While they are referred to as "unvented" crawl spaces, sealed, "conditioned" crawl spaces require a nominal air flow for ventilation. I'm not familiar with Canadian codes, but the 2018 International Residential Code gives a couple of alternatives to meet the requirement: either a continuously operated mechanical exhaust (e.g., a fan) at a flow rate of 1 cubic foot per minute/50 sq. ft. of crawl space area or a conditioned air supply delivering the same flow rate of air from the living area. Both options require a path for returning air to the common area. Typically this is achieved by installing a transfer grill located in the floor.

I take exception to the code requirement to return air from the crawl space back to the living area. The last thing you want is to introduce air which could contain soil gases, rodent droppings, insect parts, mold spores, etc. back into the living area and certainly not via a direct connection to the HVAC system which you've described. For the majority of homes the air flow rate is small and doesn't risk depressurizing the home.

Charles: I guess most "crawlspace remediation" experts/con-men would say, as part of their sales puff, that there's no way "oil gases, rodent droppings, insect parts, mold spores, etc." could possible get through their minimal-gauge plastic sheeting, for which they charge an unGodly sum!

;)

Most likely!

The "unvented" crawl space portion of the code is probably one of the least understood portions of the code--- by those supporting its inclusion in the code. Hopefully the dumb stuff included in the current version of the code will be corrected in the next code cycle. There is also a code requirement to install a dehumidifier in an "unvented" crawl space. We've been constructing homes with conditioned crawl spaces for 23 years (which required code modification approvals until the option was included in the prescriptive code) and have never needed a dehumidifier to maintain proper relative humidity.

Building Science Corp. says:

"To remove any small incidental sources of moisture, it is important that some air circulate from the living space into the crawlspace. It is for this reason the approach is called a “conditioned crawlspace” not a “unvented crawlspace.” Flows of 50 cfm per 1000 sf when the mechanical system operates are recommended."