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Energy Metrics Blog

Problems with calculating natural air changes

Over the years the method for determining natural air changes (denoted as ACHn or NACH) has improved quite a bit, but for use in energy auditing, the current method is still problematic. Factors like building height, geographical region, and wind shielding have been adjusted for, but there’s still no standard for measuring the volume of a building.

Here’s the formula:

ACH_{n} = \frac{(CFM_{50})(60)}{NV}

ACHn is the removal and replacement of a volume of air equal to the volume of the building in one hour.
CFM50 is the rate (measured in cubic feet per minute) at which air is entering the building when under a constant pressure of -50 pascals, or in other words, the leakiness of the building when a fan is blowing air out of it.
N is the correction factor for building height, wind, etc.
V is the volume of the conditioned space in the building.

Calculating the volume sounds like it should be easy. Take the area of the footprint, times the ceiling height, times the number of stories, correct for sloped ceilings, knee walls, cathedral ceilings, etc., and you have the volume. However, if you measure the interior volume of each room and add them up, you can get a significantly different number. And what do you do with a semi-conditioned basement or other semi-conditioned rooms in the building? Is that included in the volume measurement or not? These decisions can drastically change the ACHn number.

Take for example, a building I recently measured. Using the footprint method I came up with a volume of 24,630 ft3. Using the room-by-room method I came up with 20,986 ft3. Adding in the semiconditioned basement I got 30,580ft3 and 26,936ft3 respectively. That gives a range of ACHn values between 0.4 and 0.6.

Debating the shortcomings of this method is well and good for energy geeks but quickly becomes a substantive issue when it’s added to building codes. Here in Massachusetts, the code will follow the IECC’s (International Energy Code Council) recommendation of ACHn of 0.35 or less. But how will volume be calculated?

I guess we’ll have to see what happens.

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