What if there was a way to create a 20-MPH wind that would surround a building on all sides and force air at that velocity against all four wall surfaces and the roof simultaneously, so that a building’s tightness (or lack thereof) could be determined? Well, there is a way to accomplish that seemingly impossible feat, and we’re showing how in Figure One (image courtesy of ESCO Institute).


Figure One

Figure One

 This illustration shows how a blower door test is accomplished in order to de-pressurize a building. From a technical standpoint, the 20-MPH wind isn’t the method of measurement, but, when the blower exhausts air out of a building and the manometer shows the test pressure being accomplished is -50 pa WRT (we’ll get back to this shortly), that’s fundamentally what is happening. And creating this positive pressure outside the building and, therefore, the negative pressure on the inside, will accomplish an evaluation of the building envelope that allows us to find out if the building is tight, or if it’s not. Of course, if it’s not, then energy is being wasted due to an unnecessary addition to the cooling load, And, while we’re on the subject of unwanted air finding its way into a conditioned space in a building, we’ll remind you that the indoor air quality is being affected due to the un-filtered and un-controlled introduction of outside air.

What a technician needs to understand about the pressure measurement process relative to blower door testing is that the scale being used, the Pascal (pa), is a very fine measurement that provides very specific information about the pressure in a building. Here’s one way to appreciate just how fine a measurement is being accomplished in a blower door test:

 ….1 PSIG is equal to 27.70 inches of water column, the scale that most technicians are familiar with and appreciate how precise it is when measuring things like fuel pressure or static pressure in a duct.

 ….0.2 inches of water column is equal to 50 Pascals.W

Which brings us to “WRT”. It stands for “With Reference To” and it is the acronym used to describe, as our illustration is showing, the difference in pressure inside the house WRT the outside pressure. With one tube from the digital dual port manometer connected to the blower door assembly, and the second tube positioned properly to the outside of the building, the accurate WRT measurement proves to the technician that the building is being de-pressurized to the point where building envelope leaks can be detected and corrected in order to minimize energy waste and cut equipment operating costs.


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