Air-to-air vs air-to-water

[Tarrington]

It's really mysterious to me why reversible multi-split systems aren't more common in the UK. They're reasonably common in the US and some of Europe, but every supplier I've spoken to about them in the UK has answered as if they've never even considered the idea. The worst thing is, nobody has been able to clearly articulate why. Some people say they're not "powerful enough" to heat a whole house (they are) or that they're less efficient or more expensive to run than an air-to-water system (when on paper they're more efficient, because they don't need as big a temperature difference). Of course, aircon is becoming more of a need as the years pass, and that's a nice side benefit of reversible multi-splits; and we've found from our current place with direct air heating that the low thermal mass you get by cutting water out of the equation is really nice.

 

I've seen some posts on here with better information from people who have actually used them, saying that underfloor heating feels cosier and an air blower creates a draught, making the room feel colder, so you can run to a lower thermostat set-point with underfloor heating: this makes more sense. But since it's quite a big decision I did a little more research into what systems are available - especially as heating suppliers are very vague about numbers.

 

After that, it makes a lot more sense why people would shy away from air-to-air. Our house is going to be about 175 sq m and the room layout is such that we'll need 6-8 indoor units. For a start, most multi-split systems only go up to 5-6 units, so we'd probably need to split it into two systems. Second, the smallest indoor blowers I've found are rated for 2 kW of heating, even though most of our rooms would need less than 1. The vendors don't recommend using much more total capacity indoors than the capacity of the outdoor unit (the heat pump itself), so we'd end up with a system with about 15 kW total capacity, even though the house itself is well-insulated and won't need more than 6 kW (and almost certainly less, as we won't need to heat upstairs and downstairs all day). That seems hugely overspecified, while an air-to-water system would be able to do a single 6 kW water loop with no problems, and (with bypass valves) still allow some per-room temperature control.

 

In addition, while there are some air-to-water units that can also do hot water for tops, that's not something you'll get from a multi-split, so there needs to be another solution there for hot water. Given all of this, it's slightly surprising that there aren't systems that can do both: air-to-water for underfloor heating and hot water, with the capability to send refrigerant to a small number of indoor blowers for heating or cooling extra rooms.

 

What do you think about this analysis? Is the need to overspec the system as a whole the downfall of air-to-air systems? Why do you think they're not more common in the UK?

[JohnMo]

Because we historically had a few days a year when Aircon was needed, historically they were also expensive to run, unlike now.

 

Toshiba make a water heating module for A2A. But generally for commercial sized units.

 

 

[Kelvin]

My planning permission includes one for the insulated metal garage largely because I might eventually build an office inside the garage so wanted the option. 

[ReedRichards]

I think you might struggle to find someone who will install an air-to-air heat pump for you.  If anything, they are more commonly found in commercial properties and the commercial installers don't necessarily want to do domestic work.  But perhaps somebody here could make a recommendation? 

[billt]

I think you can find an installer easily, just look for air conditioner fitters. Someone's coming from a local air conditioner company tomorrow to quote for one for one room in our house.

 

As to why they're not common:

 

Not generally discussed so people don't know about them, but that is slowly changing.

 

Multi room systems become complex and expensive - you are only allowed one heat pump under PD rules so planning permission would be needed if you need more than one outdoor unit.

 

The draft might be uncomfortable and the noise of the indoor unit might be unacceptable.

 

After some discussions on here I thought about the possibility of using air to air, maybe with a heat pump cylinder, but the complexity ruled it out. We have 10 rooms needing heating and 2 circulation areas, the house is long and narrow so multiple outdoor units would be needed, it would still be difficult to sight some of the indoor units. It's simpler and cheaper to use an air to water heat pump.

[Tarrington]

1 hour ago, ReedRichards said:

I think you might struggle to find someone who will install an air-to-air heat pump for you.  If anything, they are more commonly found in commercial properties and the commercial installers don't necessarily want to do domestic work.  But perhaps somebody here could make a recommendation? 

