Heat pump calculation load

[SBMS]

We have had our SAP rating done. It’s come out with a primary energy rate of around 23kWh per sqm.  One of the ASHP suppliers has come back and intimated we might need a 1 x 12kW & 1 x 7kW system. They’ve calculated a 35 W/m2 requirement - so 16kW (over 450sqm - this includes a potential future extension). This has been done based on the worst case -2 outside +20 inside. 
 

We are trying really hard for good u values - 0.14 walls, 0.1 Roof and floor etc. 

 

Does this calculation sound correct? I appreciate this is the ‘worst case’ so the average will be better but can anyone comment on this W/m2 calculation? Is it our floor area that’s pushing us to two units?

[JohnMo]

Form factor affects heat loss, so how much you have of the inside exposed to outside walls. The closer to a square box you get, the better for heat loss, everything else being equal.

 

Next is ventilation heat loss, with MVHR you basically half your heat losses.

 

You don't give enough information to make a call.

 

We are just under 200m², with MVHR as bad a form factor as you can get, long, thin, single storey all vaulted ceilings. Our heat loss is about 3kW at -9 outside. So double that you are at 6kW, if you don't have MVHR you are getting close to 12kW worst case. 2 storey, MVHR you could be closer to 4kW at -2.

 

10 minutes ago, SBMS said:

primary energy rate of around 23kWh per sqm.

Meaningless figure for heating system design.

 

You will have a figure in the SAP that states the heat demand for each month in Watts. Use the January one, that maybe close to your heat loss.  This my figure - heat loss rate W. Look in the same place in your report.

 

 

You need a proper heat loss calculation, nothing else is really good enough. Too many variables to make a guess.

 

12 minutes ago, SBMS said:

includes a potential future extension

If it's a definite, 100% going to get done size for it, if it's really a maybe don't, you will end up with an oversized heat pump.

[sharpener]

16 minutes ago, SBMS said:

They’ve calculated a 35 W/m2 requirement - so 16kW (over 450sqm - this includes a potential future extension).

 

35W/sq m should be quite achievable for a new build. Difficult to say any more without detailed knowledge of type of building & its construction. You might think about installing the 12kW first, configured for adding the 7kW later, but wait until you build the extension. Depending on the usage pattern and as-built thermal performance it might turn out you do not need it.

[SBMS]

2 minutes ago, JohnMo said:

Form factor affects heat loss, so how much you have of the inside exposed to outside walls. The closer to a square box you get, the better for heat loss, everything else being equal.

 

Next is ventilation heat loss, with MVHR you basically half your heat losses.

 

You don't give enough information to make a call.

 

We are just under 200m², with MVHR as bad a form factor as you can get, long, thin, single storey all vaulted ceilings. Our heat loss is about 3kW at -9 outside. So double that you are at 6kW, if you don't have MVHR you are getting close to 12kW worst case. 2 storey, MVHR you could be closer to 4kW at -2.

 

Meaningless figure for heating system design.

 

You will have a figure in the SAP that states the heat demand for each month in Watts. Use the January one, that maybe close to your heat loss.  This my figure - heat loss rate W. Look in the same place in your report.

 

 

You need a proper heat loss calculation, nothing else is really good enough. Too many variables to make a guess.

 

If it's a definite, 100% going to get done size for it, if it's really a maybe don't, you will end up with an oversized heat pump.

Thanks @JohnMo.  Yes MVHR. Have found the heat loss in my sap for January:

 

 

That figure is what’s confusing me as it seems the supplier has come up with a much higher figure. We’ve got a fairly standard form factor…

[SBMS]

3 minutes ago, sharpener said:

 

35W/sq m should be quite achievable for a new build. Difficult to say any more without detailed knowledge of type of building & its construction. You might think about installing the 12kW first, configured for adding the 7kW later, but wait until you build the extension. Depending on the usage pattern and as-built thermal performance it might turn out you do not need it.

