Air-to-Air or only Air-to-Water Heat Pump?

[puntloos]

In my new high efficiency house, I will have UFH with an Air to Water (A2W). Heat pump Monobloc, but for extra cooling on hot days I think extra air cooling with FCUs is needed:

Should I use a secondary Air (to coolant) to Air system, or stick with just the Air to Water system to also drive my Fan Coil Units, which actually means Air - to Coolant - to water - to Air, hence less efficient.

When comparing Mitsubishi gear (not stuck on mitsu but they at least make both, some benefits to have all from same mfg, and easier to compare), the Air to Air is more efficient and therefore a little quieter even with the extra compressor in the FCU - but installing a separate secondary system has at least some financial, and some space (big device ourside) downsides.

What do you think? Any further pros and cons? Some detail below:

Option A:
A2W ASHP for UFH, hot water taps
A2A ASHP R32 coolant e.g. MXZ-4F83VF, 4x SEZ-M25DA 2.5kW FCUs

Arguments for this Option:
Pro: More efficient. Water is not a great heat carrier compared to coolant. Easier to make powerful devices.
Pro: Coolant line is easier to insulate, less loss from back of garden to the devices, water is more lossy
Pro: If A2W breaks, I have backup. Not a huge deal in a super insulated house.
Pro: No need to switch the ASHP from heating to cooling with all the associated hassle especially if you want some hot water during hot summer.
Pro: Surprisingly, more quiet than the equivalent Water based FCU, at least comparing PEFY-WP25VMS1-E (water) with SEZ-M25DA (R32)).

Option B:
A2W ASHP for everything, 4x in-ceiling 2.5kW cooling power fan coil units

Arguments for this option
Pro: Water coolant is safe, although R32 is not lethally toxic unless you truly douse yourself in it for ages. There's a chance R32 will be outlawed at some point in the medium/far future but right now if anything it's the #1 recommended coolant
Pro: A2W means 'standard' engineers can work with it, no coolant certification needed
Pro: Somewhat cheaper (don't need an extra 4500 GBP device, but the A2W monobloc needs to be a little bigger and pricier)
Pro: Smaller footprint in my garden, only 1 big fan device vs 2

 

[Ric2013]

If your house is to be high-efficiency, it shouldn't need daytime cooling at all.

Crikey, even my 1870s house in Essex stays reasonably cool if I open the windows at night and just shut windows during the day, taking care to draw the curtains on sunbeaten openings during the day (no external shutters...yet). Managed to keep the internal temperature down to 26 in a house in Portugal throughout a 42 degree heatwave by the same method (though that house had external shutters).

Build it with enough thermal mass and shutters. Remember than large openings on the west will be prone to evening sun and consider the orientation of the building carefully as there is much more to environmental design than gadgets. Cool it at night if it proves necessary due to future climate change. The less we are dependent on mechanical devices, the better.

 

[puntloos]

If your house is to be high-efficiency, it shouldn't need daytime cooling at all.

Crikey, even my 1870s house in Essex stays reasonably cool if I open the windows at night and just shut windows during the day, taking care to draw the curtains on sunbeaten openings during the day (no external shutters...yet). Managed to keep the internal temperature down to 26 in a house in Portugal throughout a 42 degree heatwave by the same method (though that house had external shutters).

Build it with enough thermal mass and shutters. Remember than large openings on the west will be prone to evening sun and consider the orientation of the building carefully as there is much more to environmental design than gadgets. Cool it at night if it proves necessary due to future climate change. The less we are dependent on mechanical devices, the better.

I hear ya, but the problem is that in various passivhaus threads the #1 issue is overheating. They are insulated so well (yet have so many windows.. pretty) that heat is an issue. And in particular in a smallish room, upstairs (convection..) with two adult humans sleeping..

 

[Ric2013]

Just realised I was quite negative. To be fair, the biggest problem seems to be those nights when there is no air movement; this is something that mechanical ventilation would address even with no cooling machinery.

If I understand you, you are comparing fan coils running off DX coils, chilled by an air to refrigerant unit externally, with fan coils run from chilled water produced by a remote chiller unit. Is that right?

 

[puntloos]

Just realised I was quite negative. To be fair, the biggest problem seems to be those nights when there is no air movement; this is something that mechanical ventilation would address even with no cooling machinery.

If I understand you, you are comparing fan coils running off DX coils, chilled by an air to refrigerant unit externally, with fan coils run from chilled water produced by a remote chiller unit. Is that right?

 

[puntloos]

Yes that's correct - my sense certainly is that the DX coils are a more efficient solution, but perhaps the upsides are not outweighed by downsides (extra cost, extra space)

 

[ShaunCorbs]

If you air tight you should be running an erv or hrv so put a condensing coil before the unit with dehumidification and there you go central ac

 

[Ric2013]

Yes that's correct - my sense certainly is that the DX coils are a more efficient solution, but perhaps the upsides are not outweighed by downsides (extra cost, extra space)

My sense is that you are correct. Unfortunately my days in HVAC are quite a long time ago now and I never did get to training to be a refrigeration engineer due to company going bust (essentially). The firm I worked for used to supply to construction firms, for installations in hospitals, hotels, universities: big stuff. We mainly sold air handling units (AHUs) which heat, cool, filter, and humidify or dehumidify the air, and a plate heat exchanger for heat recovery was standard. The conditioned air is then ducted to whatever rooms require it, and extract air passed through the same AHU.

I agree with Shaun that if you are cooling, you will probably want to dehumidify: if the outside air is (for example) 26° C ('normal' dry bulb thermometer) and 75% RH, the RH rises to 95% when that air is cooled to 20° C. In a way, 26°C would be more tolerable than 20°C if the air were fairly dry.

I think the point of chilled water is that a hospital, say, will have a large chiller unit and can heat some rooms and cool others and it saves the mess of having 1000 separate condensers to maintain and account for. Often, if there are fan coils, they will be 4-pipe which means they can be switched from cool to heat mode almost instantly and some fan coils are able to heat while others are cooling. My impression is that there is isn't really an advantage in using chilled water on a domestic scale, though I can see why it would be annoying to have to reverse-cycle to switch between cooling and hot water production if you take the other route. Pity you can't use an internal A2W heat pump to cool your air and heat your water in one go... or can you?

Generally, large projects used chilled water from a chiller unit, small projects (by our standards) used DX. Something on a domestic scale would probably run on a packaged split system, but that wasn't our market and we used to refer customers to Adcock Refrigeration.

 

[puntloos]

If you air tight you should be running an erv or hrv so put a condensing coil before the unit with dehumidification and there you go central ac

I was looking into that route but ducting air everywhere seemed to be much harder to control and required huge ducts etc. Figured that getting the heat energy or the cold to the specific room and only treat the air in that location.

 

[ShaunCorbs]

I was looking into that route but ducting air everywhere seemed to be much harder to control and required huge ducts etc. Figured that getting the heat energy or the cold to the specific room and only treat the air in that location.

As you passive eg air tight you will need a supply and return in each room it’s normally 2.5-3” duct have a look at zehnder