Measuring air change rates with CO2 sensors for heat pump heat loss calculation


Richard Jack of BuiltTestUK spotted an interesting aspect that I missed about the new standard that it seems to effectively halve the ventilation heat loss at the point when you add up all the rooms to calculate the zone or building total… it’s a bit strange as it suggests that you need to size the radiators for the room values but the whole house requirement would then be that much lower…

The factor that halves this is the f.i-z factor in equation 15, or the selection of qv.leak instead of qv.env.i (whichever is largest in that max selection). This same factor is present in the simplified method.

Here’s an example of what this looks like in the supplementary simplified calculation example available in EN12831-2:

As you can see they did indeed halve the individual room heat losses here to give the zone and build totals!!

A possible reason for this factor that halves the vent heat load as you add up all the rooms is that there’s an orientation factor earlier in the standard calculation that doubles the infiltration air volume rate and perhaps this is there to account for wind load on one side of a building that results in higher heat requirements in those rooms? Perhaps this later halving is then to average it all out for the whole building?

Strangely in the simplified example given it effectively halves the air change rate from an n.min of 0.5 to 0.25 for the whole building, which is then really a bit low for air quality… and certainly below part F regulation requirements for the whole building… but then it is just an illustrative example.

Correction to my post above: After reading through the standard more thoroughly I realised I was wrong in my assessment that it suggests to use the simplified method for natural ventilation, it actually says that the standard calculation method is suitable for all situations and that the simplified method is only suitable for air-tight buildings without forced ventilation. The earlier 2003 standard for the n50 blower test conversion has also just been changed to a different formulation starting with the permeability test result instead.

Example spreadsheet calculation using the EN12831-1:2017 standard method:

Here’s my attempt so far to implement the standard calculation method:

EN12831_2017_ventilation_calculation.ods (20.4 KB)

So this does perhaps give a calculation basis for lower whole building ventilation heat loss… E.g the result for our house (whole house) with part F continuous extraction levels + natural ventilation is 0.6 ACH! But individual rooms are calculated to double that ~1.1 ACH…

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