Building Pressurization 1

[engggu]

Hi experts,

For an office building, do we have a rule of thumb or recommended percentage for the pressurization? Say outdoor air 5% higher than the exhaust?

thank you

 

[EnergyProfessional]

That depends on the leakage. And also where you pressurize within the building. Note the ERV can freeze when you have too high OA intake compared to EA (depending on climate, of course).

Ideally you get a tight and well insulated building and have neutral pressure. If you have too large pressure differences, you also can get problems with doors.

So no, a rule of thumb won't work well.

 

[GT-EGR]

A value like that is useful to plan some extra outside air capacity. But it really all depends on what your control method is, how tall your building is, and how extreme your summer and winter temperatures are.

I’ve never seen an all encompassing approach because building pressure targets are seasonal and also subject to building stack effect, and also depends on how compartmentalized all your spaces are. To really engineer it I would say you target your main envelope areas with cracks like the lobbies and areas with lots of glazing, then you do a crack area calculation on how much air to create the delta P between inside and outside that you want in that area. Compare that to your 5% and you’ll see how much it makes sense.

To me this is an elusive value that you’ll get 100 different answers for. And keep in mind you may want to differ your pressure target from positive in summer to neutral/negative in the winter, and should even compensate from the seasonal stack effect of floors above/below depending on where your sensor is if you have just 1 sensor/point of control.

 

[engggu]

Thank you very much experts

 

[EnergyProfessional]

To start, you need to equalize the outside pressure over all 4 sides to eliminate wind-effect. I never see that done in practice. Next you need to find an indoor location that isn't pressurized or de=pressurized by the HVAC system. i also never see that happening. So you basically are trying ot control pressure and you have inaccurate pressure values this is based on.