Crunch time for heat pump size

[SBMS]

I’ve done my own heat loss calculations for our new build. Verrrrry roughly the spec of the house 

triple glazed windows 0.76 average

200mm cavity with eps beads at 0.15

Doors averaging around 1.0

Floor averaging around 0.1

roof has 304mm blown cellulose 0.12

going for high level air tightness

mvhr

 

the build is big - gross floorspace over 3 floors is circa 490sqm. 
 

We are going Panasonic aquarea. Wall mounted fan coil cassettes upstairs, UFH on ground floor. We have selected the Panasonic m series tcap because it natively supports dual zone (useful for cooling) 

 

Crunch time - I need to now select the heat pump. My calculations are coming out around 9kW. Heat punk is around 9.5kW. MCS installer is coming in more conservative around 12.1kW. I suspect we are somewhere between 9.5kW and 12 but we do have some unfortunate thermal bridges such as a big piece of steel over 8m of sliding doors. These thermal losses aren’t accounted for in my calcs. 
 

my dilemma - do i go for the 12kW model and hope its enough or jump up to the 16kW model?

Edited Tuesday at 21:55 by SBMS

[marshian]

25 minutes ago, SBMS said:

my dilemma - do i go for the 12kW model and hope its enough or jump up to the 16kW model?

 

Whats the difference in modulation between the two??

[JohnMo]

Even at 490m² at -3 your figures sound high. You possibly have better U values to me, your form factor will definable better than ours. So unless one side of the house is all glass, 9kW maybe a little high. At around 200m² we are 3kW. So 2.5 x 3 is 7.5kW, nudging 8kW (including DHW) at -3. So if Panasonic do a 9kW, I would be looking at that with some wiggle room. The bigger you go the bigger the pipes, flow rates to accommodate etc.

 

So definitely wouldn't be looking at 16kW.

 

But you will build houses big - are you actually going to use all of it? If not why not 2x smaller heat pumps in cascade. Then you can modulate down to 2kW and all the way up to 12kW+ should you need it.

[SBMS]

7 minutes ago, JohnMo said:

Even at 490m² at -3 your figures sound high. You possibly have better U values to me, your form factor will definable better than ours. So unless one side of the house is all glass, 9kW maybe a little high. At around 200m² we are 3kW. So 2.5 x 3 is 7.5kW, nudging 8kW (including DHW) at -3. So if Panasonic do a 9kW, I would be looking at that with some wiggle room. The bigger you go the bigger the pipes, flow rates to accommodate etc.

 

So definitely wouldn't be looking at 16kW.

 

But you will build houses big - are you actually going to use all of it? If not why not 2x smaller heat pumps in cascade. Then you can modulate down to 2kW and all the way up to 12kW+ should you need it.

Hope I’m going to use all of it! I’m the idiot putting an 8x3 in ground swim spa in so that’s a fair chunk of the floorspace as well 

[SBMS]

12 minutes ago, JohnMo said:

Even at 490m² at -3 your figures sound high. You possibly have better U values to me, your form factor will definable better than ours. So unless one side of the house is all glass, 9kW maybe a little high. At around 200m² we are 3kW. So 2.5 x 3 is 7.5kW, nudging 8kW (including DHW) at -3. So if Panasonic do a 9kW, I would be looking at that with some wiggle room. The bigger you go the bigger the pipes, flow rates to accommodate etc.

 

So definitely wouldn't be looking at 16kW.

 

But you will build houses big - are you actually going to use all of it? If not why not 2x smaller heat pumps in cascade. Then you can modulate down to 2kW and all the way up to 12kW+ should you need it.

Is it not likely my heat losses are bigger than calculated not smaller? As they don’t account for inaccuracies in installation, thermal bridging etc? 

[SBMS]

4 minutes ago, SBMS said:

Is it not likely my heat losses are bigger than calculated not smaller? As they don’t account for inaccuracies in installation, thermal bridging etc? 

Going to answer my own question here.. I guess my heat losses is worst case rare day not the average…. 
 

has anyone else ended up oversizing? Or tracked whether their heat pump was oversized?

[JohnMo]

2 minutes ago, SBMS said:

Hope I’m going to use all of it! I’m the idiot putting an 8x3 in ground swim spa in so that’s a fair chunk of the floorspace as well 

So how are you heating all of that,is that included as well in heat pump sizing?