One thing with this.. I might be misunderstanding but am wondering if we need UFH upstairs (we have a first floor and rooms in roof as well - So UFH on both floors). This adds quite a bit of cost. But if I wanted to minimise radiators upstairs I would need to work on a certain value for sizing the wet UFH loops in the slab to ensure it provided the right amount of output?

[SBMS]

9 minutes ago, sharpener said:

35W/sq m should be quite achievable for a new build

is 35w/sq m good? Average? 

 

It’s masonry 200mm cavity with eco bead. 
 

304mm posi rafters with blown in cellulose and external 22mm wood fibre sarking. 
 

175mm PIR in the floor. 
 

Glazing averaging around 0.85 but a fair amount of it to be fair. 
 

aiming for 1.0 on airtightness. House footprint is 190 sqm PA ratio of 0.35. 

Edited November 22 by SBMS

[JohnMo]

They (MCS installers) will exclude MVHR and your airtightness, they use a standard set of air changes per hour based on a leaky house. If you provide certificates for air test and MVHR install, they should accept your figures and calculate correctly (should but may not). I just didn't bother with grant, glad I didn't. Really you are looking at a 6kW unit, that will leave of headroom for DHW heating.

 

I like everything here about Panasonic units, so that would be my first port of call.

[SBMS]

6 minutes ago, JohnMo said:

They (MCS installers) will exclude MVHR and your airtightness, they use a standard set of air changes per hour based on a leaky house. If you provide certificates for air test and MVHR install, they should accept your figures and calculate correctly (should but may not). I just didn't bother with grant, glad I didn't. Really you are looking at a 6kW unit, that will leave of headroom for DHW heating.

 

I like everything here about Panasonic units, so that would be my first port of call.

Sorry @JohnMo you’re suggesting I need a 6kw unit not a 16kw?

 

am I right in thinking that January heat loss rate figure of around 4kw is a peak figure based on -9 outside? So a 6kw unit would comfortably cope?

 

sorry @SteamyTea for miscapitalisation of the Ks and the Ws on my phone typing 

[JamesPa]

24 minutes ago, SBMS said:

Sorry @JohnMo you’re suggesting I need a 6kw unit not a 16kw?

 

am I right in thinking that January heat loss rate figure of around 4kw is a peak figure based on -9 outside? So a 6kw unit would comfortably cope?

 

sorry @SteamyTea for miscapitalisation of the Ks and the Ws on my phone typing 

If your installer wont accept the figures, find another installer.  It appears that there are now some MCS installers out there who will do sensible things with heat loss, whilst still conforming to the letter of the MCS requirement. 

If a 6kW unit is what you need, you really don't want a 16kW one and even less do you want two units with the (almost) inevitable complex (and very probably highly inefficient) plumbing system that will accompany this arrangement.

Edited November 22 by JamesPa

[SBMS]

15 minutes ago, JamesPa said:

If your installer wont accept the figures, find another installer.  It appears that there are now some MCS installers out there who will do sensible things with heat loss, whilst still conforming to the letter of the MCS requirement. 

If a 6kW unit is what you need, you really don't want a 16kW one and even less do you want two units with the (almost) inevitable complex (and very probably highly inefficient) plumbing system that will accompany this arrangement.

I think that’s what I’m trying to work out - what size do I need? Is the SAP calculation accurate enough to size the heating or does an MCS installer (which they are) produce a more accurate figure?

[JohnMo]

Do your own heat loss calculation. You don't need to do room by room just the overall house. Actually calculate for own education and comfort that you know you are doing the right thing.

 

Yes from the info presented I am saying 6kW not 16kW.

 

Here a spreadsheet I did for mine. Was borrowed from Boffins Corner. Plus in your numbers and see what pops out.

 

 

Heat loss calculator - Sleepieshill house.xlsx

[SBMS]

1 hour ago, JohnMo said:

Do your own heat loss calculation. You don't need to do room by room just the overall house. Actually calculate for own education and comfort that you know you are doing the right thing.

 

Yes from the info presented I am saying 6kW not 16kW.

 

Here a spreadsheet I did for mine. Was borrowed from Boffins Corner. Plus in your numbers and see what pops out.

