## Residential Ventilation Calculation Example

Here is a step by step example to follow to determine if your house
has enough ventilation based on the 2010 version of ASHRAE Standard
62.2:

Now for the math…..

Qtotal = 0.01 X CFA + 7.5 X (bedrooms + 1)

where ” Qtotal ” is the required whole house ventilation (in CFM), CFA is the conditioned floor area you determined in step 1, and “bedrooms” is the number of bedrooms in the house. So a 2 story, 3 bedroom, 2000 square foot house will require 50 CFM of total, whole house ventilation, or

Qtotal = 0.01 X 2000 + 7.5 X (3 + 1) = 20 + 7.5 X 4 = 20 + 30 = 50 CFM

In order to meet the ASHRAE 62.2 Standard, a kitchen should have a vented range hood capable of exhausting 100 CFM TO THE OUTDOOORS (not back into the house) intermittently (fan with an on/off switch and the fan is switched on). Likewise, the full bathrooms should have exhaust fans capable of exhausting 50 CFM intermittently or 20 CFM continuously (i.e. the bathroom fan is left ON ALL the TIME and exhausts 20 CFM continuously).

If the kitchen or bathroom has an “operable window” (i.e. a window you can open to the outdoors) you get a “bonus” of 20 CFM whether the window is open or not. You only get credit for ONE window, even if you have 12 open windows in a kitchen, you still only get the bonus of 20 CFM. Also, ceiling fans do NOT count. So a full bathroom with an exhaust fan pulling 25 CFM (measured) and an operable window gets recorded as a bathroom with 45 exhaust CFM (25 CFM for the fan plus 20 CFM for the window), which is 5 CFM less than the ASHRAE 62.2 Standard UNLESS: A) the window is always open (not recommended); or B) the exhaust fan is always running (also not recommended, but possible).

Now that we know how much ventilation a 3 bedroom, 2000 square foot house should get (=50 CFM), let’s calculated how much “local” ventilation (kitchen and bathroom ventilation) the house needs:

A) One kitchen with 2 windows and no exhaust fans exhausting to the outdoors. The kitchen needs 100 CFM of intermittent (ON or OFF switch), and you get credit for 20 CFM for the windows (again, you only get credit for one window). So your kitchen needs 100-20 = 80 CFM of intermittent ventilation.

Qkitchen = 100 – 20 = 80 CFM additional ventilation required.

B) Now, the house has 2 full bathrooms: one master bathroom and one hallway bathroom. The master bathroom has an exhaust fan measured at 40 CFM, and a window. So it has 40 + 20 = 60 CFM, which is greater than 50 CFM, so it needs no additional ventilation. Notice, you don’t get a credit of 10 CFM either.

Qmaster bath = 50 – 40 – 20 < 0, so no additional ventilation is required.

The hall full bathroom has a window and an exhaust vent fan measured at 20 CFM. Again, the bathroom needs 50 CFM – 20 CFM for the fan and – 20 CFM for the window. So, the hall bathroom needs an additional 10 CFM of intermittent (ON or OFF switch) ventilation.

Qhall bath = 50 – 20 – 20 = 10 CFM additional ventilation required.

So, this house needs an additional “local” ventilation of 90 CFM:

Qlocal = Qkitchen + Qmaster bath + Qhall bath = 80 + 0 + 10 = 90 CFM additional ventilation required.

So, according to ASHRAE 62.2, you can either add the additional ventilation required to each kitchen and bathroom in the house, or you divide the local ventilation (Qlocal) by 4, and add it to the whole house ventilation (Qtotal). Why divide the local ventilation (Qlocal) by 4? Honestly, I have no idea, but it’s what is in the standard!

Qtotal = Qtotal + Qlocal/4 = 50 + 90/4 = 50 + 22.5 = 72.5 CFM

QInfiltration credit = ½ X [(CFM50/N) - (CFA X (0.02)]

where “CFM50″ is the average result from the door blower test from step 3, N is the calculated weather factor for your local area and house size, and CFA is the conditioned floor area determined in step 1.

Use the following table for determining the appropriate N for your house located in eastern Nebraska (includes both Omaha & Lincoln):

So, the taller your house, the lower the N-value because the more air infiltration your house should have as a result of stack effect, wind, weather, etc. The calculation for the N value is actually pretty complex, but this table is a good approximation. So, for a 2 story house, the N-value should be 18.4.

