Heat Loss Calculation

[Crofter]

As longer term users of the forum will be aware, I'm very much inclined to do things myself. I designed and built a small house from the ground up with absolutely minimal outside help. Partly it's because I've struggled to get trades in the past, and partly it's because I'm a cheapskate.

 

Anyway, next project is ASHP for my 70s bungalow. Almost certainly going to go A2A. I've been leaning towards a ducted system but may go with multi split.

 

Obviously a heat loss calculation is required. We just had a new EPC done and I was a bit surprised how superficial it was.

 

If I get a pro to do my HLC, how accurate is it really going to be?

Can I attempt this myself?

So far I've just run numbers through a 'radsizer' online tool from a radiator supplier, and checked the Heatgeeks' rules of thumb, both of which return a value of around 5.5kw. There are plenty of assumptions in there. In practice I'm going to be installing a 5, 6, or 7kw system, so does it really make all that much difference trying to nail this down perfectly?

[MikeSharp01]

You can probably do it yourself with the JS Harris spreadsheet.

[JohnMo]

52 minutes ago, Crofter said:

radiator supplier, and checked the Heatgeeks' rules of thumb, both of which return a value of around 5.5kw.

Rules of thumb are generally pants, and the radiator suppliers not much better, as they are just built on rules of thumb also. As mentioned above there is a spreadsheet on here. Before I used the one here I did my own long hand calcs, both gave similar outcomes.

[LiamJones]

+1 on the JS Harris spreadsheet! Very similar results to PHPP and far less complexity!

[Crofter]

Ok thanks for the reminder about Jeremy's spreadsheet.

Air tightness clearly has an enormous effect. Unfortunately I have very little idea of just how bad it is in this house.

 

It's a 1970s l bungalow, twin block walls with cavities filled. New 2G/3G windows and doors throughout. Floor is currently pretty draughty (T&G pine boards, suspended). I'm going to improve that as much as I can (probably insulation with a breather membrane stapled underneath). Loft has several layers of glasswool everywhere, must be about 300mm.

 

Any ideas for a number to pull out of the air for ACH for a house like this?

[Alan Ambrose]

>>> Unfortunately I have very little idea of just how bad it is in this house.

 

You can get it measured if you like. You can probably even diy it if it's for your own use.

 

https://en.wikipedia.org/wiki/Air_changes_per_hour about 3/4 of the way down.

 

[Crofter]

I could do, but I'm on the other side of the world right now and trying to get ballpark figures.

[Iceverge]

Start with an ACH of 7. 

 

Probably realistic if you haven't undertaken any remedial work. 

[Crofter]

Ouch!

Is 7 really a realistic value?

 

Another thought- I presume I can work backwards from known energy use and use that as another data point?

[JohnMo]

Trouble with a leakage rate done at 50Pa, that's basically the same as gale on all 4 sides of the building at all times, which isn't real.

 

From a SAP report a leakage rate of 7 at 50Pa, is an infiltration rate of 0.4217. So use that, plus any forced ventilation and MVHR efficiency of 0 if you don't have that.

Edited October 18, 2023 by JohnMo

[MikeSharp01]

Anybody looking for the JSHarris spreadsheet can find it HERE: https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=18333

 

 

[Alan Ambrose]

>>> I presume I can work backwards from known energy use and use that as another data point?

 

With some approximations and some hand waving I think you should be able to.

[MikeSharp01]

3 hours ago, Alan Ambrose said:

With some approximations and some hand waving I think you should be able to.

As Alan says, and don't forget that as hand waving generates heat you should adjust for that in your calculations NOT