What is combustion air, and why did my report flag it?

What does insufficient combustion air mean?

Combustion air is the air a gas or oil furnace, water heater or boiler pulls in to burn its fuel and vent the exhaust. "Insufficient combustion air" means its space is too small or sealed to supply it. Treat it as a carbon monoxide finding: have a licensed heating, ventilation and air conditioning (HVAC) contractor size the fix before your deadline, then ask the seller to pay for it.

Key takeaways

• The air rule adds up every fuel-burning appliance in the space.
• Owners often block interior vents with towels or rags. Check them first.
• Metal louvers count at 75% of their size in Montana State's example, so the opening must be bigger.

How serious is inadequate combustion air?

Fix it before you move in. The International Association of Certified Home Inspectors (InterNACHI), the inspectors' trade association, lists it as a cause of carbon monoxide (CO) spilling into the house. CO spillage "may occur" from "insufficient supply of combustion air to the heating appliance, as would be the case if it was located in a small, well-sealed room that lacks an outside air supply."

The Environmental Protection Agency (EPA) warns that "CO can kill you before you are aware it is in your home." If a CO alarm sounds or anyone feels sick near the appliance, the Consumer Product Safety Commission (CPSC) says to "get outside to fresh air immediately, and then call 911." Don't wait on a contractor for that, and don't run the appliance again until a licensed professional has checked it. CPSC also says to put CO alarms "on every level of the home and outside sleeping areas."

Can combustion air come from inside the house?

Yes, if the space is big enough. A New Jersey inspection firm, ARCH Inspections, notes combustion air "is usually considered readily available from the house air" as long as the furnace isn't in a small enclosed space.

The test is volume. Montana State University's weatherization training center gives the rule: "50 cubic feet per 1,000 Btu/h of the appliance input rating." Btu/h, British thermal units per hour, is each appliance's input rating. Add up every appliance that shares the space.

A 40,000 Btu/h water heater alone needs 40 × 50 = 2,000 cubic feet, and an 80,000 Btu/h furnace needs 80 × 50 = 4,000. Put both in one space and the total is 120,000 Btu/h, so the space needs 6,000 cubic feet.

A 5-by-6-foot closet with an 8-foot ceiling holds 5 × 6 × 8 = 240 cubic feet. That's far short of either figure. Openings also have to reach a room with enough air. As ARCH puts it, "It does little good to add openings to an adjacent small space."

How do you fix insufficient combustion air?

There are two routes, and a licensed HVAC contractor picks the one that fits your house.

• Openings to indoor air. On the same floor, two openings into a larger part of the house, one within 12 inches of the top of the enclosure and one within 12 inches of the bottom. To reach another floor, openings in doors or floors.
• Outside air. Ducts or an opening to the outdoors. In a basement, ARCH describes ductwork running from a screened hole in the outside wall down near the burner.

The openings are sized to the total input rating. Montana State's worked example does it for a furnace and water heater totaling 120,000 Btu/h. Drawing from the same floor takes two openings of 120 square inches each. From a different floor, it's 240 in total. Outdoor air through vertical ducts takes two openings of 30 square inches; horizontal ducts need 60 each, and a single outdoor opening needs 40.

Louvers block part of the hole. Montana State counts metal louvers at 75%, so 120 square inches of free area takes a 160-square-inch louver.

Before any of that, check the vents the house already has. US Inspect, an inspection company, says owners frequently block interior vents with sweatshirts, towels and rags because of the cold they let in. Outside vents get blocked too, with leaves, plastic bags, bird's nests and lint. Ask your inspector if that's what they saw. Either way, the contractor confirms the openings are big enough.

What does the fix cost?

It depends on the route, and a licensed HVAC contractor's written quote after they measure the room and check each appliance's rating sets your number. How to get repair estimates during the inspection period covers getting that visit booked before your deadline. While the contractor is there, have them check the rest of the system; what an HVAC inspection covers before you buy lists what to ask for.

For the other lines on your report, Kept prices every line of an inspection report for $79, usually under 4 hours.

What should you ask the seller for?

Ask for the correction by a licensed HVAC contractor before closing, or a credit for the contractor's quote. A line like this gives the seller something specific to act on:

Furnace and water heater closet: correct insufficient combustion air per licensed HVAC contractor's written scope. Provide the paid invoice and any permit before closing.

What to ask for after a home inspection shows how this line fits with the rest of your list. The buyer repair request letter template has the wording around it. Your contract sets the deadline for the request, so confirm the date with your agent.

What if the seller says no?

Don't drop this line to keep the deal smooth. It's a CO item. Counter with a credit for the contractor's quote, or a lower price by the same amount. If neither works, budget to have it fixed before anyone sleeps in the house, or cancel if your contingency allows it.

Is a water heater in a closet against code?

It depends on the code your city has adopted. Your local building department or a licensed contractor answers for your address. As one example, InterNACHI cites the 2006 International Residential Code as barring fuel-fired water heaters from a room used as a storage closet. Keep the closet clear of stored things either way.

How do I price the rest of my report?

This line waits on the HVAC contractor. The rest of your list doesn't have to. For other furnace and air conditioning items, see heating and cooling findings on an inspection report.

While you wait on the HVAC contractor, upload your inspection report to Kept and put real numbers behind the rest of your request.

Sources

• US Inspect, Combustion Air: What Is It? Why Is It Important?
• InterNACHI, Spill Switch Inspection
• InterNACHI, Backdrafting
• EPA, Carbon Monoxide's Impact on Indoor Air Quality
• CPSC, Carbon Monoxide Information Center
• Montana State University Extension, Combustion Air Examples
• ARCH Inspections, Combustion Air for Furnaces