# How do I calculate infiltration heat loss based on blower door test results?

| Posted in General Questions on

A few years ago, I was considering replacing my AC system. The current system is very old and doesn’t keep the house as cool as I want on design days, so I did my own Manual-J in order to form a point of referenced when speaking to contractors. I bought a limited-time license for the HVAC-Calc 4.0 software package.

More recently, I’ve been looking at replacing my hot water boiler, and am referring back to this Manual-J, which also gave a winter heat loss calculation. I used the correct indoor/outdoor design temperature for the region, and tried to enter very detailed information about walls, floor, ceiling, insulation, doors, windows,etc.

At time time, I could only guess at the air-infiltration rate (ACHnat), so I chose the default (i.e. average), which equates to 0.7 ACH in the winter time. However, last year a contractor performed some air sealing (primarily in the attic) subsidized by my utility company, after which the blower door test measured 2735 CFM50.

This is easy to convert to ACH50 by dividing by the volume of the house and multiplying by 60, but I’m not sure if I should include the volume of the basement or not. I have a 2-story colonial (Boston area), with a 1-story family room section, with a garage under the family room at basement level.

After getting the ACH50 number, I can divide it by the LBL factor of 14.8 from this paper:

in order to convert to ACHnat.

I ALSO found on this site, a formula to directly calculate infiltration heat loss as:
delta-T * ACHnat * volume * 0.018

Questions/Problems:

1.) As stated early, I’m not sure if I should include the basement air volume when converting from CFM50 to ACH50.

2.) I used the air infiltration heat loss formula with the ACH=0.7 assumption from the Manual-J in order to establish a reference, but this calculated value is WAY off from what is reported in the Manual-J. For example, for the 330sqft x 9ft family room (volume of 2970 cu ft), the formula gives:
67 * 0.7 * 2970 * 0.018 = 2507 BTHU, while the Manual-J reports 5776 BTUH! Obviously the difference is amplified for larger rooms. What could the Manual-J software possibly have been doing in order to come up with BTHU number so much different than the formula above?

3.) The Manual-J incorrectly assumed that I am running 120 degree air through my AC ducts in order to heat the house (I didn’t specify radiant baseboard heat at the time). I give air duct heat loss number assuming the air inside the duct is 120 and the outside of the duct is exposed to outdoor ambient 1 degree. Should i just subtract this heat loss entirely to correct the Manual-J?

Regarding the air infiltration heat loss , it just occurred to me while writing this that maybe the RIGHT answer is to just totally scrap the numbers from the Manual-J and replace this by the total blower door CFM50 (assuming I can get a good conversion to ACHnat under design conditions), and use the above formula to calculate infiltration heat loss.

I want to get this right because being an older home (i.e. late 70s) that is not super tight, it seems that the infiltration losses really dominate (i.e. account for more than half) of the total loss.

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### Replies

1. GBA Editor
| | #1

Thomas,
Q. "I'm not sure if I should include the basement air volume when converting from CFM50 to ACH50."

A. The answer can be found in my article, Blower Door Basics. In that article, I wrote, "To calculate ach50, multiply cfm50 by 60 minutes per hour and divide the product by the building volume, including the basement, measured in cubic feet."

It's fair to say that the phrase "default air infiltration rate" is fairly meaningless.

The other problem is that blower door results don't really correlate to "ACH(nat)" rates. ACH(nat) will always be a guess or an estimate, because a blower door doesn't tell you where the holes are. A hole in the basement or attic has a much greater effect on infiltration than a hole near the neutral pressure plane. A blower door can't tell the difference.

2. Expert Member
| | #2

"More recently, I've been looking at replacing my hot water boiler, and am referring back to this Manual-J, which also gave a winter heat loss calculation. I used the correct indoor/outdoor design temperature for the region, and tried to enter very detailed information about walls, floor, ceiling, insulation, doors, windows,etc."

Since you have a heating history on this place it's easy to sanity check your Manual-J heat load using fuel use against heating degree day data, as outlined in this bit o' bloggery:

If you don't feel like doing it long-hand, Borst Engineering & Construction LLC has a handy "Existing Building Energy Usage Analysis " web-widget version of a fuel-use load calculator on their site (along with other handy calculators):

Be sure to read the relevant section of their instruction manual for their web-widgets:

There is more to specifying a boiler for running hydronic baseboard heating systems than the heat load however, especially (but not exclusively) when installing a modulating condensing boiler. There isn't an agreed upon definition for "radiant baseboard heat", so a better description would be needed, along with:

How many zones?

How many feet of baseboard or equivalent direct radiation (EDR) do you have, broken down by zone?

Some of the napkin-math on those issues can be found here:

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