ASHP sizing, heat loss calcs, buffers

[Dunc]

Apologies for yet another "how big does my ASHP need to be" question; I've read a lot but understand very little.

ASHP to drive UFH for a 1.5 storey house, well insulated (U=0.1/0.12/0.1 W/m2K roof/wall/floor) and airtight (<0.6ACH) with MVHR. 

JHarris spreadsheet heating demand 1.7 kW (-5C out/21C in, to match UFH design quotes).

Various quotes from ASHP/UFH designers suggest heat losses and pumps sizes: 4.8 kW loss/8.5kW pump, 4.7 kW loss/5kW pump, 3.9kW loss/5kW pump. All seem to be using -5C out/21C internal temps, flow temp 45C, return temp 5C

SAP calc indicates 4.45 kWh/m²/yr, if that's relevant.

3 key questions:

1. Have I done something wrong in the JH spredsheet?

2. Can the ASHP be too big? i.e. supplier recommending 8.5kW pump for a load of 4.8 kW (or 1.7 if I'm right!)?

3. If the 4.7 or 3.9 kW load is correct and paired with a 5kW pump, is there enough overhead for DHW duty?

Bonus Q: One supplier suggested that fitting a buffer will "deal" with an ASHP that is oversized. Does that sound right?

[Beelbeebub]

If your heating demand is under 2kw you'll struggle to get an (air to water) HP rated for much less than 3.5kw. 

 

Has the house been built yet? Such a low demand might suit an air to air HP (aka air conditioner) instead (depending on layout, preference etc) 

[JohnMo]

4kW heat pump would be fine. Go for something like a Panasonic. You will generally get some cycling, but that is normal.

 

4 hours ago, Beelbeebub said:

Such a low demand might suit an air to air HP (aka air conditioner) instead (depending on layout, preference etc) 

Then you need something else to heat the hot water.

[Dunc]

@JohnMo are you suggesting that a 4kW pump would be fine because the heat load is being over estimated, and my 1.7kW figure is correct. Or that even if heat load is ~3.9-4.8kW a 4kW pump is sufficient?

 

@Beelbeebub not built yet but rather late to make a subtantial change in the overall plan! 

[JohnMo]

19 minutes ago, Dunc said:

@JohnMo are you suggesting that a 4kW pump would be fine because the heat load is being over estimated, and my 1.7kW figure is correct. Or that even if heat load is ~3.9-4.8kW a 4kW pump is sufficient?

 

@Beelbeebub not built yet but rather late to make a subtantial change in the overall plan! 

If it 1.7kW it would be fine. If its 3.9-4.8kW obviously not big enough, then you are looking at a bigger heat pump. The JH spreadsheet is pretty close to reality, in my experience and our house.

 

We are around 200m2, very poor form factor, (single storey, all vaulted ceilings, long thing building) not Passivhaus, we have a heat loss of around 3.5kW at -9 and a 6kW heat pump. 

[Chanmenie]

1 hour ago, Dunc said:

@JohnMo are you suggesting that a 4kW pump would be fine because the heat load is being over estimated, and my 1.7kW figure is correct. Or that even if heat load is ~3.9-4.8kW a 4kW pump is sufficient?

 

@Beelbeebub not built yet but rather late to make a subtantial change in the overall plan! 

How are you planning on building it ? Those U values for walls floor and roof are very low 

[Beelbeebub]

9 hours ago, Dunc said:

not built yet but rather late to make a subtantial change in the overall plan

Fair enough. 

 

Your specs seem good and your design point very conservative. Unless you are far north -5 isn't very common and 21C  indoors is nice and warm. 

 

Your calculated heat loss is well below the output of the lowest sized HPs. So I would suggest go with a small unit (assuming you don't have an unusually high hot water demand) use a single zone system, weather compensation, no buffer etc. 

 

If you are really paranoid then make sure you have a little extra space on amd around your installation site. If the absolute worst happens and your 4kw HP isn't quite enough you could always swap it for the next size up a 6 or 7kw unit easily. They are typically only slightly larger. So site your unit as if it were a 7kw eg don't get right up against the limits of the space around it requirements but pop a 4kw unit in. 

 

 

[ReedRichards]

20 hours ago, Dunc said:

2. Can the ASHP be too big? i.e. supplier recommending 8.5kW pump for a load of 4.8 kW (or 1.7 if I'm right!)?

Yes it can, you need to look at the modulation range and how little power you might need.  If it's 1.7 kW, can your 8.5 kW heat pump even supply 1.7 kW continuously or would it have to supply, say, 3.4 kW with a 1:1 on/off duty cycle.  And that's when it's -5 out, what if it's 10 out, how little power will you need then?

 

20 hours ago, Dunc said:

All seem to be using -5C out/21C internal temps, flow temp 45C, return temp 5C

 

You can't want 45 C flow temp for UFH and the return temp can't possibly be less than your room temperature, can it?  Typically for a heat pump the return temperature is about 5 C less than the flow temperature.  So maybe you want 35 C flow, 30C return?

[Beelbeebub]

A decent UFH setup in a well insulated house needs flow temps in the low 30's or even high 20's.

 

A flow of 45C is way too high. 

 

Make sure they put the pipes in at a decent spacing and don't thry to cheap put by having 300mm or more spacing to allow for the low outputs needed. 

[Dunc]

Thanks for the thoughts, everyone. Very helpful!

Clarifications:

@ReedRichardsTypo (I can't edit the orignal post). Companies I've requested designs from all use flow 45C and return 40C, deltaT 5C.

@Chanmenie U values should be correct, MBC Timberframe "pasive" frame with blown cellulose fill...so yes, we should be on the way to passive levels for both insulation and airtightness. Form factor is reasonable but not perfect, 1.5 storey but with a lean-too entry/utility, and thermal bridging not perfect.

@BeelbeebubYes, we are far north: Inverness-shire. We can see snow on Ben Wyvis this morning 🙂.  Great point on future proofing the installation site!

 

I'm really curious about the design temps. Only one company will share the details prior to me coughing up significant cash. This one calculated transmission heat loss of 2.373 kW and ventillaiton loss of 1.506 kW for a total of 3.879 kW (26 W/m², 9 W/m³). I asked them to recalculate performance for flow 35C / return 30C and they sent the data below...seems to suggest that with a 26C temperature differential, a flow of 35C is not sufficient?

 

[JohnMo]

Couple of issues 

The W/m2 looks huge for your heat loss value. With MVHR your ventilation heat loss, drops to 100 to 150W. Not 1.5kW. So reality your W/m2 should be half they are quoting.

 

The dT quoted is rubbish, you would be in 3 to 5 range heating the floor direct from heat pump. The smaller the dT the higher the output for a given flow temperature. So would really be flowing 30 or less. It also makes the flow rates quoted as rubbish, the lower the dT the bigger the flow rate.

 

The other thing you need to look at is your house design temp, is -5 low enough, really depends where in Inverness-shire you are. I am in Elgin, we had a month worth of days that hit -9 last year and the year before.