Thursday, April 2, 2009

The Blower Door Test

The critical part of our home energy audit was the blower door test. In so many words, this is a diagnostic that measures how much the house leaks. "Leakiness" is crucial for quantifying energy waste. This is not a question of eliminating air infiltration, but instead optimizing it. The house needs to breathe, especially so that excess water vapor can escape. If it doesn't, you develop mold problems. At the other extreme, if too much air escapes, your furnace and air conditioner have to run longer to compensate-- driving up your utility bill in the process.

The blower door test puts a physical measure on air flow in and out of the house. To conduct this test, the audit technician sets up an apparatus like the one shown in the picture:

The key component is a high-power electric fan that essentially sucks air out of the house. To conduct this test, all other doors and windows are closed, and all house fans plus the clothes dryer are turned on. The remaining gaps and penetrations are then the collective "pathway" for air infiltration. Gauges on the blower door apparatus indicate the volume of air movement, measured in cubic feet per minute (CFM). The data generated by these gauges are compared to engineering standards for how much air should infiltrate a house of a given square footage.

The results for us: A house of our size should be "breathing" at a rate of 1,800 CFM. The blower door test shows that the current rate is like 5,300 CFM-- three times what it should be. Before this test, I had only a vague understanding of my home's heat loss. My understanding now has an order of magnitude-- which is my motivation to do something about it.

Only one word of caution: the blower door test moves a SERIOUS volume of air. Nothing in the house was disturbed or damaged. But I will say that it effectively pulls dust out of every imaginable recess, like from the interior spaces between walls. Don't be surprised if the house has develops a musty odor for this reason. Fortunately, it dissipates after half a day or so.

The number-one item on my summer improvement list is to seal air leaks. We'll discuss that exercise in future posts.

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