Decoding Depressurization

The discussion of the acceptability of a ‘large exhaust device’ is one that is often overlooked or disregarded by building officials and brings up some notable concerns.  First, a section from the  Ontario Building Code (OBC) that reference depressurization or makeup air parameters:

Part 9 of the Building Code applies to residential buildings and is pretty clear that if a solid fuel appliance (wood fireplace, wood stove) or a natural draft appliance (mid efficient furnace, standard efficient boiler or even a pizza oven) is present within a home the provision for a makeup air system is necessary to protect the safety of the occupants.  Further, if radon is of concern then again it should be managed.

What does managed mean?  In the simplest terms for all CFM being exhausted, less a small provision for infiltration, a makeup air system must be installed and interlocked to the large exhaust device.  When the range turns on…..so does a makeup air fan to simultaneously re-pressurize the home……or not allow any form of depressurization.  See two attached document for illustration purposes.

Part 6 – which is enforceable when 5 bedrooms or more are present in a home….or the home is 6400sqft or larger…..or there is a solid fuel appliance present…..see attached for the verbiage of it.  Within that section it notes that the large exhaust fan should not adversely affect “other exhaust devices” which would include HRV’s or furnace venting.  To me this again would suggest that makeup air is required is all cases…..which fall under Part 6 of the OBC.


In order to manage depressurization you may need a make-up air system (MUA).  We can perform an HRAI W-3 calculation to properly size this for your situation.
April 27, 2015 by Matthew Pedersen
previous / next

Leave a comment