 

1 minute ago, SBMS said:

Is it not likely my heat losses are bigger than calculated not smaller? As they don’t account for inaccuracies in installation, thermal bridging etc? 

Depending on how you have calculated ventilation losses there can be plenty of room for error both ways. Is your thermal bridging any worse than the glazing U value? If not impacts aren't much worse than the glazing? If the steel is insulated on the inside most likely not a huge issue.

[SBMS]

4 minutes ago, JohnMo said:

So how are you heating all of that,is that included as well in heat pump sizing?

 

Depending on how you have calculated ventilation losses there can be plenty of room for error both ways. Is your thermal bridging any worse than the glazing U value? If not impacts aren't much worse than the glazing? If the steel is insulated on the inside most likely not a huge issue.

no - the spa has its own heat pump. It’s basically a big hot tub so a closed system driven from its own a2w pump. 
 

We are going to put some insulated plasterboard to reduce bridging of the steel lintel…

Edited Tuesday at 22:47 by SBMS

[JohnMo]

29 minutes ago, SBMS said:

8x3 in ground swim spa in so that’s a fair chunk of the floorspace as well 

So if that's heated - have you accounted for the net input this provides to the building. If the space has insulated external walls, the heat in this area is added to the building internals.

[SBMS]

7 hours ago, JohnMo said:

So if that's heated - have you accounted for the net input this provides to the building. If the space has insulated external walls, the heat in this area is added to the building internals.

Thanks John. I did account yes, it’s got An insulated cover 95% of The time but also the room it’s in will Be a bit warmer as well. 
 

it sounds like you would err on the 12kW side. One frustration is Panasonic don’t seem To publish their modulation figures for these machines so I don’t know what they would typically modulate down to. If they both modulated to a similar level I probably wouldn’t be as concerned. My upfront cost is a bit more for the bigger machine granted and if I didn’t use the extra kW it wouldn’t matter… but if for some reason I’ve undersized it I’m a bit stuck. 
Sound logic?

 

the installer says they haven’t come across a build that’s averaging 19w/sqm before and think they’re nervous we will be cold. They’ve used higher air change figures in their calcs I think as well (as per mcs). 

[JohnMo]

1 hour ago, SBMS said:

installer says they haven’t come across a build that’s averaging 19w/sqm before

To put that into reality that is double a passivhaus. Really look at how they have accounted for ventilation heat losses. With MVHR ventilation heat loss drops from big to negligible.

 

Another thing we are 192m² living space plus another 26m² in the insulated space. Our roof area is around 50% more than yours as is our floor area, as we are single storey and every room has vaulted ceilings. All of which (part of form factor) really add to our heat loss per m². We are nearer 13W per m².

 

With a 12kW you may need a volumiser, depending on water volume. You can always get a volumiser with immersion that gives you another 3kW if you really need it. 

 

We have an oversized heat pump that doesn't modulate well. I can get acceptable SCoP from it, around 4, if it modulated better I would be getting closed to 5. So a 25% reduction in running costs. So sizing is important.

 

Looking at heat pump monitor a 12kW appears to modulate down to about 4.5kW, cannot see any 16kW on there.

[Dillsue]

We've got a calculated 8kw loss at -2 but installed a 7kw HP on the basis that it very rarely gets to low single figure temps and being undersized means the HP should run more efficiently most of the time. It's dropped to -1 a couple of times in the last month and the HP maintained the temp OK, but it worked hard when it was cold!!!

 

We have back up heating by way of an LPG boiler and a wood burner, but would be happy running a fan heater for a few days if we didn't have the backup and the HP couldn't cope. Remember if you oversize you're stuck with potential inefficiency for the life of the HP

[SimonD]

2 hours ago, SBMS said:

They’ve used higher air change figures in their calcs I think as well (as per mcs). 

 

On this basis alone, I would go and find another installer who understands heat loss calculations and design. Unless you've made significant changes to your Heat Punk defaults, I'd also caution reliance on those.

 

Since this year, there are 2 ways to complete a heat loss with ventilation losses. 1 is the simplified method that uses a CIBSE design table which is known to over estimate ventilation losses in the majority of cases. 2 is using a detailed method, including blower test results, that is known to reduce calculated ventilation losses significantly. This method takes more time.

