One of my favorite HVAC Technicians, Chris Hughes, once told me a story about a difficult conversation he had with a homeowner about the expectations of a new HVAC system.

A similar conversation happened more than once. The homeowner made it clear that they had very high expectations of the new system. So, like a good Comfort Advisor, Chris directed the conversation to clarify expectations.

The conversation went something like this:

• Chris: OK, so you’re telling me you want perfection out of this new system.
• Homeowner: Yes, that’s right. I want it to be perfection!
• Chris: OK, I understand that. Unfortunately, you didn’t buy that kind of house.

Chris was being blunt, but he knows which customers it’s OK to be blunt with. More importantly, he was being honest. Why is that honest? Shouldn’t we expect the perfect new system install to deliver HVAC perfection?

In a word, no. We can’t expect perfection from a new system install because the equipment is only part of the system. The house, the ducts, and HVAC equipment together are a system.

This is why the whole process begins with a load calculation – we must understand the heat loss, heat gain, and other aspects of the building itself if we want to deliver High-Performance HVAC™.

That’s where a blower door can help an HVAC technician to understand the building itself. So, what does a blower door do, and how does it work?

A blower door primarily does two things. It measures the total air leakage from a house or other building, and it helps to locate the leaks within a building envelope.

What a Residential Blower Door Does

A blower door uses a variable-speed fan to depressurize or pressurize the entire volume of a house or other building. During a test, the fan measures the air that flows through it. You temporarily fit a fabric panel and frame into a residential exterior doorway so little to no air leaks around the fan. This helps to maintain the whole house at the same pressure (see Figure 1).

With the most common test, you set up the blower door fan to depressurize the house to -50 Pa (-0.2 in. H2O). At that pressure we know that all the air leaking into the house is being blown out through the fan, which is measuring the air flow. If there’s 2000 CFM going out through the fan, then we know there’s 2000 CFM leaking in through all the leaks. This is called the CFM@50Pa or CFM50.

Two Common Measurements

CFM50 isn’t the only number we use to score the leakage of the house. It’s also common to normalize leakage by using the volume of the house since larger houses usually leak more than smaller houses. The resulting number is ACH50, or Air Changes per Hour at 50 Pa. It’s the number of times all the air would get exchanged in the house if it leaked at the CFM50 rate for an hour.

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