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Difference of n50 and infiltration rate

asked 2015-09-28 04:20:07 -0600

gerald gravatar image

updated 2015-09-28 08:26:01 -0600

Could anyone explain me, what is the difference between the n50 value and the infiltration rate? In my project I got to the infiltration rate but I need the n50 value (for comparing simulation results and for a presentation). The infiltration rate is 0,11/h.

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answered 2015-09-29 09:48:15 -0600

updated 2015-09-29 12:30:07 -0600

It looks like by infiltration rate you actually mean the annual average infiltration rate, that is the air exchange under natural conditions. The given value (0.11 ACH) seems coherent with this assumption and it appears to be what you used in your energy model.

As pointed out by @aparker, the n50 or ACH50 is the measure of the air exchange through the building envelope under a 50Pa pressure difference. Under natural conditions, the pressure difference will be much lower and varying depending on the building exposure and wind velocities and directions. However, a reference pressure of 4 Pa is typically used in the US to represent natural conditions (more details about that in this paper written in 1980 by M. Sherman and D. Grimsrud).

If ACH50 is the air exchange at 50 Pa, we can say that the air exchange under natural conditions is ACH4 = 0.11 ACH at 4 Pa.

In order to convert ACH4 to ACH50 some assumptions need to be made, and the air exchange rate should be converted to air flow rate first. Let's assume a building with an enclosed air volume of 10,000 m3. The air flow through the building envelope at 4 Pa would be:

Q4 = ACH4 * Vbuilding = 0.11 * 10,000 = 1,100 m3/h

For a particular building, the relation between pressure difference and air flow through the envelope is linear, and the power law equation of flow through an orifice can be used to estimate the air flow at different pressures. Still, we need to make an assumption on the pressure exponent n which is unknown unless a blower door test was performed. This parameters represents the characteristic shape of the orifice and ranges from 0.5 (perfect orifice) to 1.0 (very long and thin crack). For a fairly air-tight envelope the exponent value would be around 0.6 or 0.7 and for a very good, air-tight envelope around 0.8 or even above.

Let's assume n = 0.75 in the present case because this looks like a reasonably good, air-tight envelope. Using the power law equation we can determine the air leakage coefficient C given the air flow under natural conditions (4 Pa).

Q4 = C * dP^n  <=>  C = Q4 / (dP^n) = 1,100 / (4^0.75) = 389 m3/h

Knowing the air leakage coefficient and pressure exponent, the air flow at 50 Pa can be estimated using the same equation:

Q50 = C * dP^n = 389 * (50)^0.75 = 7,313 m3/h

The previous two equations can be merged and Q50 can be obtained directly from Q4, without the intermediate step to calculate C:

Q50 = Q4 * (50/4)^n = 1,100 * (50/4)^0.75 = 7,313 m3/h

And because we know the enclosed air volume of the building, we can convert the air flow back to an air exchange rate:

ACH50 = Q50 / Vbuilding = 7,313 / 10,000 = 0.73 ACH

Fortunately, we can simplify all this by using substitutions and obtain ACH50 ... (more)

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Excellent answer!

aparker gravatar image aparker  ( 2015-09-29 11:52:38 -0600 )edit
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answered 2015-09-28 13:05:29 -0600

This answer assumes that n50 is the same as ACH50. ACH50 is the number of air changes that occur when the building is pressurized to 50 Pascals. This is a pretty common measurement in residential buildings, where a blower door is used to do the pressurization. This article gives a pretty good backstory about ACH50 and what reasonable and unreasonable values are.

Anyhow, your building probably isn't pressurized nearly that high, so you need some way to convert from whatever pressure your ACH value assumes to 50 Pa. This paper has some equations (and assumptions) that you can use. Of course, read about the assumptions and the equations to make sure you understand them before using them.

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Asked: 2015-09-28 04:20:07 -0600

Seen: 2,618 times

Last updated: Sep 29 '15