Exhaust fans and make-up air

Phius Certification Manager Lisa White weighs in on a common challenge for passive house designers.

When it comes to exhaust for the kitchen range/cook-top, either a re-circulation hood or direct exhaust hood is allowed in Phius certification, and among projects in certification, there has been no dominant approach — we see it both ways about equally. 

When projects use kitchen hoods that exhaust directly to the outside, it’s common practice to provide makeup air relief when the hood is in operation. This is because that additional exhaust airflow is separate from balanced ventilation system, the building is very air-tight, and without pressure relief, the exhaust airflow causes slight depressurization in the building.

Phius has been asked to clarify if makeup-air is required for projects when direct exhaust is utilized in the kitchen. The short answer is, it depends. The longer answer is below.

The ventilation balance  requirements for certification are outlined in the PHIUS+ Certification Guidebook v2.1 Section 3.5.3.3, and copied below:

Regardless of type, the ventilation system must meet one of the following requirements for balance:

1. Total measured supply and exhaust airflows are within 10% of each other. (Use the higher number as the basis of the percentage difference.)
2. The total net pressurization or depressurization from the un-balanced ventilation system does not exceed 5 Pa. The net pressurization/ depressurization that the ventilation system imbalance causes on the building is determined using the multi-point air-tightness test results graph.

Intermittent exhaust airflow rates for kitchen exhaust hoods are generally much higher than a continuous exhaust airflow rates in the kitchen.  For example, a whole house may have a total of 150cfm continuous, balanced ventilation, and may have a 125cfm kitchen intermittent exhaust hood. With this combination, option 1 above would likely never pass.

PHIUS has established a method for determining compliance with option 2 during design. A stress test must be used to see if this intermittent ventilation system would cause more than 5 Pa of depressurization in the building. For a single unit building, the stress test is simply measuring the effect of turning on the range hood. If that airflow rate causes more than 5 Pa of depressurization in the building, there must be a provision for makeup air. For multi-unit buildings, an appropriate ‘stress test’ has now been defined that is both conservative and realistic.

Try out the Intermittent Exhaust Allowance Calculator here.