So for our 2 story, 3 bedroom, 2000 square foot example house with 2 full bathrooms and one kitchen, the average door blower result was 1500 CFM50, the infiltration credit is:

QInfiltration credit = ½ X [(CFM50/N) - (CFA X (0.02)] = ½ X [(1500/18.4) - (2000 X (0.02)] = 81.5 – 40 = 41.5 CFM

So, to determine how much additional ventilation your house needs (if any), subtract the infiltration ventilation from the total ventilation. If the number is > 0, then you should add mechanical ventilation to the house. If, the difference between the total ventilation need (Qtotal) and the infiltration (QInfiltration credit) is less than zero, you don’t need to add any ventilation.

So, for this particular example house, we need to add 31 CFM of mechanical ventilation:

Qmechanical = Qtotal – QInfiltration credit = 72.5 – 41.5 = 31 CFM of ventilation. This can be done several ways, including placing the master bathroom exhaust fan on a timer to cycle on/off for 47 minutes every hour. The master bathroom exhaust fan had a measured 40 CFM, so 31/40 = 0.775 * 60 minutes/hour = 47 minutes/hour

Run time = (31 CFM/40 CFM) X 60 minutes/hour = 47 minutes/hour.

This can also be accomplished by adding a kitchen exhaust fan that exhausts OUTSIDE the house at 100 CFM. This would decrease the local ventilation requirement from 22.5 CFM to 2.5 CFM, and decrease the need ventilation from 72.5 CFM to 52.5 CFM. Then the kitchen fan can be placed on a timer to run 32 minutes per hour. Alternative, a whole house ventilation system can be installed.

**For existing houses (i.e. not new construction)**:**Step 1.**Calculate the square footage of conditioned floor area (CFA) in your house. Please included finished attics and basements where there are registers delivering air from a furnace, air conditioner, or air handler. This is your conditioned floor area (CFA).**Step 2**. Hire an energy evaluator or qualified contractor to perform a door blower test in accordance with ASTM E-779 Standard. This is a 2-way (pressurization and depressurization), mutli-point door blower test. The technician will pressurize the house at several different pressures, then repeat the test by depressurizing the house at several different pressures. Then, the technician will provide you with an average air infiltration rate at a standard pressure of 50 Pascals. The air infiltration rate should be expressed in cubic feet per minute at 50 Pascals pressure (CFM50).Now for the math…..

Qtotal = 0.01 X CFA + 7.5 X (bedrooms + 1)

where ” Qtotal ” is the required whole house ventilation (in CFM), CFA is the conditioned floor area you determined in step 1, and “bedrooms” is the number of bedrooms in the house. So a 2 story, 3 bedroom, 2000 square foot house will require 50 CFM of total, whole house ventilation, or

Qtotal = 0.01 X 2000 + 7.5 X (3 + 1) = 20 + 7.5 X 4 = 20 + 30 = 50 CFM

**Step 3.**The energy evaluator or qualified contractor should also measure the flow rate through the exhaust fans in your kitchen and full bathrooms (if any). The exhaust fan flow rate should be expressed in cubic feet per minute (CFM).In order to meet the ASHRAE 62.2 Standard, a kitchen should have a vented range hood capable of exhausting 100 CFM TO THE OUTDOOORS (not back into the house) intermittently (fan with an on/off switch and the fan is switched on). Likewise, the full bathrooms should have exhaust fans capable of exhausting 50 CFM intermittently or 20 CFM continuously (i.e. the bathroom fan is left ON ALL the TIME and exhausts 20 CFM continuously).

If the kitchen or bathroom has an “operable window” (i.e. a window you can open to the outdoors) you get a “bonus” of 20 CFM whether the window is open or not. You only get credit for ONE window, even if you have 12 open windows in a kitchen, you still only get the bonus of 20 CFM. Also, ceiling fans do NOT count. So a full bathroom with an exhaust fan pulling 25 CFM (measured) and an operable window gets recorded as a bathroom with 45 exhaust CFM (25 CFM for the fan plus 20 CFM for the window), which is 5 CFM less than the ASHRAE 62.2 Standard UNLESS: A) the window is always open (not recommended); or B) the exhaust fan is always running (also not recommended, but possible).