 

 

Heat loss calculator - Sleepieshill house.xlsx 32.02 kB · 6 downloads

Thanks @JohnMo - I have done the heat loss calculations. I might be missing it but where in the spreadsheet does it show a ‘peak energy input’ requirement that would help me size the heat pump? I can only see total monthly losses. 

[JohnMo]

1 hour ago, SBMS said:

show a ‘peak energy input’

Cell B39

[SBMS]

22 minutes ago, JohnMo said:

Cell B39

Thanks @JohnMo - found that just before your reply 👍

 

Think the current supplier is just going off a stock standard insulated house.  They've calculated:

I get 4.5kW - although what would be a sensible amount to include for DHW in worst case?

[SBMS]

I think I've worked out probably what the supplier is specifying.  If I change the Air changes figure from 0.5 to, say 5, the energy requirement jumps to 12kW which is probably what they are basing it off.

 

Can I ask - the figure that is on the SAP which is the "Measured/design AP50" (air permeability) - is this the same as the 'Air changes per hour' figure on Jeremy's spreadsheet? If not, what is the difference? We are aiming for 0.5 to 1.0 on the air pressure test - is this the same as air changes or are they different? If different, what's the link between the air pressure test figure and the 'air changes per hour' figure in the heat loss calculator?

[JohnMo]

10 minutes ago, SBMS said:

Measured/design AP50" (air permeability) - is this the same as the 'Air changes per hour' figure on Jeremy's spreadsheet?

No they are different things. The figure on the spreadsheet is ventilation rate via the MVHR.  There is an additional not heat recovered leakage rate, this will also be in the SAP report near the airtightness score, it's about a 1/10th of the AP50 figure

[SBMS]

6 minutes ago, JohnMo said:

No they are different things. The figure on the spreadsheet is ventilation rate via the MVHR.  There is an additional not heat recovered leakage rate, this will also be in the SAP report near the airtightness score, it's about a 1/10th of the AP50 figure

I think I see now that the AP50 designed as per my sap is 1.0. Does jeremys calculator capture the air tightness of the building? I understand the MVHR ventilation rate would assume a perfectly sealed controlled house. But if say, there’s additional ventilation loss (say another 0.5ACH) how does it account for that and where would it be entered?

 

Im worried I’ve misunderstood this part of the heat loss calculator. 
 

If my score for AP50 was 1.0 And my surface area of the house is 450m2. Then if the volume of the house is 1200m3 then the ACH50 would be circa 0.375. If my MVHR was ventilating at a rate of 0.5ACH then does this mean my total air change figure becomes 0.875??

 

How does the heat loss calculator account for uncontrolled ventilation loss based on airtightness in addition to controlled ventilation from MVHR?

[JohnMo]

10 minutes ago, SBMS said:

Does jeremys calculator capture the air tightness of the building?

No, but on a good airtightness house it is very small.

 

If you want to calculate 

If the volume of a house is V m3, and the air change rate is n ACH (Infiltration rate) then the total amount of air passing through it per hour will be n × V m3. This air needs to be heated up through the temperature difference ΔT between the external temperature and the internal temperature. The energy required to raise one cubic metre of air through one kelvin is 0.33 watt-hours, i.e. its heat capacity per cubic metre is 0.33 Wh m–3 K−1. Thus the total ventilation heat loss, Qv , will be:

 

Qv = 0.33 × n × V × ΔT watts

 

Do not use the AP50 figure in this calculation, replace n for Infiltration rate in section 2 of the SAP.

[JohnMo]

1 hour ago, SBMS said:

although what would be a sensible amount to include for DHW in worst case

I use a simple assumption 

 

You allow two hrs for DHW. So on the coldest day you need 4.5kW for 24 hrs, so 108kWh. To do this in 22 hrs you need 4.9kW of energy. So you need at heat pump that can put out 4.9kW at your lowest design temp.

[SBMS]

50 minutes ago, JohnMo said:

No, but on a good airtightness house it is very small.

