Non PassivHaus new homes - Space heating requirements?

[IanR]

On 14/12/2022 at 09:00, anonymous said:

as it should be possible for me to go from an air tightness of 7.6 to 5 quite easily, it not below that (Which is obviously what I'd want, cost permitting) 

 

On 14/12/2022 at 11:20, anonymous said:

Tbh I'm looking to use PassivHaus principles as much as possible~~~~

 

In that sense, air tightness and an MVHR seem key at the very least 

 

MVHR is key to PassivHaus principles due to the Max allowable air leakage rate of 0.6 ACH @ 50 Pa. The principle is to reduce energy loss via air leakage to the minimum reasonably achievable. A consequence of that is that MVHR is then required to keep the air in the house "healthy".

As a rough approximation 0.6 ACH is typically around 1m³/h.m²@50Pa

 

For English Building Regs, houses with an infiltration rate greater than 3m³/h.m²@50Pa do not require whole house mechanical ventilation as it's considered there's sufficient natural air leakage to keep a healthy air environment within the property.

If you are above 3m³/h.m²@50Pa infiltration rate then increasing this further with an MVHR system will cause additional energy losses via the additional air changes caused by the MVHR, although +80% of the energy should be recovered at the expense of the electricity running the fans.

 

MVHR is typically an expensive retrofit, and unless you plan to target sub a 3m³/h.m²@50Pa infiltration rate it will never pay for itself.

Unless you plan to achieve PH targets, a more cost effective plan would be to achieve a >3m³/h.m²@50Pa, not install MVHR and use room extractors, perhaps with heat recovery, in wet rooms. Although I personally would want adjustable trickle vents in my windows to provide some level of additional, controllable ventilation if required.

For info, I have a Airflow MVHR system, and the installation was designed by Airflow themselves. I remember one of their parameters being a Maximum infiltration rate of 5m³/h.m²@50Pa otherwise they wouldn't supply their MVHR system, with a recommendation of below 3m³/h.m²@50Pa.

Edited December 23, 2022 by IanR

[ProDave]

4 minutes ago, IanR said:

As a rough approximation 0.6 ACH is typically around 1m³/h.m²@50Pa

Is there an easy way to convert?

 

So my 1.4m³/h.m²@50Pa air test result may not be as bad as I thought,  so might only be about 0.85 ACH?

[SteamyTea]

24 minutes ago, ProDave said:

Is there an easy way to convert?

 

So my 1.4m³/h.m²@50Pa air test result may not be as bad as I thought,  so might only be about 0.85 ACH?

Easy enough to work out.

Calculate the (leaky) area and volume.

Divide volume, m³, by the product of the m³.h^-1.m^-2 and the area, m².

 

 

The thing that stops it being easy is that the leaky area and volumes of houses is different.

So taking my house, which is solid floored and terraced and just comparing it to the end of terrace net door, for the same m³.h^-1.m^-2 @50Pa will give different numbers.

 

Knocking up a spreadsheet quickly and it shows that at 1.5 m³.h^-1.m^-2 @50Pa, my house would have an ACH figure of 1.5 and my neighbours at 1 ACH.

So care has to be taken if the m³.h^-1.m^-2 is used on its own and in isolation.  Bit like U-Value for windows or walls.  Great to have a low window U-Value, unless you cover all the walls in glass.

[IanR]

46 minutes ago, ProDave said:

Is there an easy way to convert?

 

So my 1.4m³/h.m²@50Pa air test result may not be as bad as I thought,  so might only be about 0.85 ACH?

 

As @SteamyTea says, it needs to be worked out for each house unfortunately.

 

ACH is based upon the volume within the thermal envelope and the m³/h.m²@50Pa figure is based upon the gross inner surface area of the thermal envelope, ie. floor, walls and ceiling/roof.