In the fan pressurisation method, the pressures are generated artificially by a fan which blows air into (or extracts air from) the building. The fan is connected to a convenient opening, usually a door, and the flow rate increased up to a maximum and then allowed to decrease. The fan assembly itself is either pre-calibrated, so that the flow rate is known for any given fan speed, or the fan assembly is fitted with pressure tappings or a restrictor plate from which the flow rate can be measured. The resulting internal to external pressure differences are measured with suitable pressure sensors inside and outside the building. The result is a graph of flow rate against pressure difference which, because it is a power law, is a curve. Figure 14.1 is a schematic diagram of the experimental arrangement, and shows the type of graph that is produced. Because the air flow is under direct control, and because both air flows and pressure differences are measured directly, this is a controllable and repeatable method, and as such has been chosen as the preferred method in the regulatory framework. However, the test procedure has some drawbacks:
• In order to be able to make reliable measurements, the pressure differences used in the test must be sufficiently high, partly to overcome the naturally occurring pressures due to wind and stack effect, and partly to improve the accuracy of the measurements. Pressure differences up to 60 pascals are normally used, which is much higher than the maximum of about 8 to 10 pascals created by wind or stack effect.
• The curve obtained as the building is depressurised usually differs from that obtained during pressurisation, leading to some uncertainty in the interpretation of the results.
• The high pressures may cause the external fabric to leak in a way which is not normal. For example, draught excluding seals which are designed to resist an externally applied pressure may be forced away from their seating by the internally generated pressure of the fan, causing the air leakage to be exaggerated. This can be compensated by reversing the air flow and depressurising the building, so that the air flow is inwards. This is often done when testing small buildings such as dwellings, but is difficult or even impossible with large buildings.
• In a large building, the internally generated pressure distribution will vary according to location with respect to the entry point of the fan, and this variation may be significant, especially if the building contains internal walls and floors.
• In large buildings, it may be difficult to generate sufficient air flow to reach a pressure difference which can be measured with acceptable accuracy.
There are also some difficulties in interpreting the test results:
• Because of the power law nature of the relationship between air flow rate and pressure difference, the constant k in the equation is an index rather than a precise measure of the area of the openings in the external envelope.
• Measurements at the pressure differences normally generated naturally are prone to excessive error; therefore results at much higher pressure differences, around 50 pascals, are needed to give good repeatability and reliability.
• The behaviour of the leakage paths during a pressurisation test at 50 pascals may not be typical of their behaviour in normal conditions; for example, the air flow in some leakage paths may be laminar at low pressure differences, but become turbulent at the much higher pressure differences used in a test.
Etheridge and Sandberg  give a fuller discussion of the possible errors associated with fan pressurisation measurements.
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