Determine the Airflow Required for Whole-Building Ventilation
Whole-building ventilation replaces a preset amount of stale, indoor air with ventilation air from outside. It is designed to work quietly in the background to remove moisture and indoor pollutants. ASHRAE Standard 62.2 assumes that a home will receive outside air from infiltration at a rate of 2 cfm/100 sq ft. Mechanical ventilation is used to provide an additional calculated amount of whole-building ventilation.
ASHRAE Standard 62.2 offers two methods to calculate the required airflow for whole-building ventilation in cubic feet per minute (cfm). Using the formula below will usually be more precise than using the prescriptive table (ASHRAE Standard 62.2 Table 4.1a) below, but both methods are acceptable. An optional calculation method is described for reducing the whole-building ventilation rate in an existing house with high infiltration.
Calculating airflow rates for an apartment building? View additional information.
The required whole-building ventilation rates below are only useful for sizing continuously operated systems. If you're planning a whole-building ventilation system with intermittent operation, the fan must be sized according to the operation schedule. The shorter the ventilation periods, the more airflow required to provide equivalent whole-building ventilation. (See the intermittent ventilation run cycles.)
Skip the optional calculations. Seal the house as tightly as possible. Install local exhaust ventilation in the kitchen, bathrooms, and any other areas with high pollutant loads. Provide whole-building ventilation of at least the rate determined by the formula or table method.
A whole-building ventilation fan, typically 30 to 130 cfm, is different than a whole house cooling fan, which is a 3,000-5,000 cfm fan used for night cooling during hot weather. References to a California code requirement for insulated shutters on whole house fans refer to the large opening into the attic required for high-volume whole house cooling fans.
The required flow rate for whole-building ventilation can be calculated using the following formula from ASHRAE Standard 62.2:
|whole-building continuous ventilation rate in cfm =|
+ (number of bedrooms + 1) x 7.5
A second way to determine the required whole-building ventilation flow rate in cfm is to use ASHRAE's prescriptive table:
|ASHRAE Standard 62.2 Table 4.1a
Continuous Whole-Building Ventilation Rate in cfm
|Number of Bedrooms|
|0 - 1||2 - 3||4 - 5||6 - 7||> 7|
|1501 - 3000||45||60||75||90||105|
|3001 - 4500||60||75||90||105||120|
|4501 - 6000||75||90||105||120||135|
|6001 - 7500||90||105||120||135||150|
Note of Caution
A blower door's annual estimate of air leakage is an average over all seasons of the year. Using Optional Calculations to reduce the whole-building ventilation rate based on this average means the home is likely to be seriously underventilated during mild weather.
This method is commonly used in federally funded, low-income weatherization programs for existing houses that have high infiltration rates. The required whole-building ventilation rate can be adjusted downward when the house is very leaky and the target for air tightening is higher than ASHRAE 62.2's default air leakage rate of 2 cfm/100 sq ft of floor area. It can only be used when the contractor can use blower door testing equipment and fan flow testing equipment, and it only applies to existing houses (not new construction). A contractor can choose to do one or both of the optional adjustments to the whole-building mechanical ventilation rate.
Sample House: To illustrate how to calculate the adjustment, here's an example using a 1,500 sq ft house with 3 bedrooms
|whole-building ventilation rate in cfm =|
|1500/100 + (3 + 1) x 7.5 = 45 cfm|
|45 cfm is the required whole-building ventilation rate that you will adjust.|
After completing air sealing, perform the final blower door test to get the predicted annual leakage number in cfm. Compare the predicted annual leakage number to the default of 2 cfm/100 sq ft. If the annual leakage number in cfm is larger than 2 cfm/100 sq ft, half of the difference can be deducted from the continuous mechanical whole-building ventilation rate.
Sample Calculation: Using the 1,500 sq ft sample house again, we calculated that it needed 45 cfm of continuous mechanical whole-building ventilation. The final blower door test, performed after all the air sealing work was completed, predicts 40 cfm average annual leakage. ASHRAE Standard 62.2 assumes that the 1,500 sq ft house has 2 cfm/100 sq ft of air leakage, or 30 cfm. Because the house is leakier than ASHRAE's default assumption, the contractor can adjust the whole-building ventilation rate down by half the difference between the default (30 cfm) and the measured (40 cfm) leakiness.
|Correction for leakage = (40 cfm predicted – 30 cfm assumed) = 10 cfm|
So half of 10 cfm, or 5 cfm, can be deducted from the whole-building ventilation rate. The final adjusted continuous mechanical ventilation rate is 45 cfm – 5 cfm, or 40 cfm.
ASHRAE Standard 62.2-2010, Appendix A Allows a
Second Adjustment Calculation to the Whole-Building Ventilation Rate
Because this calculation is not part of ASHRAE Standard 62.2-2007, it cannot be used for Title 24 compliance. The option applies only to existing homes or apartments (not new construction) that have inadequate kitchen and bathroom local exhaust ventilation. The calculation is used by some federal weatherization crews when:
- the existing kitchen range hood and/or bath fans are missing or do not have adequate flow to meet local exhaust requirements AND
- it is not possible to install or upgrade the missing or underperforming fans
Determine the amount of local exhaust currently provided in the kitchen and bathroom. In the example, the house has no kitchen ventilation and one old bath fan with airflow of 30 cfm. For an intermittently operated kitchen range hood and bath fan, ASHRAE Standard 62.2 requires a minimum of 100 cfm for the range hood and 50 cfm for the bath fan.
Sample Calculation: In the sample house above, assume no kitchen range hood is installed and the bath fan airflow is measured at 30 cfm. The house is missing 100 cfm of kitchen ventilation and 20 cfm of the required 50 cfm bathroom ventilation, for a total of 120 cfm of missing local exhaust ventilation. ASHRAE 62.2-2010’s Appendix A allows the whole-building ventilation rate to be increased to cover the deficiency in kitchen and bathroom local exhaust fans (120 cfm). Adding 25% of the missing local exhaust (25% of 120 cfm) to the continuous whole-building ventilation rate will meet the local exhaust requirements in the example.
25% of 120 cfm is 30 cfm.
30 cfm + 45 cfm (whole-building ventilation rate) = 75 cfm
So in the example, under ASHRAE Standard 62.2-2010, Appendix A, a continuously operated 75 cfm fan for whole-building ventilation will also meet the requirement for local exhaust ventilation.