Jump to content

Fabric and ventilation heat loss calculator

Jeremy Harris

Recommended Posts

On 17/01/2022 at 16:40, SteamyTea said:

If your house is that leaky, no need to put the MVHR in.


I would have cast that slightly differently.  If you are truly losing 4 ACHP with no heat recovery, then this will be by far the largest source of heat loss.  Think of your house as a system: it has to be in balance.  There no point in having fantastic windows, etc., to keep the heat in whilst at the same time letting it escape through every crack in the house if there is the slightest wind.


I have a passive-class house which is maybe 40% larger than yours, and our energy expenditure is just over 11 MWh p.a., (This is all electric, of which about 60% is resistive space heating – I can't make a case for installing an ASHP even though we've allowed for one in our initial  design).  Even with ~ 90% heat recovery, our MVHR circulation accounts for about a  ⅓ of our heat losses. 


I would suggest as an option that you consider retaining the MVHR, but look at what mitigation you can do to get your ACHP under 1.  You will find this will get you a more balanced system, and a more comfortable house as well as lower bills.


BTW, in my case my FP contract with OVO will more than double in October which tilts the cost benefit case and so we may be installing an ASHP over summer. ?

Edited by TerryE
Thanks Nick :-)
Link to comment
Share on other sites

12 minutes ago, TerryE said:




I was under 4 MWh last year (again).  When I moved here, 17 years ago, and even have a keen interest in energy usage, I was using 11 MWh.year-1.

I still use resistance heating (no gas), have original timber windows and back door, though re-glazed to a 16mm gap, rather than a 4mm gap.  Added some insulation to the loft, fixed easy to find leaks (got one I just cannot seem to cure), the rest of the savings have been better management of my usage.

  • Haha 1
Link to comment
Share on other sites

Yup Nick, however we've got a largish detached 4 double bedroom + ward storage / drying room which we keep at ~ 22½ °C all day, all  year -- this may seem profligate, but we're getting old now and I just like being able to walk around in bare feet and shorts when I want without feeling cold; my wife and son ditto.


As we discussed on the forum previously, if you don't have fundamental design flaws in your build (unsealed air-gaps, missing insulation, cold bridges, etc.) then the macro heat-loss is pretty straightforward: just the sum-product of area × U-value × ΔT for external surfaces + the air heating of cold replacement air -- which is basically what JSHs spreadsheet represents.  The overall heat-loss is driven by the cost trade-offs on the coefficients in this calculation that you can control.  


As  you and others have shown, it is quite possible to significantly reduce heat losses by retrofit, but in my experience this is a lot easier to achieve in a new self build where the owner has far more control over the design itself and can monitor build quality.  In our case the extras incurred for going to passive standard were pretty much offset by savings elsewhere (e.g. no CH system other than the in-slab UFH loops -- my entire heating system cost about £2½K and is pretty much maintenance-free).


Even though 90+% of our electric heating is done at off-peak rate E7 tariff, with the expected tariff increases when our fixed price contract finishes, the ~4-5 MWh that I could save by installing an ASHP might just about cover its investment cost so long as I do the install myself. 


As Joe said on another post, IMO dot&dab boarding out counts as a major design flaw, as this is rarely done as the videos on YouTube demonstrate, and too many internal thermal images of such external walls show that these voids often act as a large plate heat-exchangers dumping heat into void behind the plasterboard where the external draft air circulating here carries it out as convective losses.  If you must use a conventional blockwork + insulation + external brick skin, then going for wet plaster on block if a far safer option, as well as making sure that all interfaces are properly taped during 1st fix.

  • Like 2
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...