@lizzie that is useful info, thanks.  I’m currently considering external shutters to try and reduce solar gain as our house typically heats up to over 32c in summer - living areas and bedrooms.  Aircon is a possibility but we really want to avoid more energy expenditure and another box outside.  Shutters seem to work quite well in sunnier climes.

@andrewj

Shutters are very effective, but do darken the room even when ‘open’ and ‘bring the room in’. I have a set of 4 that live in my garage for 8 months and lean against my south facing lounge when needed. (Lot this year!) Happy to send a pic if it helps.

Regarding comments about having A2A rather than A2W, as I have both I would only comment that the peace and quiet when you turn the AC off is a bit of a relief. Sure that one would become accustomed over time but silence can be golden imho.

@watcher actually, a picture from inside the living room on a sunny day and cloudy day, at midday, with the shutters open would be very useful.  What you mention is one of the concerns we have.  What AC do you have and how old is it?

Cloudy one done, might get the sunny one tomorrow or definitely Saturday.

Midea 3.5kw, 2.5 years old.
It really is not noisy, quieter than a large stand fan we run, but maybe like being in the kitchen when the fan oven is roasting Sunday lunch…nice when you switch it off to eat!

@old_scientist I’m thinking along the lines of a multisplit external unit with 4 internal units.

Has anyone any views or experience of the Daikin Perfera range please? After the news this morning that with the imminent El Nino likely to cause record temperatures next year, I wish to be ready for the discomfort. Until now, I wasn’t aware that the Air Con market will attract VAT next year - under such circumstances, this does not sound like a vote winner! Regards, Toodles.

About the original question:

Flow rate for MVHR is adequate for ventilation, but way too low for air conditioning.

Heat capacity of air is only 1200 J/°C/m3, so consider an air conditioner with 2kW cooling power:

- Intake indoors air at 21°C

- Blows cooled air at 5°C

- DeltaT: 16°C

- Flow rate = Power / (DeltaT * Heat capacity) = 0.1 m3/s = 360 m3/h

This would be for one split unit in one room, then multiply by the number of split units to get total required air flow. For a house with a few splits, it'll have to move more than 1000 m3/h of air to be effective.

Ventilation systems will max out at a much lower flow rate, perhaps 150-200 m3/h, and with heat recovery deltaT is much lower, perhaps 5-10°C. The result is very low cooling power, and it has to be spread over the whole house. So MHVR is not effective against heat gain, best it can do is perhaps shave off a few °C.

Also it does not dehumidify, whereas a real air conditioner does... and removing humidity is even more important than cooling: warm dry air is comfortable with a fan, but warm moist air is not. In fact using an AC to get warm dry air (say 25-27°C) in combination with a fan results in decent comfort while using less power than setting the AC to 20°C.

@bobflux Thanks very much for that information and advice; air con is the answer then! I have read that some units incorporate dehumidifiers so must take a closer look at those. Regards, Toodles.

Dehumidifying is a side effect of cooling the air, so pretty much all air conditioning units are also dehumidifiers...

It blows air over the evaporator fins, and if the fins are below the dew point, humidity will condense into liquid and be evacuated.

It takes quite a lot of energy to boil/evaporate water, in fact water has one of the highest heat of vaporization per kg among liquids, about 2.2 MJ/l or 0.7 kWh/l... and to condense water vapor into liquid, the same amount of heat must be removed by the aircon.

In warm dry air, sweat evaporates fast, and while doing so it removes this large amount of heat from the skin, so humans wearing light clothing feel fine. Even with 30-35°C, if the air is dry, it is possible to ride a bicycle at high intensity outdoors without overheating because you get free wind cooling. Indoors, on a stationary bike, it is not possible without a fan and dry enough air. If you want to pick a gym, pick the one with the large ceiling fans over the bikes.

