Correct sizing of a heat pump

[KnightPhoenix (Zak Wilcoxen)]

@TrystanLea

ssume that actual performance in terms of capacity available is at least 10% below the figures here (based on my own experience). So at 45C water outlet temperature and somewhere between -7 and 2 the datasheet suggests 15-16kW. Minus 10% = 13.5 - 14.4. Which hopefully gives you an even healthier buffer of 20-30% if your heat loss is correct. My own 5kW R410a Ecodan is technically about 1.3x oversized yet gives good results, you really just want to avoid the 2-2.5x oversizing that is pos

You may find this useful - Another Heat Pump Spreadsheet: Beyond the Rule of Thumb | Protons for Breakfast

The ‘Rule of Thumb’ is splendidly simple : one just divides the previous year’s gas consumption by 2,900 to give the heat pump size in kilowatts. So if a dwelling used 10,000 kWh of gas the previous year, then one would estimate that it needed a 3.4 kW heat pump.

This is obviously based on the same heating requirements and the same outdoor temperatures as last year (from memory it was a little mild). All of the MCS calculations will rightly assume every room is heated and all to around 18-22°c based on MCS guidance. If you were zoning or only heating part of the house this would be distorted.

[TrystanLea]

@HollyCottage

am I right in interpreting the image you posted as showing it could drop to an output of only 3.7kW with a COP of 3.5 at 7oC outside temperature?

Yes at 35C flow temp, the minimum is 4.2 kW and at a cop of 4.45 that means ~ 950W minimum electrical input. You will probably see 1000-1100W with pumps included…

@HollyCottage

About 21,000kWh/yr in 2000/21 before the internal insulation work and 18,000kWh/yr

If we assumed 20,000 kWh of gas.

If we use the divide by 2900 rule of thumb that suggests ~ 7kW

The dive by 2900 rule on my own heat pump would suggest a capacity requirement of ~4kW (I have a 5kW heatpump) and max heat requirement closer to 3.5 kW.

What does your heat loss come out with from heatpunk with an air change rate of 0.5 for the rooms?
One way to clarify the air change rate question is to get an blower door test done, but that’s obviously another cost and delay.

[HollyCottage (Simon Flood)]

@TrystanLea

Yes at 35C flow temp, the minimum is 4.2 kW and at a cop of 4.45 that means ~ 950W minimum electrical input. You will probably see 1000-1100W with pumps included…

4.2 - 14kW seems a pretty good output range that’ll encompass most seasons/circumstances. Presumably once my heating requirement falls below 4kW and if the pump starts cycling frequently I could elect to have some off periods rather than have it running on/off for 24hrs/day

[HollyCottage (Simon Flood)]

@TrystanLea

What does your heat loss come out with from heatpunk with an air change rate of 0.5 for the rooms?

Sorry - can’t see how to change this on Heatpunk. Is it related to the ‘build date’?

[TrystanLea]

You may have to set it to a custom room e.g?
Ah when you select the room type you can change the air change rate in the custom room section:

[MyForest (David Bowen)]

I’ll chime in as a 14kW Ecodan R410A owner.

Ours cannot modulate down far enough to match our heat demand. The minimum I’ve seen is about 1.8kW input and about 6kW output. It refuses to sit at that low level for very long though. It’s usually bottoming out at 2kW producing 7kW.

However, that’s not a problem for us because we turn it on and off. For example, today the pumps have been:

Main: 36.3%
Space Heating: 28.9%
Hot Water: 7.4%

So we’ve actually used 900W on average and produced just over 3kW.

I could elect to have some off periods rather than have it running on/off for 24hrs/day

This chart is the amount of time we run the radiators compared to the average daily temperature - focus on the green line:

So you can see we practically never run it 24hrs/day. Even on 2023-12-01:

You can see it turned off around 15:00 because it was too warm. (there was also a saving session which is the big gap in the day).

If I zoom into the part where it ran continuously we can see it was still only averaging 6.5kW output:

so it would need to be a lot colder to see it trying to produce 14kW. However, I’m also sure it won’t actually produce 14kW when it’s cold due to defrosts and reduced CoP. Note that our flow temp at -3 °C effective outdoor temp was still only 39 °C so I’ve always considered the design flow temp of 45 °C to be somewhat pessimistic.

Our heat demand calc was 14kW based on previous oil usage.

I continue to be fine with my over-sized pump and having it cycle. Admittedly I wasn’t happy with it cycling every few minutes so you can see we have it doing something like 12 minutes on / 45 minutes off on a day like today.

As you can see here, the room temp continues to rise a bit even when the heat pump is off because the radiators are still warm. Admittedly I do turn the pumps off so I’m not sending that warmth back outside again.

