Exhaust fan in room with gas water heater and furnace

Inspected a townhouse with a laundry room that has an exhaust fan and also has the gas water heater and gas furnace in the same room. There appears to be sufficient combustion air (through vent to crawl space) and draft air (through vent to attic).

My question is…

doesn’t the running of the exhaust fan while the water heater and/or furnace are operating (burning) cause a potential backdraft situation that could pull combustion air into the laundry room and potentially into the rest of the home?

Or is this not a problem because of the combination of:

  • the combustion and draft vents
  • draft diverter (water heater) and draft hood (furnace)
  • the force of hot air rising
  • the relatively weak overhead exhaust fan.

I can see it either way…

Any input would be appreciated…

Hi Jeff,

How large is the utility room? How big is the fan? Are there levour vents on the door? Some things to consider. What I don’t like the most is drawing combustion air from a crawl space.

Erol Kartal

Hi Erol,

Utility room is about 8’ x 8’
Fan is about 9" x 9" like a standard bathroom fan
No vents in the door. Standard hollow core interior door

Getting combustion air from a well ventilated crawl space is pretty standard in California.

Water Heater 40k Btu/hr Furnace 80k Btu/hr 120k total
Combined upper and lower vents into utility room is approximately 168 sq inches. More than adequate.

I would have said OK if it had vented doors - now I don’t know. :neutral:

I have been in well vented crawls that smell like car exhaust. All kinds of pollutants in damp ground. Mold spores, animal feces etc; Just have a bad feeling about drawing air from them.

Erol Kartal

A worst case scenario test should be performed. That is starting exhaust fans and all clothes dryers while measuring draft. Then start water heater then shut down. Then start furnace and then shut down. Then start furnace and water heater together. Undiluted flue gas carbon monoxide is to be tested for elevated levels. Loss of draft or combustion gas spillage during this test needs to be corrected.

Add a 240 BTU’s for a gas dryer too.
What I see as a potential problem is that natural draft appliances don’t have positive pressure on the flue so that means that a powerful vent fan could in essence "pull " products of combustion down the flues back into the room. If the fan where very powerful.
For louvers, calculate @ 80% free area.

The unknowns are how tight the room is : Door / windows sealed and or if the combustion air is available at all times… You need a low and a high ducting / vent system to maintain air flow. Sounds like that is there , 1 square inch for every thousand BTU’s and working on a 10:1 air gas ratio for proper combustion.

What happens with the fan is not working … Where is the air flow… You could try this: With the FAU and the water heater running use fake smoke or something like it at the draft hoods to visually inspect behavior of draw at hoods…Then try it with fan running… All things to question… Need pictures of room , door, vent , fan…:smiley:

When providing combustion air only grills should be used, no registers. Whether the grill is wood or metal does make a difference in sizing.

Per Peoples Energy of Chicago:

Louvered wood doors consider 25 % free area.
Metal louvers and grills consider 75% free area.

So a metal louver 10"x 10" = 100 square inches/ take 75% of that and you get 75 square inches.:smiley: