A2A size

[Crofter]

Fitting an underfloor ducted air to air heat pump.

Installer has offered the choice of 7kw or 10kw.

The 7kw can go down to 1.1kw minimum, whereas the larger unit can only drop to 2.1kw.

I'm tempted to go for the smaller one, my understanding is that the lowest output is quite important as when this is too high, the system will start to cycle and efficiency will suffer.

The smaller unit is also significantly quieter, and a little cheaper too.

 

I know it's an almost impossible question to answer without a proper heat loss calculation. I've had a stab at it and generally 7kw seems like plenty, although the real figure will depend on how much heat we're going to lose through air leakage, which appears almost impossible to determine. 

 

So, what wins out, higher output vs lower modulation?

[PhilT]

any possibility that you might extend in the next few years?

[Crofter]

7 minutes ago, PhilT said:

any possibility that you might extend in the next few years?

There is, but it would be a loft conversion, and I'd likely be installing a second system to handle that, probably with a couple of wall mounted units.

[PhilT]

55 minutes ago, Crofter said:

There is, but it would be a loft conversion, and I'd likely be installing a second system to handle that, probably with a couple of wall mounted units.

Is this an existing or new property and, if existing, what is your annual energy consumption on  heating, in kWh?

[Crofter]

27 minutes ago, PhilT said:

Is this an existing or new property and, if existing, what is your annual energy consumption on  heating, in kWh?

Existing, but I can't separate out the heating and hot water usage, both are direct electric.

[PhilT]

2 hours ago, Crofter said:

Existing, but I can't separate out the heating and hot water usage, both are direct electric.

What's the total

Edited July 9 by PhilT

[Crofter]

20 hours ago, PhilT said:

What's the total

Took a while to dig it out but we seem to be averaging about 840kwh per month in the heating/DHW meter. 

Unfortunately this is partly based on estimated readings, and the occupancy is the house might not be representative of future use- it was used as a holiday let for the last three years, so people had little incentive to save energy, but also it was mostly empty through the coldest months (although there was nobody around to turn the heating down).

[Crofter]

Any takers? 

Basically my choices area down to a unit which will modulate between 1.1-7.3kw, or one that does 2.1-10.4.

There's a third option which can do 2.1-8.1 and has done nice air purifying features, and it's a tiny bit cheaper. 

 

I've heard that the ability to modulate down to a very low level is quite important for efficient running.

[PhilT]

17 minutes ago, Crofter said:

Any takers? 

Basically my choices area down to a unit which will modulate between 1.1-7.3kw, or one that does 2.1-10.4.

There's a third option which can do 2.1-8.1 and has done nice air purifying features, and it's a tiny bit cheaper. 

 

I've heard that the ability to modulate down to a very low level is quite important for efficient running.

Too little info so far. The energy use data you gave is way too rough even for a ball park sizing estimate so, as things stand, your heat loss calc is your best guide.

[atlantication]

840 kWh/month, taking off 3 kWh/d for DHW equates to pretty much exactly 1 kW constant load - 730 hours per month.

 

Assuming this is averaged over a year, and conservatively that the worst month takes 1/3 of the total heat demand. The max monthly demand would only be an average of 4 kW. With a 7 kW unit there would be more than enough headroom for intermittent use/cold snaps assuming the house has at least a little insulation.

 

The best way to confirm would be to multiply your heat loss out by the Heating Degree Days for your location/year and see if that vaguely matches up with the electric input. That way you can be more confident in your calculations.

 

One other caution - would be a good idea to look at the datasheet of the system, make sure the heat output at low temperatures still meets the 7 kW.

[severnside]

I would say go for higher power as it does modulate to fairly low. You can take advantage of cheaper tariffs and run it for limited time instead of running 24hours. A2A keeps flow temperature reasonably low so there is not much extra efficiency loss compared to A2W if you try to run your UFH or radiators at higher temperatures. We have A2A with ducted system and few on wall units. We slightly overspecced it. I don’t think there is too much efficiency loss with bigger units. Also bigger units will probably have lesser defrost cycles

[Crofter]

In the absence of a proper heat loss calculation, I've been looking at a few online tools.

A radiator sizing tool gives me 8.9kw

Heatgeek's 'rule of thumb' suggests anywhere between 4.4-7.2kw.

 

We are in a pretty exposed location in NW Scotland. Starting to lean towards the 10kw to give a bit of extra margin. But I'm also aware that with ASHP bigger isn't always better...