Heat Loss Calculation for ASHP - Air Changes

[benben5555]

Hi, just got our heat loss calculation back for our ASHP. It details the room sizes, insulations levels, window sizes etc and finally the air changes per hour. All of the living rooms and bedrooms have been set to 0.5ACH and the kitchen and utility areas to 1.5ACH. Is this standard practice? We are aiming for a good air tightness (1.5-2ACH), but we will not be anywhere near 0.5ACH.

 

I saw something on Heat Geek saying that the minimum requirement for a new house is 0.5ACH, but surely the heat loss calculation cannot be based on this which would be better than Passivehouse standard.

[JohnMo]

MCS seem to make up the numbers and are are populated on their spreadsheet - one of the reasons they always oversize, on well built and well insulated houses.

 

But you have to careful that you are comparing apples with apples. There is a difference between building regs air tests, passivhaus air tests (done pressurised to 50Pa) and natural ventilation rate used for heat loss calculations. 

[benben5555]

I found this conversion table on an Australian website. If the heat loss calc is using natural ventilation rates, 0.5 is the equivalent of 10ACH which is crazy and must be really affecting the calculations. The bathrooms at utility were calculated at 1.5 which would be the equivalent of 30ACH!

 

[JohnMo]

As I said you need to compare apples with apples.  MCS spreadsheet just take the ACH and uses it in the ventilation heat loss calculation. 1.5 ACH assumes you have a vent fan in that room nothing more or less. The calculation shown is for the ventilation heat loss - cell G05

 

[JamesPa]

On 29/12/2023 at 22:23, benben5555 said:

Hi, just got our heat loss calculation back for our ASHP. It details the room sizes, insulations levels, window sizes etc and finally the air changes per hour. All of the living rooms and bedrooms have been set to 0.5ACH and the kitchen and utility areas to 1.5ACH. Is this standard practice? We are aiming for a good air tightness (1.5-2ACH), but we will not be anywhere near 0.5ACH.

 

I saw something on Heat Geek saying that the minimum requirement for a new house is 0.5ACH, but surely the heat loss calculation cannot be based on this which would be better than Passivehouse standard.

For what it's worth the actual measured heat loss in my 1930s house (albeit with some later extensions) can only be explained by assuming 0.5ach, which is frustrating because most of the MCS brigade insist on using 2-3.  Double glazing without trickle vents, internal wall insulation, blocked chimneys and solid flooring downstairs is presumably sufficient to reduce the natural ventilation to this level.  There are a couple of odd corners where mould is a problem (and where I plan to fit some extract), but by and large it's OK.