That's true, a lot of the people we spoke to (including the main building contractor we're using) can only do air-to-water. Even the place we eventually found who can do both spent about an hour trying to talk us out of it (which is weird when that's their main USP).

[George]

My preferred system would have been a multisplit a2a for the upstairs bedrooms and an a2w unit for hot water and downstairs ufh. 

Two small units very rarely run concurrently. 

 

However, the rhi grant wouldn't be compatible with this set up so I had to go for a whole house a2w system. 

[Caddy]

I've just been gathering quotes for an A2A system and most of the people have been commercial installation companies that are moving into the domestic market. I have a small house with 5 rooms and one outdoor unit is fine for me but I agree they seem to sum the indoor units and size the outdoor unit from that. Not sure this will work well for highly heat efficient houses with small heat requirements.

[Radian]

3 hours ago, Tarrington said:

Our house is going to be about 175 sq m and the room layout is such that we'll need 6-8 indoor units.

 

I don't want to get a reputation for being a zealot about A2A, so I'll say some bad things about them for a change: The colours of the outdoor units aren't very pretty and they can look a bit 'downtown Shanghai' on the outside of a building. But seriously, they're not ideal if you need 6-8 indoor units to serve lots of separate rooms (as you've already surmised). However, having recently installed a couple to heat two largish rooms in a new extension over two floors, with intermittent occupancy, I really can't imagine a better solution.

 

In fact, every time I make use of them I can't help but wonder how I could use them to heat our main house. The reason being the speed they heat a space and the tiny amount of energy they use to do it. But I keep coming up against the roadblock of having separate rooms rather than large open-plan spaces. What I am considering is a single A/C unit at the top of a central, vaulted stairway/hall. This space could do with responsive heating in the winter and cooling in the summer and doesn't present any comfort issues with breezes. I've got to work out how much of the gas central heating it would offset to make sure it would make economic sense. But I think a hybrid system such as this should be at the back of everybody's mind.

 

 

[Kelvin]

It was why I thought about it for my insulated garage/workshop. Seemed like a reasonable way to heat/cool the space that wasn’t very expensive to buy or to run but could heat it up quickly enough. 

[crispy_wafer]

We use A2A at work, in the offices and lobbies etc, I blast them up to temperature pretty early then turn the fans right down on them, so no audible noise or noticeable draughts, so that, the high temperatures last summer and the fact that we'll get some heat from them is just a bonus, the bedrooms dont need to be above 18, anything more than that I struggle to sleep.  I have noticed that the warm air doesn't get through doorways particularly well, so our unheated hallway is also a thought.  However because we are having ducted and the unit has 3 in/3 extract, we have 2 in/2 extract to the bedroom, so maybe I could use the redundant one in the hallway, but it would only work of course when the bedroom is in use, better than nothing though

[HughF]

They’re also considerably less popular because you need to find an fgas installer to do the work. A monobloc can be installed by anyone with a bit of plumbing experience.

[RichardL]

Part way through going to A2A,   I installed one multisplit for the lounge/home office last year under permitted development.

That was a test - especially for sub zero temps over the winter.

It certainly pulls the juice when its very cold - but the plan is as electric use goes up my oil use goes down and I'll probably end up in a similar cost space but without the home burned hydrocarbons.
 

The experience was good enough to progress with more this year - its a different heat from radiators of course - but seems like a good option vs A2W and the limited / constant heat output. A2A has the potential for no radiators rather than larger ones - admitted there are still boxes on the wall for heat though.

Currently dependent on planning for more boxes outside - I'll end up with one at either end of the house - broadly living area/bedrooms split.


Re installers - they do like to put the internal units on external walls so you need a bit of thought over where they go and especially if you want to hide trunking etc - but its all possible.

