Fabric and ventilation heat loss calculator

[SBMS]

4 hours ago, JohnMo said:

You still have to careful your not just wasting £300, with sh!te in sh!te out. 

 

I would get UFH in, which I think is done, make provision for ducts for pipes and cables through wall. Seal them up until after air test. Get house air test done, get MVHR in and commissioned, then if going down MCS route go get the survey done. Without all that in place there is a good chance it will be treated as default values for ventilation heat loss.

 

Or just don't bother MCS at all, spend £2500 on a suitable sized heat pump, buy a 200L direct cylinder - install. Power cable and communication cable to outside unit and two pipes to UFH manifold. Then claim VAT back as it's a new house. Little over £3k in materials claim around £600 vat back, so £2400 plus a little labour.

Agreed, you need the input data (fabric data etc) to be correct - but the same is said for Jeremy's tool. From what I've seen the modelling software for MCS new versions is more accurate as it accounts for PSI values.  The newer MCS standard also allows specifically for an Air permeability input (the below is Heatpunk's):
 

 

There's a good guide that explains this: https://help.heatpunk.co.uk/books/heat-pump-task-and-system-performance/page/ventilation-calculations-in-heatpunk

[JohnMo]

30 minutes ago, SBMS said:

There's a good guide that explains

But it still takes no credit for MVHR so as pretty rubbish, for anything better than an air test score of 3, which then has MVHR.

[Great_scot_selfbuild]

On 07/01/2026 at 11:19, JohnMo said:

So parts list gets super simple. UFH and manifold, couple of ball valves for manifold isolation, 2x anti freeze valves, strainer/ball valve at ASHP (required for warranty) plus an isolation valve on flow out from heat pump. Fill system with inhibited (corrosion and biocide inhibitors - not antifreeze or glycol). Expansion vessel and relief valves are in monobloc.

 

A direct cylinder (half the price of a heat pump cylinder).

 

I paid £2500 including vat for Hiaer monobloc, you can claim the vat back on a new build.

@JohnMo your reply her (along with others) has prompted me to spend quite a bit of time over the last week doing some reviewing of previous quotes, get a couple of them updated, as well as investigate sourcing the ASHP and DHW myself without the grant and have a plumber install it.

 

The majority of them quote between £12 - £14,500 before the grant, resulting in near to £5k after the grant.

 

I've a quote for the supply and commissioning from an MCS supplier for ~£5,500 before grant and so the only cost would be our install cost (the quotes typically have the install broken down as ~£2,500 - £3,500).

 

What do you think of this Cool energy HP? (this would 'only' cost us the install costs). To me, the numbers appear to suggest it modulates up a huge amount (9kW), but not down very well; I'm still on a steep learning curve with all this.

 

I'm attracted by having an installer who is familiar with the make/model of the HP as so many threads are associated with poorly set up HPs. I looked into the Haier one - my main reticence with the less common makes, or going it on my own it the support for them afterwards. 

 

Thanks

[JohnMo]

29 minutes ago, Great_scot_selfbuild said:

What do you think of this Cool energy HP? (this would 'only' cost us the install costs). To me, the numbers appear to suggest it modulates up a huge amount (9kW), but not down very well; I'm still on a steep learning curve with all this.

Long reply

Sorry it's huge. You need good modulation at the 7 degrees point. But it only modulates down to 4.32kW at 7 degs. Our winter average temp is around 7 degs so your heat pump spends most it's time in that range of output. Your heat demand at design point is lower than that. But their next size down, in real terms, is too small for your demand.

 

So you will get cycling and have a hard time setting it up. Your scop is likely to be high 3s. But if you get a more appropriate sized heat pump your SCoP should be low 5s to mid 5s.

 

My current heat pump is 6kW, but since purchasing I have found it's actually a software limited 10kW, modulation is similar to the Cool Energy unit. I have got it running acceptably, but it's just took big a mild temps. So that has a few impacts - Your target flow temperature is higher to make up for cycling. It was 7 degs yesterday target flow temp was 29 degs, cycling was 40 mins on 90 mins off. So I got a cop associated with a 29 Deg flow temp, CoP yesterday was 4.6.

 

However is it didn't cycle and ran steady state, flow temp target would only need to be 23 degs and CoP would be closer to 6.3. A saving of 2.3kWh as a good estimate on one day.

[jack]

For those using SketchUp, here's a potential alternative way of working out heatloss, based on SAP principles:

 

https://forum.buildhub.org.uk/topic/46635-preliminary-sap-and-heat-loss-tool-for-sketchup/