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 11 hours ago 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 10 hours ago 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