This table compares a small, typical, and large house if they all had the same ACH50 score. ACH50 is important because that’s what most states use for their building codes, but there are other ways to normalize leakage.

Understanding ACH50 – a Number Line

Now let’s see where that 6.8 ACH50 would fall on a number line of typical houses and against the building codes. Figure 2 shows that 6.8 ACH50 would barely pass today’s building code in Florida since it’s just under 7 ACH50, the maximum allowed.

In general, a more airtight house is always better. In a tighter house, it’s always easier to control the temperature, the humidity, and the air quality. This is simply because the air we condition doesn’t leak out as fast and get replaced with unconditioned outdoor air.

When houses get extremely leaky, approximately eight ACH50, it becomes very difficult to keep the house conditioned so that it’s consistently comfortable.

So Why Should an HVAC Tech have a Blower Door?

One common answer to this question is because a blower door will help dial in your load calculations and make them more accurate. This is true, but the benefits are sometimes overstated.

If you don’t know how leaky a house is, you’d make an air tightness guess somewhere in the middle of the scale, to keep the error to a minimum. You wouldn’t assume it’s really leaky or super tight; something approximately eight ACH50 would be a good guess.

If you do that, the uncertainty in your load calculation would rarely be more than ±15%. That’s a lot smaller than the capacity steps in the equipment you select.

You might see as high as 25%, but only for the heating load and only in very cold climates. Remember, properly sizing heating equipment isn’t as critical as properly sizing cooling equipment.

Heating equipment usually comes in increments of 20,000 Btuh, while cooling equipment comes in increments of 6000 Btuh (half a ton). So, the bottom line is that the lack of a blower door test will rarely change the size of the equipment you choose.

Anticipate, Find, and Solve Comfort Problems

If improving your load calculations isn’t the main reason to have a blower door, what is? Diagnostics! The house, ducts, and equipment all work together as a system. A blower door can help you become the master of that system.

Combined with an infrared camera, a blower door can locate major and minor leaks in the house. Locating leaks can be very important because air leakage in real-world houses isn’t like we assume it is in a load calculation. A load calculation assumes the insulation thickness is the same in all the walls and all air leaks are uniformly distributed in the exterior surfaces.

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