Determination of the packing degree of buildings (air exchange coefficient n) by using BlowerDoor equipment
The packing degree of buildings is one its characteristic property that is described by air exchange coefficient. It accounts for convective heat leakage (different from leakage caused by heat conduction that can be measured both for objects in real exploitation conditions, and also for ready made objects in standatised conditions).
For determination of the packing degree of buildings, ventilator of special type with continuously changable frequency is used. It is complected together with measurement instruments for pressure and productivity of ventilator V' (m3/h). The ventialator also includes framework and textil for handy packing of system at some opening (window, doors, etc.). Such an equipment is called Blower Door. The measurements are performed creating sequentially the pressure that is by 50 Pa lower and higher than outside in correspondence with standart EN ISO 9972. A number of additional conditions should be maintained for sufficiently accurate measurements. Most important is that the preesure difference between outside and inside due to atmospheric phenomena (wind and convective flux of air created by temperature difference) should not exceed 5 Pa. Therefore, summer is more suited for testing of packing degree, because of small temperature difference between inside and outside.
The preparation of rooms for measurements depends on the aim of measurements and condition of building during them:
- If the measurements are performed before exploitation, then one should check whether the outside shell is closed in order to escape from errors. The drainage should be packed as it is not filled with water.
- The additional ventillation should be switched off and openings of ventilation system packed, measuring the packing degree of outside shell. The ventilator should be expoitated in certain regime measuring the efficiency of ventilation system for influx and outflux.
- The part of building should be isolated and openings closed performing measuring in it. Such kind of measurements in one or more rooms allows to determine whether windows and doors are qualitatively built in.
Measuring the air exchange coefficient in this way at given pressure difference 50 Pa
n50=V/VN (1/h) ,
where VN air volume in the room and V efficiency of ventilator (m3/h), the necessary packing degree n can be calculated for modelling of heat leakage:
n = n50·eV,
where eV coefficient characterising resistance of building to wind, and other standartised characteristic values of packing, e.g:
q50 = V/Ashell; np50 = V/Abase,
where Ashell and Abase area of outside shell and basement.
In order to find the openings in the outer shell of building, it is commendable to use thermographic diagnostics together with simultaneous creating of high/low pressure in building.
Standartised value of n without additional ventiallation is 0,8 (1/h). The measuremnts carried out in various public and living houses before reconstruction show significantly larger air exchange coefficient n=1,0-1,4 (1/h), that essentially increases the heat leakage. For example, value of n in the gangway of typical block house can exceed even 10 (1/h), i.e. the chimney effect takes place. Installing the controlable ventilation system, the packing degree can be essenially higher n<0,3 (1/h) depending on the use of the ventillation system.
The problems in measurements can occur, if the packing of building is poor and its volume is large, because in this case the pressure difference 50 Pa (in correspondence with standart EN ISO 9972) cannot be made with one Blower Door equipment iekąrtu. In this case, the measurements can be made twice at lower pressure difference (25 Pa) and taking into account that V'~Dp:
The dependence of air flux on pressure diffrence at various degrees of packing