Crawl Space Combustion Appliances - Exhausting Gas from a Crawl Space
If there is a furnace or water heater in a crawl space that requires air to burn for combustion of gas or oil, you may need to allow air to enter the crawl space for that purpose. This air, once used to burn fuel, goes up the chimney. The idea is if you create a vacuum in your crawl space because you sealed it up so tight, then you will not have a draft to take exhaust gases up the chimney from your furnace or water heater.
We are talking about appliances that are "atmospherically vented", meaning they take in air for combustion from around their metal enclosures from the room they are in. Your water heater and HVAC unit are most likely atmospherically vented.
Fresh air combustion draws in air from inside the building to burn and exhausts it up the chimney. There is enough air leakage in a basement, even without vents, that adequate combustion air is not a problem.
"Normal air infiltration" in a building is considered adequate by the building code for combustion air supply. However, a crawl space is smaller than a basement, and you are sealing it up tight as part of your strategy to stop moisture. If there are combustion appliances located in the crawl space, there has to be a way for them to get air to maintain proper draft all of the time.
You must not depressurize a space where combustion appliances are located.
One easy way to provide combustion air is to install two vents in the crawl space ceiling to the first floor. This way air can be drawn down into the crawl space if the combustion appliances need it. Yes, this kills the idea of sealing openings in the crawl space ceiling to slow the stack effect, but if you have combustion appliances in the crawl space, it's better than the alternative, which is to leave a vent to the outside open where "evil" unconditioned (hot, cold, wet) outside air can enter. The International Residential Code 2000 allows for the installation of vents to adjoining indoor spaces for combustion air.
A combustion air supply unit is a good option. One unit called the "In-forcer™" is a fan unit wired to your burner circuit. When the burner turns on, the fan blows in make-up air, and when the burner is off, a damper closes to prevent outdoor air entry. For more information go to www.tjernlund.com.
For new homes, or when replacing your water heater or HVAC unit, you should use "direct vent" appliances, instead of "atmospherically vented" ones. Direct vent appliances have a place where you can attach a dedicated combustion air supply duct to the outside.
It is important to have a (CO) carbon monoxide alarm if you have combustion appliances or attached garages.
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