[Gone West]

13 hours ago, RichardL said:

Part way through going to A2A

We are thinking about the possibility of installing A2A as a replacement for oil. We are in a bungalow so all the pipes can run through the loft to each room. The only problem we can see is how to run the condensate pipes as none of the heating units will be on an outside wall and we don't want internal ducting. Did you have a problem with the condensate pipe?

[RichardL]

22 minutes ago, Gone West said:

We are thinking about the possibility of installing A2A as a replacement for oil. We are in a bungalow so all the pipes can run through the loft to each room. The only problem we can see is how to run the condensate pipes as none of the heating units will be on an outside wall and we don't want internal ducting. Did you have a problem with the condensate pipe?

If you have good loft access a ducted A2A may be a good option... only one unit in the roof then large diameter pipes to each room with dampers to control which gets heat.

Then only one connection from outside to inside - and one condensate run, probably from the roofspace towards the eaves and into gutters?

Re your question - my existing units are positioned so they have a downhill run to an outside wall, in a loft space.
It will be a challenge for the rest of the house and I think at least one bedroom unit will need a pumped/uphill condensate run.

[Radian]

24 minutes ago, Gone West said:

The only problem we can see is how to run the condensate pipes as none of the heating units will be on an outside wall and we don't want internal ducting. Did you have a problem with the condensate pipe?

You can get condensate pumps that will allow you to route the drain along with the refrigerant and power lines.

[RichardL]

On 26/01/2023 at 13:54, HughF said:

They’re also considerably less popular because you need to find an fgas installer to do the work. A monobloc can be installed by anyone with a bit of plumbing experience.

I agree with your point - but I guess its a double edge sword that monobloc's just need a plumber whereas aircon is more specialised.

There are cowboys everywhere who will falsify experience - but anything that stops the 'double-glazing salesman' opportunist jumping into the market  without training is a good thing IMHO - (hopefully they are all occupied with conservatory roof insulation)

[JamesPa]

Nothing wrong with a2a, I have it at work in a couple of buildings.  It's cheap to install and you get warm air quickly.  It delivers a good CoP.  Multisplit, for some reason, seems to be more expensive than multiple single units.

 

However the fans do make a noise and some might not like the breeze.  Also, without a reserve of hot water, the defrost cycle can be a bit cool.  Domestic buildings tend to have more smaller rooms than commercial which isn't such a great match to A2A, but a big open-plan house might be.  Its really about what you prefer and layout.  Given that wet systems are well established in the domestic market I would expect A2A to remain a minority, but that doesn't mean it is not worth considering.

[crispy_wafer]

44 minutes ago, Gone West said:

We are thinking about the possibility of installing A2A as a replacement for oil. We are in a bungalow so all the pipes can run through the loft to each room. The only problem we can see is how to run the condensate pipes as none of the heating units will be on an outside wall and we don't want internal ducting. Did you have a problem with the condensate pipe?

 

Ducted units a possibility with vents into rooms from above, because the fan coil units are suspended from rafters, the condensate pipes will run downwards to a good drain location 

[RichardL]

21 minutes ago, JamesPa said:

 

...However the fans do make a noise and some might not like the breeze...

Absolutely aligned with this comment & based on experience with a home multi-split -  if a little white-noise -  fan noise annoys, or you have heating on in bedrooms at night then think twice.

It's a quieter version of that constant hum in any large office space, not constant since it runs on a thermostat - unless its cold outside
You can turn down the fan to non-audible - but if its sub zero outside the fan will be 30-50%.

Front room with the telly on or normal talking I doubt you'd notice.
Bedrooms to heat up in the morning to get up and evening before turning in  - but off overnight - again perfect.

Side thought-  if you enjoy finding that hot draft spot in a shop under the aircon outlet or by the door air curtain then certainly consider A2A

Edited January 30, 2023 by RichardL

[mike2016]

Considering this question for my own house - it's going to be a new build, quite small only 100m2 and I can get a Daiken with a 90l/120l hot water cylinder water heating option too. I think A2A works best in a retrofit though. Another primary benefit is direct cooling. I could live with the slight noise and draught, in an ideal world I'd get A2A for the bedrooms / office and A2W for the ground floor! 