 

If you want to calculate 

If the volume of a house is V m3, and the air change rate is n ACH (Infiltration rate) then the total amount of air passing through it per hour will be n × V m3. This air needs to be heated up through the temperature difference ΔT between the external temperature and the internal temperature. The energy required to raise one cubic metre of air through one kelvin is 0.33 watt-hours, i.e. its heat capacity per cubic metre is 0.33 Wh m–3 K−1. Thus the total ventilation heat loss, Qv , will be:

 

Qv = 0.33 × n × V × ΔT watts

 

Do not use the AP50 figure in this calculation, replace n for Infiltration rate in section 2 of the SAP.

Thanks @JohnMo  this figure can’t be entered into Jeremy’s calculator though?

[SBMS]

1 hour ago, JohnMo said:

No, but on a good airtightness house it is very small.

 

If you want to calculate 

If the volume of a house is V m3, and the air change rate is n ACH (Infiltration rate) then the total amount of air passing through it per hour will be n × V m3. This air needs to be heated up through the temperature difference ΔT between the external temperature and the internal temperature. The energy required to raise one cubic metre of air through one kelvin is 0.33 watt-hours, i.e. its heat capacity per cubic metre is 0.33 Wh m–3 K−1. Thus the total ventilation heat loss, Qv , will be:

 

Qv = 0.33 × n × V × ΔT watts

 

Do not use the AP50 figure in this calculation, replace n for Infiltration rate in section 2 of the SAP.

Just trying to work out the correlation between the air permeability (AP50) and what n should be? Im Assuming that ‘n’ is not 1.0 but then I don’t really understand how to derive the infiltration rate from the air permeability AP50 figure?

 

the SAP does have a figure for infiltration rate of 0.05 but I don’t know how this is derived?

[JohnMo]

1 minute ago, SBMS said:

infiltration rate of 0.05

That's the figure to use, base on site exposure prevailing winds and exposed area. So number of Watts used will be very small.

[SBMS]

10 minutes ago, JohnMo said:

That's the figure to use, base on site exposure prevailing winds and exposed area. So number of Watts used will be very small.

I think I get it can I check? 
 

Given an air permeability AP50 of 1.0, the ACH is 1/20 of this so 0.05. Fed into the calculation above this would mean at delta t of 29 degrees for a 1200 cubic vol house the heat loss would be 574 watts (so this would be the added load on the ASHP?)

 

Also - when passivhaus talks about the ‘maximum air permeability is 0.6ACH 50’ - am I right in thinking this isn’t the same as the air permeability figure seen on the SAP or done on the blower test (AP50) - although they frequently seem to be very similar numbers?

 

 

Edited November 23 by SBMS

[JohnMo]

4 minutes ago, SBMS said:

added load on the ASHP?)

Yes

 

4 minutes ago, SBMS said:

0.6ACH 50’ - am I right in thinking this isn’t the same as the air permeability figure seen on the SAP or done on the blower test (AP50)

Correct they are different, PH is ACH, building regs is m³ per m². Can be very different numbers.

[SBMS]

58 minutes ago, JohnMo said:

Yes

 

Correct they are different, PH is ACH, building regs is m³ per m². Can be very different numbers.

Got it.  What would a good permeability (AP50) value be?  I am conscious that that it's not part of Jeremy's spreadsheet.. Those who aren't targeting good levels of airtightness could have their assumptions thrown.  For example, if I target an AP50 of 1.0, which for my dimensions equates to 0.58ACH50 this is 574 watts of additional heat required from the ASHP.

 

If I changed the AP50 to 5, then the heat loss is nearer to 2.7kW. Be good if the calculator allowed this as an input?

 

I assume that MCS when doing their sizing calculations should take into account the AP50 (or ACH50) targets to generate an accurate figure?

 

FYI - I found this diagram useful for other readers to calculate between ACH50 and AP50:

 

From https://thefifthestate.com.au/columns/spinifex/why-we-should-change-our-building-air-tightness-metrics/#:~:text=It's really a very similar,m3) of a house..