Posted By: WillInAberdeenMHRV must comply with an obscure BRE guide from 1994, which suggests that (leakage+deliberate) ventilation should add up to 0.5-0.7ach, and that leakage will be 0.7ach for a typical house, or 0.2-0.35ach for a specially detailed house. So MHRV should be sized for 0.3ach or less, maybe 0, which is hardly prescriptive!

Are the leakage numbers there values for testing at 50 Pa or values for actual infiltration under 'ordinary' conditions? (However 'ordinary' conditions are defined)

A value of 0.2-0.35 ACH sounds like a plausible test value at 50 Pa for a PH (i.e. a specially detailed house). According to our PH certification PHPP we have a test result of 0.42 and a consequent infiltration of 0.033 ACH, i.e. a bit less than 8% of the test value. The effective ventilation rate will be 0.033 plus the MVHR rate 0.38 = 0.413 ACH so I can't see how the rules make sense taking either interpretation of the leakage number.

It's not great that the Scottish building standards require compliance with a 25-year old document which is not available as an electronic text. Athough some scans of original paper copies are available online, including in the BRE shop. It was written before passivhouses were a thing!

In short it suggests that 'natural infiltration rates [in the existing housing stock in 1994] as low as 0.2 ach are the exception rather than the rule; 0.7 ach is probably typical.... Generally a mean natural infiltration rate of 0.2 ach will have an air leakage rate at 50 Pascals of about 4 ach.'

4ach @50 Pa is roughly 10m³/h/m². For comparison the airtightness target in the 2020 building standards for setting TER is 7m³/m² @50Pa (=~3ach @50Pa) - so new buildings are expected to be detailed to be more airtight than was seen as exceptional for the average housing stock in 1994. Clearly you can aim for better than 7m³/m² @50Pa and that will help you beat the TER but require more artificial ventilation.

The other referred document in the building standards is CIBSE B2 which advises against sizing ventilation based on ACH figures at all, or by implication m³/h/m² or l/s/m² figures. So a bit of a tangle.

an02ew might have meant: "A dwelling is DEFINED as less airtight and can rely on natural ventilation at a designed air permeability of >5 m3/m2hr OR an as built >3 m3/m2hr."

What's also interesting is that the guidance doesn't seem to admit of the possibility of a passive ventilation system, although probably it could be proposed and supported by an expert advisor.

VA document

From the example calculation, looks like the 5m³/m²h figure is at test conditions, and is the same part L figure that an02ew quoted a few posts above, 29th Jan.

They actually subtract 0.04l/(s m³) which is a totally obscure unit but is equal to 0.15ACH.

(Edit : does that actually comply with the new building regs? Supply rates look too low?)

Scottish Standards are being updated now, and will look like the English ones

Posted By: djhThe document appears to behttp://www.vent-axia.com/files/catdownloads/part%20fand%20l%20brochure%20web.pdf" rel="nofollow" >http://www.vent-axia.com/files/catdownloads/part%20fand%20l%20brochure%20web.pdfand that refers to the 2010/13 version of the regs. So it's now pretty much out of date. I have no idea whether that matters. Sorry haven't looked any further.

There doesnt seem to be any more informative in the revised partF just advise to seek expert advise, which is a bit of a waste of time considering its a 2121 revised doc dealing with ventilation. Could it not decide an optimum AC/h rate from which it should be easy enough to work back from the AT results and calculate the air exchange rate for the MVHR instead of the arbitrary figure in the above documant.