Hi

Does a very averagely sealed house really leak air at a rate of 105 air changes per hour? Or is 5.25ACH more likely?

105 ACH comes from my calculations thus:

The number entered into SAP for air tightness is usually 3-8. If you go for 5, you get the following answer:

SAP multiplies its air tightness figure by 20 to get Q50. Q50 is m3 of air change per m2 of building envelope area per hour. (So Q50 is divided by 20 for SAP entry).

Now then.

If you have a simple house with ground floor area of 5m x 8m which is 5m high, this means that you have a volume of 200m3 and an envelope area of 210m2. If you enter into SAP a pressure test of 5, this means that Q50 = 100 (multiply 5 by 20 = 100).

Therefore SAP pressure test of 5 is 100m3 of air change per m2 of envelope area, which in this example gives the sum 210(m2 of envelope area) x 100(m3 - q50) = 21,000m3/hour. Divide this by the house volume (200m3) and you have an ACH of 105.

I don't know much about this but 105 air changes an hour seems very high. If I don't multiply the SAP figure by 20, the result is 5.25ACH which feels more realistic.

Is this right or have I made this up?

The wisdom of the collective Brain would be much appreciated.

Ta

Posted By: delboySAP multiplies its air tightness figure by 20 to get Q50.

Testing is normally done @50Pascals - to change the rates from normal pressure to the 50Pa or vice versa pressure means multiplying or dividing by 20 (ish - some people may argue about the exact multiplier but this one seems to be the one that's used most often). I suspect your figures were already @50Pa so you've made them 20x too large. 5-ish ACH@50Pa sounds about the right ballpark - not a particularly good figure, but better than the UK average. You need to get under 3 (or ideally 1.5) before a HRV makes any economic sense.

Posted By: CWattersAren't pressure tests done with deliberate ventilation blocked? Hopefully your deliberate ventilation (trickle vents or MHRV) will totally dominate your accidental ventilation.

Doubtful - a few square cm of open HRV vents would be much smaller than the typical effective leakage area of most buildings. Of course, it's better to block them up so that you're actually measuring the building itself. A de-pressurizing test also has the advantage of automatically sealing any vents that have flaps on them that open automatically on usage.

Paul in Montreal

Q50 - air tightness and n50 ac/hr are "deemed" interchangeable for SAP .

Q50 is a function of the enclosing surface area and n50 is a function of volume . The enclosing surface area and the volume of most domestic buildings is numerically similar - it is in the example you posted above .

The "50" in both cases refers to the 50 paschalls pressure of the air tightness test . Building regulations require air tightness in the form of Q50 - max permissible 10m3/m2/hr . Divide by 20 to get ac/hr - at normal pressure

Paul must be correct - you are starting with a Q50 figure 5.25 - equating to ac/hr 0.265 at normal pressure

105 ac/hr would mean that the roof and windows are not in place yet .....