Note that mine is a PUHZ-HW140VHA2 which uses R410A not R32 so you’ll see different behaviour from yours.

It’s nice to see everyone helping you with some good advice on this thread.

[HollyCottage (Simon Flood)]

Thanks @TrystanLea - got that to work. Changing every room to 0.5 reduces my heat loss to 10.5 kW. Guess however you’d expect an older stone property with two chimneys (albeit with chimney balloons) and a log burner might have a higher than average air change rate?

[KnightPhoenix (Zak Wilcoxen)]

Unfortunately, the only way to know for sure would be a blower door test.

[TrystanLea]

@MyForest

I’ll chime in as a 14kW Ecodan R410A owner.

Thanks @MyForest

@MyForest

The minimum I’ve seen is about 1.8kW input and about 6kW output

That’s good to know, so much higher than the datasheet suggests there.

@MyForest

However, that’s not a problem for us because we turn it on and off.

That’s the key to getting past the minimum modulation issue, is good on/off cycling control like you have implemented.

@MyForest

However, I’m also sure it won’t actually produce 14kW when it’s cold due to defrosts and reduced CoP

It would be great to do some max output testing on the newer R32 units. I imagine youd see similar to what Im seeing on my R410a 5kW, maybe 10% below databook capacity?

[TrystanLea]

@HollyCottage

Guess however you’d expect an older stone property with two chimneys (albeit with chimney balloons) and a log burner might have a higher than average air change rate?

It suprised me that Im seeing air change rates below 0.5 in an uninsulated mid-terrance solid stone house, with no real effort on draught proofing. Measured with both a blower door test and CO2 decay rates (I will post more on this at some point when I get a chance to write up).

Hard to know how to call this @HollyCottage, lots of datapoints pointing towards 14kW being the right size. Gas consumption might suggest 11kW is fine, perhaps even the 8.5kW?? if your actual max heat demand is closer to 7kW. Backup wood stoves would also suggest a lower capacity is likely fine, but then you dont usually get to take that into account with an MCS install…

[HollyCottage (Simon Flood)]

@TrystanLea

@MyForest

and reduced CoP

It would be great to do some max output testing on the newer R32 units.

Very happy to share my data once I’m up and running.

[HollyCottage (Simon Flood)]

@TrystanLea

Gas consumption might suggest 11kW is fine,

Yes although some rooms don’t heat up very well with current configuration (small rads on microbore pipework) and doesnt take into account additional 36m2 new extension - so suspect our gas consumption might be giving an underestimate.

[HollyCottage (Simon Flood)]

@TrystanLea

lots of datapoints pointing towards 14kW being the right size

Think I’m going to go with 14kW unit as planned. The system is being installed in our garage so guess there will be some small additional losses from piping everything through to the house.

Will be interesting to see how low it can modulate down too. Also having buffer tank fitted so am I right in thinking that can help prevent short-cycling?

[johncantor]

@MyForest

I continue to be fine with my over-sized pump and having it cycle. Admittedly I wasn’t happy with it cycling every few minutes so you can see we have it doing something like 12 minutes on / 45 minutes off on a day like today.

Have you ever tried to quantify the advantage you might get with a say 8.5kW unit on the ‘average’ winters day. i.e. by the ‘on’ cycling being say twice the time, you flow temperature should be say e.g. mid low 30s as opposed to 40

[greentangerine (Nick Ireland)]

@HollyCottage

Will be interesting to see how low it can modulate down too.

My R32 14kW Ecodan runs down to around 900W.

[HollyCottage (Simon Flood)]

@greentangerine

My R32 14kW Ecodan runs down to around 900W

That’s good to know. Giving you an output of around 3kW without short-cycling?

[greentangerine (Nick Ireland)]

Yup

I’m the Weymouth install on heatpumpmonitor.org if you want to look at the stats.

NIck

[johncantor]

Be mindful that the lowest steady-state output is when its cold out. When everyone wants low output… at say +10c out, then the minimum output seems typically no lower than around 40% ??? Another thing is that when they start to cycle, then they often ‘rev-up’ after start. Shame they are not clever enough to realise that the rev-up is not needed at that time.

[MyForest (David Bowen)]

@johncantor

Have you ever tried

No, because I’m on the RHI scheme for another three years so I can’t change things and seeing those numbers would make me sad.

At the moment I’m OK “taking one for the team” and demonstrating how to cope when it’s oversized so when others end up in this situation they have a way to cope.

It’s good that you are all are working to get a better fit.

[Ken_Bone]

Ive noticed this going from my old 16kw Samsung, which could modulate down to 650watts input, this is a R410a unit.
I now have the new Clivet R290 12kw, it cannot modulate down below 1.2ish kw input.