similar approach, is if it were possible to combine a smaller number of these more complex/larger/expensive in-room units, to provide cooling in a few select rooms, with existing radiators in the rest of the rooms, running the loop hot to drive the radiators for heating during winter in the same way as regular A2W heat pump, then running the loop at room temperature in the summer to allow the in-room units to operate to provide cooling. That would be a really great solution.

Is there a section or even one discussion here that compares air-to-air brands and models and installers?

For those of us who have decided we want one, just like everybody who has an air-to-water heat pump.

There isn’t as far as I know, @djh, but you’re welcome to start one.

That said, I suspect it’ll be like A2W; brand differences likely inconsequential compared with the effect of a good installer and good system design. However, you’re right that there is a distinct lack of publicity of good installers, so that on its own would make a new thread worthwhile.