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Purpose of the work
How long do you have to wait after having adjusted the fan during an air permeability measurement until the pressure difference has practically reached its final value? Which parameters determine the required waiting time?
Method of approach
The pressure difference on the building envelope as a function of time can be described by a differential equation, which generally has to be solved with numerical methods. Variants with different parameters had been calculated for an idealized fan and ideal weather conditions.
Content of the contribution
The results for the calculated variants will be presented as well as the influence of the parameters that determine the waiting time:
- air change rate at 50 Pa
- target pressure difference
- flow exponent
- initial pressure deviation
- absolute air pressure
- ...
Results and assessment
The air change rate at 50 Pa is the decisive parameter. If it is 0.1 /h, for instance, you have to wait 70 seconds at each measuring point; if it is 0.01 /h, the necessary time is 700 seconds, i.e. 12 minutes.
Two crucial questions have not yet been clarified:
- What does the situation look like for a real air handling unit where the flow rate falls as the pressure difference rises?
- Does the situation tend to be adiabatic or isothermal in practice? To which extent does the indoor air temperature approaches the temperature of the surrounding surfaces during the measuring time?
Conclusions
Measurement service providers should observe the 7 second rule: If you divide the number 7 by the numerical value of the air change rate at 50 Pa (in 1/h), you will receive the waiting time in seconds for each measurement point. This topic should be investigated in greater detail in theory and practice.
For further information please contact Dipl.-Phys. Joachim Zeller at: joachim.zeller@t-online.de