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I have concerns about replacing fresh air to my central forced air furnace after installing exhaust fan system

Rory Kennedy | Posted in Energy Efficiency and Durability on

Renovated the basement in my bungalow (1425 sqft). I have installed an exhaust system in the basement that has 4 vent diffusers (2 in games room, 2 in the home theater) this is powered by an inline exhaust fan (438 cfm) and ducted to the exterior of the house with 6″ ductwork.
My concern is am I removing too much air from the house and impacting the combustion air required for the furnace? There is a 6″ freshair intake to the return air of the furnace and the house is 35 years old and has typical “natural leakage”. Should I upgrade the intake (bring in more) or am I worring over a marginal amount of air loss.

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  1. GBA Editor
    Martin Holladay | | #1

    When designing a ventilation system, the first step to is to calculate the ventilation rate. Most people use the formula provided by the ASHRAE 62.2 standard (7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area). For example, if your house has 4 occupants and measures 2,800 sq. ft., you need 30 cfm + 84 cfm = 114 cfm. (For more information on this topic, see Designing a Good Ventilation System.)

    Your exhaust ventilation system is almost certainly oversized. It is very rare that a home needs 438 cfm of ventilation. So the first step is to downsize your fan.

    Once that's done, you probably want to hire a HERS rater or BPI rater to perform a combustion safety test on your furnace.

  2. Expert Member
    Dana Dorsett | | #2

    There's a pretty severe energy penalty for that degree of excess ventilation too. If you're going to blast away like that, at least have it under occupancy sensor control!

    If it's 20F outside 70F inside, (50F difference) every 100cfm (6000cubic feet per hour) is blowing away 0.018BTU/degree-ft x 6000cfh x 50F= 5400 BTU/hr of sensible heat (not including moisture issues.) Since you're running about 300cfm more than you'll EVER need that's a huge background load, even at lower delta-Ts.

    During the cooling season that can represent a gia-NORMOUS latent load on the AC, even when it's not very hot outside, in most eastern US climates.

  3. Rory Kennedy | | #3

    Because the fan is already installed (taking it out would be a pain) could I regulate the air flow with a rheostat. The exhaust is specifically planned for cigar and cigarette smoke when entertaining?

  4. Dick Russell | | #4

    Martin, I see various "ASHRAE 62.2" formulas out there, and of course giving different results. For example, this one:,d.dmg&cad=rja

    says the ventilation rate should be:
    CFMfan = 0.01Afloor + 7.5(Numberbedroom + 1) + (alternative compliance supplement) - (Infiltration credit)

    which would be only 1 cfm per 100 sqft of conditioned floor space plus an allowance for each bedroom rather than for each occupant. Can you shed some light on the differing formulas and which is the proper one to use?

  5. Expert Member
    Dana Dorsett | | #5

    Unless the fan motor was designed for speed control via dimmers, you're likely to burn up either the fan or the dimmer (or your house) if you try it.

    Putting in on a duty-cycler/interval timer or putting it under occupancy sensor control would keep it from generating truly huge energy downsides, but the right thing to do is to yard it out and start over- it's ridiculously oversized for the application.

    Even ASHRAE 62.2 has no real science behind the numbers, and isn't necessary for maintaining indoor air quality if you manage your indoor pollutant sources prudently. (No smokin' in the boyz-room, etc.)

  6. Rory Kennedy | | #6

    The reality is there will be "pollutants" ie: smoke etc and the diffuser vents are located to maximize the draw from the heavy areas as quickly as possible. The motor is rated for speed control so there is no concern regarding the electrical functioning of the unit or the use of a rheostat. Having said all that and realizing that, through discussion, the unit is oversized for the job. Will I need to supplement the intake air to the forced air furnace. (concerns about "back drafting")

  7. Aaron Gatzke | | #7

    Maybe they only need an off/on switch as he says it is specifically used to remove cigarette and cigar smoke while entertaining. It sounds as if it may not be in use all the time?

