a2a Sizing A2A heating

[gregh]

Anyone had experience with sizing A2A heat pumps (ie. Mini/multi splits) for heating loads?

 

Have had a few quotes for fitting out our house now, and sizes from contractors have varied quite a bit.

 

I’ve tried my best to approximate heat loss requirements using MCS heat pump sheet, as well as Jeremy’s sheet from the forum, and approximating existing radiator input. Often coming out with significantly lower numbers than any of the contractors. 
 

A couple examples:

(House is detached chalet bungalow from late 90s, brick with filled cavity walls on GF, timber frame with 50-100mm insulation on FF. Located in South East.)

 

Our lounge is a 25m2 room with 3 external walls and decent amount of glazing. MCS calc indicates probably 2kw losses, radiators (3) seem to add up to about 2.2kw. Contractors suggesting a 3.5Kw or 5Kw wall unit. Seems like 2.5Kw at most would be sufficient??

 

Looking to do ducted to 3 bedrooms upstairs, one about 20m2 and the others both 12-14m2. MCS seems to be about 1.2kw for the big one and 500-700w for the other two. Contractors wanted to do a separate 3.5Kw unit for each bedroom, and one contractor said 5Kw for the large bedroom. I am thinking we could get away with a single 3.5Kw unit ducted to all 3 rooms! (Daikin even does a plenum with motorised dampers to control individual room temps to a degree)

 

So am I missing something? Or are the contractors just vastly over sizing (as seems to be the case with many in A2W?) The units themselves aren’t massively different in price as you go bigger, but the running costs almost certainly will, particularly if they end up significantly oversized.

 

Any input is appreciated 

[JamesPa]

11 minutes ago, gregh said:

Anyone had experience with sizing A2A heat pumps (ie. Mini/multi splits) for heating loads?

 

Have had a few quotes for fitting out our house now, and sizes from contractors have varied quite a bit.

 

I’ve tried my best to approximate heat loss requirements using MCS heat pump sheet, as well as Jeremy’s sheet from the forum, and approximating existing radiator input. Often coming out with significantly lower numbers than any of the contractors. 
 

A couple examples:

(House is detached chalet bungalow from late 90s, brick with filled cavity walls on GF, timber frame with 50-100mm insulation on FF. Located in South East.)

 

Our lounge is a 25m2 room with 3 external walls and decent amount of glazing. MCS calc indicates probably 2kw losses, radiators (3) seem to add up to about 2.2kw. Contractors suggesting a 3.5Kw or 5Kw wall unit. Seems like 2.5Kw at most would be sufficient??

 

Looking to do ducted to 3 bedrooms upstairs, one about 20m2 and the others both 12-14m2. MCS seems to be about 1.2kw for the big one and 500-700w for the other two. Contractors wanted to do a separate 3.5Kw unit for each bedroom, and one contractor said 5Kw for the large bedroom. I am thinking we could get away with a single 3.5Kw unit ducted to all 3 rooms! (Daikin even does a plenum with motorised dampers to control individual room temps to a degree)

 

So am I missing something? Or are the contractors just vastly over sizing (as seems to be the case with many in A2W?) The units themselves aren’t massively different in price as you go bigger, but the running costs almost certainly will, particularly if they end up significantly oversized.

 

Any input is appreciated 

A2A is often sized, in a commercial context, for cooling which is more challenging than heating.  That may be part of the explanation.  Having said that it does sound a bit excessive.  If they modulate down efficiently then efficiency might not suffer, no idea how A2A performs in this respect.  

Edited April 11, 2023 by JamesPa

[RichardL]

My learnings so far - perhaps obvious/perhaps not?

Sizing If you know a room's BTU requirement you can convert that to kW and get an idea of the A2A heat input required.
Google will convert - or there will be a constant, google again, to get an idea.
For mine there were limited options anyway,  1.8, 2.5, 3.5 -  small, decent, huge room basically.

Note - as above the kW on the units are specified as cooling - but google for <unit name>.pdf install or combination and you can find the combination tables which give info on heat vs. cool inputs  etc. (I did for Mitsubishi Electric anyway)

Replacing central heating
You don't need an internal unit in every room or where you currently have a radiator - A2A blows warm air through the house, especially useful if you have corridor or open plan layouts.

The floor based units seem noisier / blowier (but still quiet TBH) than the wall ones - may be because the wall ones are further away though?

You can have more -or- less kW capacity on external vs internal multisplit - but there are limits / rules etc.

You can't really put an A2A inside unit in a bathroom - so need IR panels or electric immersion radiators etc with their own control system. 

