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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 you used it in your energy model to represent the annual average infiltration rate.

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 assume that 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 9,000 m3. The air flow through the building envelope at 4 Pa would be:

Q4 = ACH4 * Vbuilding = 0.11 * 9,000 = 990 m3/h

Given 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) = 990 / (4^0.75) = 350 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 = 350 * (50)^0.75 = 6,581 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 = 990 * (50/4)^0.75 = 6,581 m3/h

An 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 = 6,581 / 9,000 = 0.73 ACH

Fortunately, we can simplify all this by using substitutions and obtain ACH50 directly from ACH4 without making assumption on the enclosed air volume of the building

ACH50 = ACH4 * (50/4)^n = 0.11 * (50/4)^0.75 =  0.73 ACH

This formula makes it possible to convert air infiltration rates between different pressures, but bear in mind the results is very sensitive to the pressure exponent! Unless this parameter is obtained from a blower door test, one should be very careful with making an estimation on n. For instance, with n = 0.7 we would obtain ACH50 = 0.64 ACH. And with n = 0.8 we would get ACH50 = 0.83 ACH

In conclusion, given an infiltration rate of 0.11 ACH under natural conditions you could report n50 / ACH50 to be 0.73 ACH +/- 15%

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 it in your energy model to represent the annual average infiltration rate.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 assume 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 9,000 10,000 m3. The air flow through the building envelope at 4 Pa would be:

Q4 = ACH4 * Vbuilding = 0.11 * 9,000 = 990 10,000 = 1,100 m3/h

Given 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) = 990 1,100 / (4^0.75) = 350 389 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 = 350 389 * (50)^0.75 = 6,581 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 = 990 1,100 * (50/4)^0.75 = 6,581 7,313 m3/h

An 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 = 6,581 7,313 / 9,000 10,000 = 0.73 ACH

Fortunately, we can simplify all this by using substitutions and obtain ACH50 directly from ACH4 without making assumption on the enclosed air volume of the building

ACH50 = ACH4 * (50/4)^n = 0.11 * (50/4)^0.75 =  0.73 ACH

This formula makes it possible to convert air infiltration rates between different pressures, but bear in mind the results is very sensitive to the pressure exponent! Unless this parameter is obtained from a blower door test, one should be very careful with making an estimation on n. For instance, with n = 0.7 we would obtain ACH50 = 0.64 ACH. And with n = 0.8 we would get ACH50 = 0.83 ACH

In conclusion, for a given an infiltration rate of 0.11 ACH under natural conditions you could report n50 / ACH50 to be 0.73 ACH +/- 15%