Posted by: lcowie | November 16, 2010

Working with a Blower Door and Manometer

When people think of home energy audits, they are generally familiar with blower doors. Nobody seems to ever mention the manometer, which is the multi-functional device that measures air pressure and airflow using two independent pressure sensors. The blower door and the manometer go hand-in-hand. It would be impossible to use the blower door properly, without the use of the manometer. The most common manometer is the DG-700 Pressure and Flow Gauge, which comes automatically with the Minneapolis Blower Door.

In case you aren’t familiar with it, a blower door is a diagnostic tool used to depressurize a building to measure the amount of airflow at a constant pressure and identify leaks in a home. You have to use the blower door because under natural conditions, it is difficult to determine the amount and source of air coming into and going out of a house.

It is best to learn the fundamentals of these devices, rather than just memorizing what buttons you press. You need to understand the fundamentals because much of your home energy audit evaluation will depend on what these devices tell you.

A blower door depressurizes a house to a constant of -50 Pascals, which is a point of reference from which to take measurements. The -50 is somewhat of an arbitrary number. The Canadians started the blower door game by depressurizing their homes to -50 Pascals, and the United States followed suit.

What the blower door does is to measure airflow in cubic feet per minute (CFM) while at -50 Pa. This tells you how much fresh air is available for occupants and allows you to identify where air is leaking into and out of a house. To compare, imagine trying to fill a bucket full of holes with water. If you work really hard to fill the bucket, then there is a hole. Likewise, for homes, if the blower door works really hard, there is a leak in the home.

As I mentioned above, the manometer uses two independent pressure sensors. In that sense, you can look at the manometer like it is two devices. The left side tells you the pressure inside the house with reference to the pressure outside the house. You need this to know that you have depressurized the house to 50 Pa.

The right side of the manometer tells you the pressure inside the fan with reference to the pressure inside the house. You need to know this so you can tell how much air is moving through the fan when the house is depressurized by 50 Pa.

The air pressure going through the blower door can be used to calculate volume of air leaving the home.


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