Control is by default - not good - expect to pay extra or add on
Control on A2A is generally bad - its, by default, based on IR remotes for each inside unit - a far cry from a zoned timer system and you risk a proliferation of clocks through the house or dependency on apps and all the layers of software and network that need to work to keep them happy.

100% Research the wired options for control - especially if you can pickup and self install those parts after the difficult A2A install itself.

Location
The installers like to put the units on outside walls (generalism) - but can be persuaded otherwise just needs a little planning and legwork to keep their lives inside their comfort zone while also getting what you need.

Google searches that I found useful:
BTU to kW
<model> install pdf
<model> user pdf
<make or make multisplit> combination pdf

Sometimes it picks up australian documentation or European - if so can be a guide if not completely accurate -  depends who left documents publically available

Edited April 11, 2023 by RichardL

[S2D2]

I fitted a 3.5kW unit, it will modulate down to 400W power draw before cycling (not specified on data sheet) and would be cycling an awful lot in a 12m room as that's still about 1600W heat output.

 

One thing to remember is that the CoP will drop at the lowest temps so it will be sized for that. Often this is not specified in any detail on the datasheet either. Even still, that sounds overspecced. Ask to see their calculations and compare them to your own assumptions.

[gregh]

1 hour ago, JamesPa said:

A2A is often sized, in a commercial context, for cooling which is more challenging than heating.  That may be part of the explanation.  Having said that it does sound a bit excessive.

 

Thanks yes I was wondering about sizing for cooling rather than heating. Based on the heatwave last summer (our only summer in the house so far), I would say GF needs very little cooling. Large eaves overhanging most of the perimeter, including south/west, so solar gain reduced. FF is another story, there were a few unbearable nights last summer so may consider that on what we do up there.

[Gill]

In a similar position with the one installer that has come back to me. 

 

6 kW living room unit - that's probably on a par with our two large storage heaters 

Same for the sunroom which is currently only used when the temps are mild and any use of cooling would be moderate 

The bedrooms they propose using 2 kW units. 

 

They're suggesting 3 outdoor units as the highest capacity is 11 but that this may actually be an approximate and that it would be best to work to capacity of 9. Very confusing. 

 

I'm fairly sure I can drop the sun room to 3. I'd ideally like to drop the living room size for better efficiency but I'll need to revisit heat loss calculations I did before the triple glazing went in. 

 

[RichardL]

Think about when the inside units will run -  do you need 100% bedroom capacity when you need 100% living space capacity?
If not - size accordingly - perhaps a smaller external banking on the above and referencing the combination charts for your manufacturer.

The bedrooms likely only running first thing and in the evening, the core house mostly off during the day mon-fri and only my office and front room on all day in the week. If you know roughly how you will use it you can be tight or slightly oversize on the outside units.

A sized example
Overall approx 175m2 house (I know huge variables every house is different)

 

A2A:

10kW outside running 2.5 + 3.5 + 3.5 inside  which is the core of the house & home office,

5.4kW outside running the master bedroom and other end of the house 2.0 bedroom + 2.5 back door area

Bathrooms on IR panels & electric towel rails
2x bedrooms mostly unused so doors open for airflow - perhaps IR if they need it eventually but decided against individual cooling for them

2x outside units really just due to the size of the house and to keep installation practicable/outside trunking and inside routing minimal + the 5.4 outside was put in first last year to gain experience re phasing out oil.


Central heating to A2A:
10 radiators being replaced with 5 indoor A2A units,
2 towel warmers going electric only + supplemented with IR panels

Mostly designed around heating / cooling almost a secondary bonus.

[gregh]

8 hours ago, RichardL said:

Think about when the inside units will run -  do you need 100% bedroom capacity when you need 100% living space capacity?
If not - size accordingly - perhaps a smaller external banking on the above and referencing the combination charts for your manufacturer.

The bedrooms likely only running first thing and in the evening, the core house mostly off during the day mon-fri and only my office and front room on all day in the week. If you know roughly how you will use it you can be tight or slightly oversize on the outside units.

A sized example
Overall approx 175m2 house (I know huge variables every house is different)

 

A2A:

10kW outside running 2.5 + 3.5 + 3.5 inside  which is the core of the house & home office,

5.4kW outside running the master bedroom and other end of the house 2.0 bedroom + 2.5 back door area

Bathrooms on IR panels & electric towel rails
2x bedrooms mostly unused so doors open for airflow - perhaps IR if they need it eventually but decided against individual cooling for them

2x outside units really just due to the size of the house and to keep installation practicable/outside trunking and inside routing minimal + the 5.4 outside was put in first last year to gain experience re phasing out oil.


Central heating to A2A:
10 radiators being replaced with 5 indoor A2A units,
2 towel warmers going electric only + supplemented with IR panels

Mostly designed around heating / cooling almost a secondary bonus.