Now that we know how much ventilation a 3 bedroom, 2000 square foot house should get (=50 CFM), let’s calculated how much “local” ventilation (kitchen and bathroom ventilation) the house needs:

A) One kitchen with 2 windows and no exhaust fans exhausting to the outdoors. The kitchen needs 100 CFM of intermittent (ON or OFF switch), and you get credit for 20 CFM for the windows (again, you only get credit for one window). So your kitchen needs 100-20 = 80 CFM of intermittent ventilation.

Qkitchen = 100 – 20 = 80 CFM additional ventilation required.

B) Now, the house has 2 full bathrooms: one master bathroom and one hallway bathroom. The master bathroom has an exhaust fan measured at 40 CFM, and a window. So it has 40 + 20 = 60 CFM, which is greater than 50 CFM, so it needs no additional ventilation. Notice, you don’t get a credit of 10 CFM either.

Qmaster bath = 50 – 40 – 20 < 0, so no additional ventilation is required.

The hall full bathroom has a window and an exhaust vent fan measured at 20 CFM. Again, the bathroom needs 50 CFM – 20 CFM for the fan and – 20 CFM for the window. So, the hall bathroom needs an additional 10 CFM of intermittent (ON or OFF switch) ventilation.

Qhall bath = 50 – 20 – 20 = 10 CFM additional ventilation required.

So, this house needs an additional “local” ventilation of 90 CFM:

Qlocal = Qkitchen + Qmaster bath + Qhall bath = 80 + 0 + 10 = 90 CFM additional ventilation required.

So, according to ASHRAE 62.2, you can either add the additional ventilation required to each kitchen and bathroom in the house, or you divide the local ventilation (Qlocal) by 4, and add it to the whole house ventilation (Qtotal). Why divide the local ventilation (Qlocal) by 4? Honestly, I have no idea, but it’s what is in the standard!

Qtotal = Qtotal + Qlocal/4 = 50 + 90/4 = 50 + 22.5 = 72.5 CFM

**Step 4.**Now we calculate the “infiltration credit”, i.e. how much natural ventilation the house gets through air leaks. This is where the door blower results come in:QInfiltration credit = ½ X [(CFM50/N) - (CFA X (0.02)]

where “CFM50″ is the average result from the door blower test from step 3, N is the calculated weather factor for your local area and house size, and CFA is the conditioned floor area determined in step 1.

Use the following table for determining the appropriate N for your house located in eastern Nebraska (includes both Omaha & Lincoln):

**Stories**1 1.5 2.0 2.5 3 or greater**N value**22.6 20.0 18.4 17.2 16.3So, the taller your house, the lower the N-value because the more air infiltration your house should have as a result of stack effect, wind, weather, etc. The calculation for the N value is actually pretty complex, but this table is a good approximation. So, for a 2 story house, the N-value should be 18.4.

So for our 2 story, 3 bedroom, 2000 square foot example house with 2 full bathrooms and one kitchen, the average door blower result was 1500 CFM50, the infiltration credit is:

QInfiltration credit = ½ X [(CFM50/N) - (CFA X (0.02)] = ½ X [(1500/18.4) - (2000 X (0.02)] = 81.5 – 40 = 41.5 CFM

So, to determine how much additional ventilation your house needs (if any), subtract the infiltration ventilation from the total ventilation. If the number is > 0, then you should add mechanical ventilation to the house. If, the difference between the total ventilation need (Qtotal) and the infiltration (QInfiltration credit) is less than zero, you don’t need to add any ventilation.

So, for this particular example house, we need to add 31 CFM of mechanical ventilation:

Qmechanical = Qtotal – QInfiltration credit = 72.5 – 41.5 = 31 CFM of ventilation. This can be done several ways, including placing the master bathroom exhaust fan on a timer to cycle on/off for 47 minutes every hour. The master bathroom exhaust fan had a measured 40 CFM, so 31/40 = 0.775 * 60 minutes/hour = 47 minutes/hour

Run time = (31 CFM/40 CFM) X 60 minutes/hour = 47 minutes/hour.

This can also be accomplished by adding a kitchen exhaust fan that exhausts OUTSIDE the house at 100 CFM. This would decrease the local ventilation requirement from 22.5 CFM to 2.5 CFM, and decrease the need ventilation from 72.5 CFM to 52.5 CFM. Then the kitchen fan can be placed on a timer to run 32 minutes per hour. Alternative, a whole house ventilation system can be installed.