It's possible to cool without dehumidifying (ie without condensing water) but whatever is doing the cooling must be above the dew point, which is quite high in summer, maybe 16-18°C. So that doesn't cool much at all, unless the heat exchanger is enormous (ie, a UFH slab with cold water in the pipes). To get usable cooling output from a split unit of reasonable size with a quiet fan, the evaporator has to be pretty damn cold. The one in my mobile unit is just above freezing. So it catches a lot of condensation, and it has to dehumidify. 

Air leaks bringing in hot outside air will cost you a bit more electricity to run the aircon, it's not so much, because air has quite low thermal capacity. But if the leaks bring in hot humid air, that's a different story, as the water hidden in that air can easily suck half the AC's power or more just to condense it. Humidity from air leaks has a huge influence on the electric bill. Single hose mobile units are particularly affected by this, as they blow out 300-500m3/h of air, which is replaced by hot humid outside air, the result being 50-70% of the power is wasted just to remove the humidity that was let in.

@bobflux It just go to show, ‘You get nought for nought’ when it comes to physics. 😉 Regards, Toodles.

Posted by: @toodles

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@bobflux It just go to show, ‘You get nought for nought’ when it comes to physics. 😉 Regards, Toodles.

Nought for nought is the very best you can do, as soon as you try to get anything at all you get less because entropy continually increases.

This is sometimes parodied as 

You can't win you can only break even

You can only break even at absolute zero

You can never reach absolute zero

Or several variations on the same theme, apparently called Ginsberg's theorem but really just a restatement of the laws of thermodynamics.

Posted by: @toodles

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@bobflux Thanks very much for that information and advice; air con is the answer then! I have read that some units incorporate dehumidifiers so must take a closer look at those. Regards, Toodles.

There is scope for product development here.  I for one don't want another ODU and another set of pipework around the house, even to just the two or three most important rooms, particularly as I already have the infrastructure required to extract heat from any point in the building, given the right terminal device.  There are a couple of products emerging which could use the existing infrastructure to extract the heat whilst also providing 'full' as opposed to 'light' cooling.  Both are essentially water to air heat pumps.

Quatt produce an add on to a water based heating system (the truncated cone thing) which connects to the water pipes and otherwise does what the standalone aircon units do (but without the pipe going outside and hence without drawing air in).  Quatt dont seem to sell this standalone, only with their own heat pump, but clearly it could be designed to work with any heat pump.

Octopus recently posted a facebook video where they had done the same by taking one of the standalone aircon units and adding a heat exchanger (connected to the radiator circuit) instead of the pipe out the window.  Towards the end of the video they teased the idea of bringing out a product based on this.

Panasonic and several others make 'water loop' aircon units which also run off chilled water.  Designed for apartment blocks they are a wall mounted self contained water to air heat pump.  These units tend to be about 3k each, which is rather expensive when compared to the £400 you pay for a standalone aircon unit, but reflects their intended use.

Also there are wall mounted monoblock air to air indoor units with two 150 or 200mm vents on the back, no pipework necessary

I cant help feeling that there is space for one, possibly all, of these in the market.  What I personally would ideally like is a version of the water loop unit where the compressor is disabled in winter (to reduce noise), so it just works as a regular fancoil.  The price needs to come down though!

Its a shame we don't (in the UK) plumb radiators TBOE (top bottom opposite ends).  If we did then they would work better for cooling (you need to reverse the flow relative to heating, but that's not particularly difficult with some ball valves).  Still nothing like aircon and still doesn't remove water vapour, but useful background.

If only we were starting from scratch we could design a system that actually worked altogether!

@toodles

I would recommend an A2A system. We have Daikin Comfora units (2.5 kW and 3.5 kW). On the hottest day of 2026 the system dumped 36.5 kWh of heat out of our house and used 4.7 kWh of electricity to do so, at an efficiency of 7.7. Cost about £1.20 on standard rate electricity, or a bit less than half that in lost solar PV export in our case. Our house is a 1970's 4-bed detached - not open plan except for the though lounge and kitchen/breakfast room, still the two units are easily enough to keep the whole house down to comfortable temperature (around 24C) in heat wave conditions - assisted by a couple of extra fans to move the cool air around.