A2A + A2W ASHP or a more powerful A2W ASHP?

[puntloos]

2 options, which one would you pick for my passivhaus design and why (and am I missing any arguments?)

 

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: 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 1kW 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 needs to be a little bigger and pricier)

Pro: Smaller footprint in my garden, only 1 big fan device vs 2

 

Final thoughts:

- I know some people consider any FCU overkill, however we've all heard the issues with locally(e.g. bedroom at night) overheating passivhauses , so I'm pretty set on having FCUs. But, I suspect even 1kW would probably suffice in most cases, even though in some rooms more power might be nice. 

- The power of the FCUs is a weird one, the 2.5kW R32 FCU is more quiet than the 2.5kW water one, the 1kW water one is, on 'low' setting equally quiet than the 2.5kW R32 one, but to reach 1kW, the water one really needs to make itself angry noise-wise while the R32 will never break a sweat.

- I don't think I care too much about the maintenance issues, yes getting a person out will be harder but it's probably rare anyway.

 

Edited March 23, 2023 by puntloos

[SteamyTea]

If you just heat the air in the building you will have less overall thermal losses, which points to an A2A as opposed to A2W with UFH.

2 hours ago, puntloos said:

Water is not a great heat carrier compared to coolant

Not sure what you mean there, water has an excellent specific heat capacity.

[puntloos]

On 23/03/2023 at 05:42, SteamyTea said:

If you just heat the air in the building you will have less overall thermal losses, which points to an A2A as opposed to A2W with UFH.

Not sure what you mean there, water has an excellent specific heat capacity.

The fundamental problem is that if you want to cool the air (given 'FCU' - not talking about UFH here), with an A2W ASHP only:

- You can't surgically cool an area when heating elsewhere

- You can't cool while the ASHP is heating water (I imagine not a huge problem normally)

 

But most importantly - efficiency. Assuming all other things equal there is one extra stage in the track:

A2W ASHP: Air -> Coolant -> Water -> Air

A2A ASHP: Air -> Coolant -> Air

 

This seems to be borne out from my research - same brand ASHPs - the A2A has a COP of 4.7, the A2W is 3.2, and the R32 FCUs are much more quiet (if I calculate "DB per Watt") than the Water ones.

[SteamyTea]

5 minutes ago, puntloos said:

The fundamental problem is that if you want to cool the air (given 'FCU' - not talking about UFH here), with an A2W ASHP only:

- You can't surgically cool an area when heating elsewhere

- You can't cool while the ASHP is heating water (I imagine not a huge problem normally)

Yes.

as you say, it boils down to the efficiency in the end.

[saveasteading]

As ST says.

Air to water to air requires an additional fan, or several, using more power.

On the other hand it has a quicker response so may be used less...which doesn't show in the COP.

We had a big system in our self build office block. It worked well enough. An extra bonus was a quick warm up on Monday mornings, and off ASAP. If it had been underfloor, the windows would have been opened half the day to lose the captured heat..

And practically, there was an electric heater in-line for extremes and the effect was very quick which is good for morale.

 

Wasn't so good on cooling, and I never worked out why.

[SteamyTea]

50 minutes ago, saveasteading said:

Wasn't so good on cooling, and I never worked out why.

I have never run the numbers, but generally, when temperatures are high, so is the RH. Condensing water takes a lot of energy.

2256 kJ/kg.

Water between 0⁰ and 100⁰C is around 4.18 kg/kg.⁰C. 

Quite a difference, even allowing for the relatively small mass of water present in a kg of air.

 

[saveasteading]

And it was simply underpowered for an office, and 15 years ago.

[markocosic]

A2W for underfloor; hot water, and chilled ceilings via plasterboard?

 

https://www.variotherm.com/en/products/modular-wall-heatingcooling.html

 

No noise at all. The premium option. 😉

 

 

The SEASONAL average COP of the solutions is not so different.

 

You wouldn't be lifting above 35 degC in heating season so 4.5ish nominal seasonal average COP for A2W

https://midsummerwholesale.co.uk/buy/mitsubishi-heat-pumps/mitsubishi-ecodan-5kw-ashp

https://www.theheatpumpwarehouse.co.uk/shop/heat-pumps/air-source-heat-pumps/vaillant-arotherm-plus-5kw/

 

I wouldn't go with an R32 or R454 A2W heat pump these days. R290 all the way. Mitsi, Pana, and Samsung are all a couple of years behind the curve on R290. Launching "this year" at ISH 2023 but unlikely to be in stock until 2024. Vaillant, Nibe, etc al are your best bet.

 

 

If budget allows I'd pop in a ground source unit. 6 kW of heating / cooling / hot water sat under the kitchen sink on in a (genuinely) refrigerator sized unit:

 

https://electrek.co/2023/03/25/award-winning-apartment-heat-pump/

https://www.qvantum.com/ISH23/GSHPM

https://www.qvantum.com/ISH23/GSHP

 

These are going to be all the rage in newbuild apartments in the not too distant future so availability of "low energy house" sized units should start getting good. And you're recharging your ground loop in summer somewhat by dumping heat into it.

 

In the meantime chuck a cheap A2W at the problem? Depends on timescales I guess.

[puntloos]

1 hour ago, saveasteading said:

As ST says.

Air to water to air requires an additional fan, or several, using more power.

On the other hand it has a quicker response so may be used less...

Not sure why it has a quicker response?