Still heavily leaning on A2W for my house as it's a 3 bedroom with a bath and I'm not sure the smaller A2A tanks would work well on the hot water side and take too long to recharge. An A2W is better suited for my situation. The other con though is cost, the A2W is closer to 20K all in with the underfloor heating loop. An A2A would be much cheaper. 

Treated floor area 101.0 m²

Heating demand 14.91 kWh/(m²a)

Heating load 11.06 W/m²

I could run refrigerant pipe, Electrical & Drains to all the proposed locations so it's easy to retrofit later but would I ever use it? 

https://www.daikin.ie/en_gb/product-group/air-to-air-heat-pumps/multiplus.html

 

What do people think about using the heat pump manufacturers integrated cylinder vs getting a separate one? Any advantages / benefits there? Thanks! 

[RichardL]

FWIW - I'm retrofit and moving off oil fired heating and hot water.

Wasn't keen on A2W for various reasons so the plan is A2A for heating and probably a self contained integrated cylinder heat pump unit for hot water - i.e. separate the systems - run them both from electric but lose the concept of a hot water circuit from a boiler for heating and hot water.

Re cylinder heat pump for DHW
Less integration provides options - suddenly it's an electric cable as the requirement to transfer energy around the house rather than a 22mm pipe full of hot water and a boiler burning oil to provide it.

The idea of a single hot water unit with cold in/hot stored/out electric feed and COP 2-3 etc seems good to me - simple install (if I can work out its vent requirements). Ideally it would take warm air from the bathroom/kitchen - but retro fit again that may not be practicable.

Solar PV heats the water march-sept  with a bit of luck.

Re the point on retro vs new build - phasing
A2A provides a phasing option - i.e. the experiment with lounge/office last year on heating, expand this year then at some point that final component for hot water meaning the oil can completely go.

Hopefully I avoid mis-sizing the heating or any part of the oil phase out since the oil is still there in the interim.

There's no particular rush though - the oil boiler can pickup the gaps as I upgrade.
Each room will lose its radiator and can phase that too including the redecoration etc.

[SteamyTea]

On 25/01/2023 at 17:19, Tarrington said:

every supplier I've spoken to about them in the UK has answered as if they've never even considered the idea

As far as I know, A2A has never attracted a government subsidy.

 

On 25/01/2023 at 17:19, Tarrington said:

low thermal mass

What is this?

 

If, say your bedrooms need half the amount of power than the smallest emitter, emits, then can't you pop it in the loft and duct half the air to each room.

Edited January 30, 2023 by SteamyTea

[Kelvin]

Coming back to the question of you’ll find it hard to get someone to fit it. These guys do a complete purchase and install service for not a lot of money. It’s unclear if they cover Scotland though as despite their map saying they do the person I spoke to said they don’t but is checking for me. 
 

https://www.appliancesdirect.co.uk/content/air-conditioner-installation

 

My slight issue is the size of the space at 10.5x6x4.2 (ridge height) so to size it for the space would likely require two units. My plan is to create an office within the garage anyway so will likely just size it for that. 

[joth]

3 hours ago, SteamyTea said:

On 25/01/2023 at 17:19, Tarrington said:

every supplier I've spoken to about them in the UK has answered as if they've never even considered the idea

As far as I know, A2A has never attracted a government subsidy.

 

They certainly didn't meet the criteria for the obsolete RHI as that mandated "use of a liquid" to provide space heating (and various exclusions around use for cooling, depending on the variant of RHI)

 

The new BUS potentially could go towards a2a unit so long as:

- it can supply the FULL heating demand of the property

- it can also provide the DHW demand

- it is MCS certified install

 

All three of those are currently pushing people towards a2w, but it could change as AFAICT the BUS itself does not preclude a2a.