Combustion Air?

Wonder if there is enough combustion air for this furnace? The furnace and hot water tank are in a utility room together. The door to this room is solid. The only air coming into this room is in the lower right hand corner of this picture.

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Is the ceiling open joist work or closed in with drywall?
Where does the fluepipe/s leave the area?
Any signs of backdrafting/smoke spillage?


  1. Is that air supply from the exterior or the building envelope?
  2. Is the solid door hard to open when the furnace is working?
  3. What type of furnace is it, high efficiency, natural draft, induced?
  4. Is the chimney lined if used?
  5. More info is required dude

[FONT=Verdana]Certainly these appliances are spillage susceptible if natural draft.[/FONT]

? 1 The air is coming from the attic.
? 2 No
? 3 Natural draft
? 4 Yes
This is something I came across.

"Combustion air (“make-up air”) is required for gas burning appliances at a rate of 50 cu ft / 1000 BTU or else external ventilation is required. External ventilation openings should be within 12 inches (305 mm) of the top of the enclosure, and one within 12 inches (305 mm) of the bottom of the enclosure. Inadequate supply of combustion air results in incomplete combustion; products of incomplete combustion include carbon monoxide. "
Do any of you do this extent of calculations?

When I find mechanical’s with combustion chambers, my report recommends a louvered door.

Hopefully HVAC experts will correct these theories if wrong. :stuck_out_tongue:

A possible thumb rule would be make-up air or combustion air should be at least equal in cross sectional area to exhaust air. So a water heater with a 3" vent should have a 3" air supply?

Louvers reduce the surface by 40 or 50%?

Far better to err on the side of more than enough?

John Kogel

Here’s some more confusing info on the subject, :p.

Whoever wrote that was a genius!

Add: “Sizing calculations were not conducted. The room housing the appliances may be inadequately ventilated for the purposes of providing combustion air and we recommend an evaluation and repair by a licensed plumbing or HVAC contractor.”