 

Then there is an option to deal with thermal bridging. If you are using approved construction details and/or have a SAP calc (which you must have anyway) which also include a bespoke PSI value calculation, you can used reduced thermal bridging values from SAP, for example - not only is this essential from a building perspective, but it improves the accuracy of heat loss estimates too. 

 

Modifying all these will help you find a more appropriate heat loss figure.

 

Ignore anyone who tries to tell you what MCS says as that's usually a sign of ignorance. You need someone who understands its nowadays based on BS EN 12831-1 2017, but also be cautious about anyone who just says their tool complies with this standard - ask them to show you their inputs. 

[JohnMo]

16 hours ago, SBMS said:

- the spa has its own heat pump. It’s basically a big hot tub so a closed system driven from its own a2w pump. 

Have you considered using a single heat pump? Size the spa heat pump big enough for the house, use the volume of the hot tub as a heat source for the house via a plate heat exchanger and mixer valve. Heat pump does two jobs, keeps hot tub temp stable, and or DHW via a diverter valve.

 

 

[-rick-]

1 minute ago, JohnMo said:

Have you considered using a single heat pump? Size the spa heat pump big enough for the house, use the volume of the hot tub as a heat source for the house via a plate heat exchanger and mixer valve. Heat pump does two jobs, keeps hot tub temp stable, and or DHW via a diverter valve.

 

I thought pool heat pumps had fairly different optimisations from domestic ones so trying to mix them would not be advisable. @SBMS wants cooling anyway so I think needs a separate system on that basis.

[JohnMo]

14 hours ago, -rick- said:

thought pool heat pumps had fairly different optimisations from domestic ones so trying to mix them would not be advisable.

Use a normal domestic one, your just heating a buffer as far as it's concerned. Depending on hot tub size a big one. To keep the ASHP inhibited and pressurised you may need to plate load the hot tub, but size the Plate Heat Exchanger (PHE) correctly, you have a 2 Deg approach temp. Then you have no issues with DHW heating, it just switches the diverter valve as normal. Add a second zone for cooling, so it bypasses the hot tub or have the hot tub cooler in summer and as cooling buffer. Heat pump doesn't care if it doing heating or cooling it just moving the refrigerant 4 port valve to a different position (as long manufacturer hasn't disabled the cooling function).

 

Most heat pumps have the functionality to have different zones as cooling only, heat only or both. And the functions can be independently set for each zone.

[-rick-]

You'd need a very big PHE. The flow through pool heatpumps is huge compared to CH.

 

If you have to upsize the domestic heatpump you may also lose out financially. Pool heatpumps are super cheap compared with domestic ones.

 

Feels like this is something so outside your normal heat pump installers wheelhouse, that you'll either be paying them a huge amount to learn on the job and develop a bespoke solution for you or you'll scare them all off.

 

 

[JohnMo]

21 minutes ago, -rick- said:

Feels like this is something so outside your normal heat pump installers wheelhouse,

Maybe, but most houses on here are outside their comfort zone, ventilation heat loss just too small, to allow s tick the normal tick box. Hence plenty are specified way over sized.

[PhilT]

6 hours ago, Dillsue said:

Remember if you oversize you're stuck with potential inefficiency for the life of the HP

the counter argument is that at the coldest temps, for a given heat output, an oversized heat pump works more efficiently than undersized - manufacturer's data tables back that up. During the coldest days you could be using 10 x as much electricity as the mildest heating days. Think about the maths, for example, 10% more efficiency x 100kWh on the coldest days = a 10kWh saving, whereas 10% inefficiency x 10kWh on the mildest days means only a 1kWh loss.

[JohnMo]

28 minutes ago, PhilT said:

Think about the maths, for example, 10% more efficiency x 100kWh on the coldest days = a 10kWh saving, whereas 10% inefficiency x 10kWh on the mildest days means only a 1kWh loss.

But do you really get that many days at -3, compared to days at 7 to 10 degs.

[PhilT]

9 minutes ago, JohnMo said:

But do you really get that many days at -3, compared to days at 7 to 10 degs.

It would seem that for modern scroll compressors, for a given range of output, a larger compressor is more efficient than a smaller one across the entire ambient temperature range. This is the Ecodan data table. The Vallant Arotherm+ shows a similar pattern when comparing the 10kW and 12kW to the 7kW model. Up front cost is obviously a consideration.

