Crawl space air

Any special requirements regarding foundation vents for supplying a medium efficiency gas furnace in an under floor area, e.g., not less than 2x the required openings?


The minimum combustion air requirement for the furnace can be found in the installation manual. If the crawl space does not have enough vents to meet this, they can always add a combustion air pipe from the exterior.

Best regards,

Don’t forget to add in the combustion air for the water heater or any other gas fired appliance.

CodeCheck has the current model requirements for combustion air which can be used as a guideline, absent specific local provisions to use as a guide if known … JMO

Hi Robert where you been hiding have missed you welcome back.:slight_smile:

Hey Crarlie, been pretty busy lately so I have been mostly lurking in the educational and structural related areas … but I try to check in on the HVAC topics occasionally since it is one of my specialty areas.

Concerning the equipment in a crawl space, the area should be vented for moisture escape anyway (unless conditioned), which can provide the required combustion air. For C.A. from an outdoor source a general rule of thumb is to look for vents up high with about 1.0 sq. inch per 3,000 BTU’s of equipment rating. So if the equipment is rated at say 120,000 BTU’s, then there would need to be 40 sq. inches of intake vents.

Even a crawl with a good floor vapor retarder (e.g. rat slab) requires air vents with 144 sq. inches per 1,500 s.f of area.