Passive House residential ventilation seems like a fairly straightforward topic on the surface. You install a whole-house heat-recovery ventilator (HRV) or energy-recovery ventilator (ERV), exhausting stale air from the wet areas of the house and supplying fresh air to the primary living and sleeping areas. (I’ll use the term HRV to represent both types of equipment throughout the remainder of this article). Unfortunately, the Passive House design guidelines don’t always align with California’s ventilation requirements.
The Passive House Institute (PHI) calculates the Design Air Flow Rate as the largest of three separate calculations:
- – 30% of the total Ventilation Volume, which is calculated as the treated floor area (a quantity similar to conditioned floor area) times an average room height of 8.2 feet;
- – The number of occupants times 18 cubic feet per minute (CFM); or
- – The whole building exhaust air requirement, where specific exhaust rates are assigned to kitchens (35 CFM), full bathrooms (24 CFM), shower-only bathrooms (12 CFM), and water closets (12 CFM). Laundry rooms may optionally be assigned 12 CFM as well.
Typically, the Standard Ventilation Rate for both supply and exhaust, assuming a balanced ventilation system, is then calculated at 77% of the Design Air Flow Rate. You can break up each 24-hour day into up to four separate flow rates, but this is typically not done in residential projects. A common scenario is to assign one hour or less to high-humidity events like baths, showers, and cooking, where the flow rate is boosted above 77%, with the remaining hours staying at the Standard Ventilation Rate.
The whole-building Air Changes per Hour (ACH) is then calculated as the average of the flow rates for each of the four parts of the day. The minimum ACH must be at last 0.3, considered the minimum rate for hygienic purposes. Rates higher than 0.4 might be flagged as potentially resulting in lower-then-desired indoor humidity during the winter.
Once a Standard Ventilation Rate is selected, airflows are allocated to the individual rooms in the house. How much air gets allocated to each room depends on which of the three ventilation calculations determined the Design Air Flow. For small houses, especially with multiple bathrooms, the winner may be the room-by-room exhaust rates. For larger houses, the Ventilation Volume or the number of occupants may be the determining factor. Each situation is unique.
California has its own separate residential ventilation requirements. There are two: whole-house ventilation, and local bath and kitchen exhaust, all drawn from the ASHRAE 62.2 standard. The whole house rate is based on the conditioned floor area and the number of bedrooms. The local bath exhaust rate is 50 CFM, switched, or 20 CFM continuous. The local kitchen exhaust rate is 100 CFM, switched, or 5 ACH based on the kitchen volume.
In a California Passive House, a HRV is normally used to satisfy the Energy Code’s bathroom exhaust minimum of 20 CFM. (It would be silly to install a separate bath fan to satisfy that requirement if you have a HRV.) The allocation of the Standard Ventilation Rate to each bathroom needs to meet this requirement. A HRV is not typically used to satisfy the Energy Code’s local kitchen exhaust requirement. It’s advisable to keep the HRV kitchen exhaust inlet away from cooking locations so as not to get fouled with cooking grease, and use a separate range hood.
When designing a residential Passive House, you need to make sure that the two ventilation standards are reconciled prior to locking in a house’s mechanical design. It could spare you some headaches.
Steve Mann, Home Energy Services
Certified Passive House Consultant, Tradesperson, Certifier, and Trainer
LEED AP+ Homes, Certified Energy Analyst, LEED for Homes Rater, and California HERS Rater.