 

1 hour ago, saveasteading said:

which doesn't show in the COP.

We had a big system in our self build office block. It worked well enough. An extra bonus was a quick warm up on Monday mornings, and off ASAP. If it had been underfloor, the windows would have been opened half the day to lose the captured heat..

I assume this was not a high-insulation place?

1 hour ago, saveasteading said:

And practically, there was an electric heater in-line for extremes and the effect was very quick which is good for morale.

 

Wasn't so good on cooling, and I never worked out why.

 

Yes at the end of the day I'm trying to decide 

- A2A powering FCUs + A2W doing UFH/DHW 

- A2W powering FCUs, UFH, DHW

 

End of the day I think the big question for me is:
- Comfort - how quick can the system react - thinking of situation of hot day, open doors.

- Equipment, internal an external boxes (2 ASHP units or one big one)

[saveasteading]

4 minutes ago, puntloos said:

Not sure why it has a quicker response?

It heats the air directly isntead of the floor or radiators.

6 minutes ago, puntloos said:

assume this was not a high-insulation place?

It was for its time. Not now.

And commercial targets were lower than domestic.

 

[puntloos]

7 hours ago, saveasteading said:

It heats the air directly isntead of the floor or radiators.

Ah yes absolutely, I thought you were comparing water-based vs R32 FCUs but yes, both much faster than UFH

[BlackMountainBuild]

This is a good question, assuming you’re starting from scratch / new build - presumably it is cheaper and more responsive to have an air2air, and air2water / immersion system for hot water. If you have air2air and MVHR do you have double the ducting, is this actually a potential problem or am I completely misunderstanding this? My experience has been installers advocate for air2water and UFH - is this just what the UK is used to?

[puntloos]

I think we have to somewhat untangle the "coolant" used and the medium you're cooling. If you're sending out cooled air, that's fast but 'temporary' (as many of us know, the moment you turn on an AC unit the room heats up again in 15 mins)

 

The coolant - either actual refrigerant, or water, has efficiency benefits I'm frankly not completely clear on how impactful they are. It's easier to shield, so running water outside isn't great. Coolants can contain more heat energy per volume, and certainly have less loss pumping it around. 

 

My current front runner is actually https://shop.systemair.com/en-GB/syshp--mini--split--odu--12--q/p611468 since it uses R32 coolant outside, and might be able to use a mix of water for UFH and coolant for FCU. 

 

[RichardL]

New build - I'd have a scenario with no wet heating - it seems simpler somehow to have self contained A2A as a sort of module, rather than lacing water pipes, even UFH, through a house.

Perhaps the romans had the right idea all along with underfloor hot air ducts?

I would also look into ducted air for the A2A generally, and route hot bathroom/kitchen etc air out via a heat exchanger or exhaust type hot water heat pump - i.e. use some of your waste heat and ventilation for the DHW.

(Apologies If this was already discussed/discounted - I only skim read the thread.)

[SteamyTea]

2 hours ago, RichardL said:

Perhaps the romans had the right idea all along with underfloor hot air ducts?

I saw a video, from Japan, where the flat had a secondary floor that sat on the primary one.

There was a void in that floor to run pipes and cables in.  Quite a good solution for a flat as you cannot easily get 'underneath' to access services.

Would be fairly easy to design something with a steel or aluminium plate that could conduct thermal energy effectively.  Then pump in warm air.

[puntloos]

12 hours ago, RichardL said:

New build - I'd have a scenario with no wet heating - it seems simpler somehow to have self contained A2A as a sort of module, rather than lacing water pipes, even UFH, through a house.

Perhaps the romans had the right idea all along with underfloor hot air ducts?

I would also look into ducted air for the A2A generally, and route hot bathroom/kitchen etc air out via a heat exchanger or exhaust type hot water heat pump - i.e. use some of your waste heat and ventilation for the DHW.

(Apologies If this was already discussed/discounted - I only skim read the thread.)

Many american houses are built with 'forced air' but somehow they always feel somewhat strange. Air is too dry, feels like there's always either a hot burst or a cold draught. 

Also air doesn't heat block too well. 

I'm still tempted to do both, but it seems that a water based device might be able to do it all, as long as you also use FCUs

 

But my main question remains - should I use a classic (R32) device or stick with water?

Putting it differently assuming for a moment @RichardL's approach and only really do air heating or cooling, should the coolers be driven by water, or R32? And why/why not?

Edited April 8, 2023 by puntloos

[SteamyTea]

45 minutes ago, puntloos said:

Air is too dry

I actually prefer that, but you can get humidity devises that control it.

Vapac is one make I have used in the past, but for a steam room.

[ReedRichards]

9 hours ago, puntloos said:

Many american houses are built with 'forced air' but somehow they always feel somewhat strange. Air is too dry, feels like there's always either a hot burst or a cold draught. 

The reason the air is too dry is because the starting point is outside air that is much colder than is typical in the UK so with a much lower moisture content in the first place.  It's not the fault of the heating system, except insofar as the usually don't add deliberate humidification.

[puntloos]

8 hours ago, ReedRichards said:

The reason the air is too dry is because the starting point is outside air that is much colder than is typical in the UK so with a much lower moisture content in the first place.  It's not the fault of the heating system, except insofar as the usually don't add deliberate humidification.

 

I did wonder about at least allowing for a humidifier but in my experience adding water to pretty much any equation tends to make nature take over (rust, scale and/or funghi)..