Do you find having a unit in the bedroom disruptive at all?

[SteamyTea]

9 hours ago, gregh said:

I would say GF needs very little cooling.

Mine is the same.  One word out of place and she is like a fridge.

[Gone West]

10 hours ago, RichardL said:

You can't really put an A2A inside unit in a bathroom

Why is that? Can't you use an in ceiling unit?

[RichardL]

2 minutes ago, Gone West said:

Why is that? Can't you use an in ceiling unit?

My installer didn't seem keen - you're putting mechanical systems motors etc inside a hot humid wet environment - I guess its not ideal.

Ducted is an option...

(Can't maybe was too strong -  there's always options )

[RichardL]

1 hour ago, gregh said:

Do you find having a unit in the bedroom disruptive at all?

TBH - the install is pending a firm to drill some holes.
However
I don't heat the bedroom at night - so its going to be a warm up before we get up in the morning, and another in the evening before turning in.
For cooling probably similar - but for cooling the comfort will massively outweigh any fan noise.

They're very quiet units unless the room temp is a long way off the target temp.
If you talk softly you drown the fan noise on low settings.

Try it and see though - the above is very subjective.
I put A2A in 2 rooms last year to check out noise levels, winter performance etc before extending.

[joth]

10 minutes ago, RichardL said:

My installer didn't seem keen - you're putting mechanical systems motors etc inside a hot humid wet environment - I guess its not ideal.

Ducted is an option...

Ducting a supply vent to the bathroom seems ok except that will push wet air around to the rest of the house (to wherever the central pathway to the ceiling plenum is). Having a supply/extract pair in the bathroom would work better for airflow, less disruption to any MVHR balancing, but then you're pulling a lot of wet air through the FCU heat exchanger. This will generate lots of condensation (when in cooling) and seems likely to end up with liquid in the ducting and corrosion to the metal. 

 

I guess the main concern is for heating, in which case the ideal use is running it prior to rather during in the room usage. But that only works if usage keeps to a strict schedule.

 

I can't recall ever seeing a/c in a hotel bathroom.

I'd personally keep to UFH for bathrooms

 

Edited April 12, 2023 by joth

[crispy_wafer]

Just to add my 2p for what it's worth, I've 2*2.5 (1 ducted,1 wall mount), and 2*3.5 ducted units going into the 3 bedrooms and 1 open plan kitchen/diner area, as mentioned above, once you settle on an installer and a chosen manufacturer you can download the brochures and see the condenser unit specs/and connection limitations. 

 

This is from MHI, but shows you the capacity limitations, there is also a page which shows acceptable combinations of indoor units too.

 

My installer is happy for me to have an 8kW condenser installed as the maximum connection capacity is 13.5kW, though he did suggest a 10kW unit may be a better fit.

 

Reason being that my use case will probably not involve running all units at full whack all at once, if that does happen then the indoor unit capacities are downrated (even downrated, I suspect they'll be fine on all but the most extreme of days, when it may just take a little longer to heat, or cool), the condenser isn't purchased yet, so still time to change my mind on this...

 

My use case is that this is for supplemental heating for downstairs if needed, but primarily installed for cooling during the summer, & autumn, and upstairs will be primarily used for background heating in the winter & spring when required, but cooling in the summer, autumn.  We'll have an ASHP for UFH downstairs, but no heating other than towel rads in the 2 bathroom spaces upstairs.

[TonyT]

15 hours ago, S2D2 said:

I fitted a 3.5kW unit, it will modulate down to 400W power draw before cycling (not specified on data sheet) and would be cycling an awful lot in a 12m room as that's still about 1600W heat output.

 

One thing to remember is that the CoP will drop at the lowest temps so it will be sized for that. Often this is not specified in any detail on the datasheet either. Even still, that sounds overspecced. Ask to see their calculations and compare them to your own assumptions.

 

you wouldn’t mind sharing the make and model would you, I’m looking at one for the downstairs hall for winter heating, maxing the unit out on the cheap night time slot( at the moment) and on and off as required.

 

i see it supplementing the GCH 

[S2D2]

3 hours ago, TonyT said:

 

you wouldn’t mind sharing the make and model would you, I’m looking at one for the downstairs hall for winter heating, maxing the unit out on the cheap night time slot( at the moment) and on and off as required.

 

i see it supplementing the GCH 

Yes I covered it in this thread:

 

See the second page for my experiences and performance data, my aim was to offset as much gas usage as possible with a single DIY install unit.

 

Let me know if you have any questions.

[TonyT]

Thanks, I’ve been MIA for a while!