  8. John Brooks | | #8

    I think Martin has already answered your "Backdraft" question

  9. Rory Kennedy | | #9

    That is correct. It is an event specific system. I should have been more descriptive, however I am still going to have some analysis on the furnace concerns. I appreciate the feedback from some obviously well versed people. I will incorporate a rheostat on the unit, I will evaluate the intake air requirements that were address earlier in the chain. If you can forsee additional bumps on this ventalation road I would like to hear them. Thanks

  10. GBA Editor
    Martin Holladay | | #10

    Dick Russell,
    It's probably time for me to write a new article on the ASHRAE 62.2 standard.

    For years, I advised people to follow the original formula from the first approved version of ASHRAE 62.2 (7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area). Then I received an e-mail from Max Sherman, a senior scientist at LBNL and one of the authors of the standard, stating: "The total ventilation in ASHRAE 62.2 is 3 cfm/100 sq.ft. plus 7.5 cfm/person, not 1 cfm/100 sq. ft. In the 2010 version there was a built-in infiltration credit of 2 cfm/100 sq. ft. to get the number you quoted for mechanical systems. In the current version, however, that default infiltration credit is gone. You can, however, get an infiltration credit based on a blower-door measurement."

    As you can tell, this makes it harder to summarize the standard in a sound bite or simple formula. I hope to explain these details in depth when I write the article.

  11. GBA Editor
    Martin Holladay | | #11

    The ASHRAE 62.2 ventilation standard specifically states that it is not intended to account for indoor smoking. Most health experts advise that cigarette and cigar smoking should occur outdoors.

    If people want to smoke indoors, it's possible that a huge exhaust fan may be necessary. (I still think it makes more sense to step outside to smoke, however.) But once you install a huge exhaust fan, you had better be sure that it doesn't cause your furnace to backdraft. It's time to hire a specialist to conduct a combustion safety test.

  12. Expert Member
    Dana Dorsett | | #12

    FWIW: I'd be shocked if the speed control on the fan is actually a rheostat (=variable resistor). If it's an AC motor it's likely to call for a specific type or model of thyristor based dimmer type control, if it's DC drive there are other types of control. Rheostats are grossly inefficient fire hazards, and haven't been used for this type of application in 50 years or more. (Even incandescent lighting dimmers moved over to all solid-state SCRs & triac thyristor type control by the mid-1960s.)

  13. Rory Kennedy | | #13

    Thanks Martin that is my intention. In response to Dana, who clearly does not think anything positive about this venture? The "rheostat" is a speed control dimmer type switch that is produced by the manufacturer of the fan unit, forgive my parlance.

  14. Expert Member
    Dana Dorsett | | #14

    Don't get me wrong, I'm all for designing in mechanical ventilation- it's absolutely the right thing to do as you tighten up a home. It's the OVER-ventilation problem that's an issue, especially when you still have atmospheric drafted fossil burners in the house- your concern about makeup air is valid!

    If it's possible to hard-wire the speed control to only run at low or mid-speed, not high, you may be able to avoid the potential backdrafting & spillage issues, but it still needs to be tested. All backdraft testing needs to be done with all exhaust fans (including clothes dryer) running, and tested with the air handler running AND not running, if you also have an atmospheric drafted hot water heater.

    In our own 1920s bungalow (a work in progress) at the point that it was time to swap out the boiler we made the conscious decision that all combustion appliances needed to be sealed combustion, with combustion air piped into the unit, which allows us to air-seal to the -nth degree without running into those issues. When we installed a wood stove, it too that dictum was followed, limiting ourselves to only units with outside combustion air kits. Now it doesn't matter- we can run all the kicthen/ bath/other exhaust fans with the clothes dryer running to boot, and there's no chance of backdrafting (as long as the door to the woodstove is closed, which it should be at all times other than when loading.)

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