11.2 "Mid range " vs 8.5 "Max" @ 35°C	In kW	In kW	Out kW	Out kW	COP	COP
	11.2 kW	8.5 kW	11.2 kW	8.5 kW	11.2 kW	8.5 kW	% dIff.
outside temp °C            -15	2.7	3.4	6.7	7.3	2.5	2.15	16%
-10	2.9	3.7	8.1	8.5	2.8	2.3	22%
-7	3.0	3.6	9	8.8	3.05	2.45	24%
2	2.4	3.0	9	9.7	3.75	3.2	17%
7	1.9	2.3	9	10.5	4.8	4.55	5%
12	1.4	2.3	9	10.7	6.25	4.6	36%
15	1.3	2.3	9	11.5	7.05	4.9	44%

[jamesdiyer]

I think for something like that cascade of two machines could be nice. Low modulation for mild temps and can maintain internal temp and a second machine for when really cold. It's going to be really important to get the right design and install done. 

 

Different machines modulate to different levels and others cycle better than others. I've an oversized vaillant and it works good. I should have had a cascade setup on reflection though, as the only time we are cold (and not actually cold but not perfect) is during mild temps. 

 

The specification pdf on that Panasonic doesn't seem to give a minimum heat output figure at X degrees C, which is what you really want to know to decide between sizes. 

 

Without being crass, you're clearly not poor. If I were in your shoes I'd find a really hot installer who knows their stuff and pay them the money to deal with the problem and guarantee it - I'm sure you've better things to do with your time than deliberate this. 

Edited 21 hours ago by jamesdiyer

[Beelbeebub]

51 minutes ago, PhilT said:

It would seem that for modern scroll compressors, for a given range of output, a larger compressor is more efficient than a smaller one across the entire ambient temperature range. This is the Ecodan data table. The Vallant Arotherm+ shows a similar pattern when comparing the 10kW and 12kW to the 7kW model. Up front cost is obviously a consideration.

11.2 "Mid range " vs 8.5 "Max" @ 35°C	In kW	In kW	Out kW	Out kW	COP	COP
	11.2 kW	8.5 kW	11.2 kW	8.5 kW	11.2 kW	8.5 kW	% dIff.
outside temp °C            -15	2.7	3.4	6.7	7.3	2.5	2.15	16%
-10	2.9	3.7	8.1	8.5	2.8	2.3	22%
-7	3.0	3.6	9	8.8	3.05	2.45	24%
2	2.4	3.0	9	9.7	3.75	3.2	17%
7	1.9	2.3	9	10.5	4.8	4.55	5%
12	1.4	2.3	9	10.7	6.25	4.6	36%
15	1.3	2.3	9	11.5	7.05	4.9	44%

I think the evaporator coil size to power output makes a difference too. 

 

You quite often see the smaller of the two outputs using the same "frame" size is more efficient. 

 

 

[SBMS]

Thanks everyone for your input. 
 

Have already got swim spa sorted and appreciate the advice John but the spa comes with a pump included and don’t want to confuse the spa installer and the heat engineers so will keep them separate!

 

I think I’m going to go for the 12kW but I have also asked Panasonic to do a full heat loss calculation as well so interested to see what they come back with. Both mine and heatpunk predict 9-10kW so I think a 12kW will work. It’s £1500 less for the heat pump as well - not to be sniffed at.
 

I did hesitate momentarily because I wondered if the cooling load could end up above this… but I’ve had a very lengthy thread elsewhere with @JohnMo where he said my predicted level of cooling was mad so 12kW cooling through fan coils should also be enough for the hot days…

 

[Dillsue]

6 hours ago, PhilT said:

the counter argument is that at the coldest temps, for a given heat output, an oversized heat pump works more efficiently than undersized - manufacturer's data tables back that up. During the coldest days you could be using 10 x as much electricity as the mildest heating days. Think about the maths, for example, 10% more efficiency x 100kWh on the coldest days = a 10kWh saving, whereas 10% inefficiency x 10kWh on the mildest days means only a 1kWh loss.

Would the 10% inefficiency represent an oversized heat pump not being able to modulate its output low enough to match a more usual winter heat demand and then start short cycling 24/7??

 

I appreciate that an oversized heat pump might perform better in cold weather if that's what manufacturers say, but cold weather only makes up a small proportion of the heating season. The rest of the time it's mild and an oversize heat pump could be short cycling for months. Do the manufacturers performance figures cover the situation where demand is less than the minimum modulated output?