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Community and Q&A

Airtightness and Combustion Backdrafting

Cortland15B | Posted in General Questions on

Hello, I just had a blower door test done on my 2800 sqft house and the results were 4.1 ACH. I want to improve on this but my water heater is still using internal air for combustion. The house has 1 bathroom fan and the kitchen is a charcoal filter recirculating range hood as of now. How tight until I need to start worrying about backdrafting problems? The furnace is HE and has external intake and exhaust. Thanks!

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  1. canada_deck | | #1

    Have you checked to see how it is doing right now? Do you have a CO monitor near the hot water tank? Have you tried any of the techniques to see if there is already any backdrafting (preferably with the bathroom fan on and all windows closed.)

    Is it possible for you to make the mechanical room very air-tight vs the rest of the house and then install a vent to the outside for make-up air just for that one room?

    1. Cortland15B | | #3

      The water heater is in an open basement and it vents out the chimney with a sleeve in it. No utility room to vent like that. There is a CO monitor a few feet away from it, no issues with it so far. I’ve held up things checking for backdrafting and haven't seen any issues though the bathroom fan hasn’t been running.

  2. Andrew_C | | #2

    Short answers:
    You are now tight enough that you should have a mechanical ventilation system. This should not dissuade you from trying to get down to 3ACH50 if you still have some reasonably low-hanging opportunities to improve airsealing.
    However, if you’re not going to put in the ventilation system first, I’d suggest doing what you’re going to do soon anyway and get a new water heater that is either a) electric or b) some sort of direct vent variant. While you’re at it, get rid of your gas range and gas dryer, if you have them. You still want to vent all-electric houses that are <3ACH, but the dangers will be lower and air quality better than if you’ve got gas appliances.
    These opinions are perfectly true, even if they are viewed through the bottom of my near empty glass after most of the Big Ten teams have been eliminated from the tournament. ;(

    1. Cortland15B | | #4

      Are you talking about a HRV when you say mechanical ventilation? I read somewhere that it wasn’t needed until you get below 3 ACH.

  3. GBA Editor
    Kiley Jacques | | #5

    The International Residential Code says when a house measures 5ACH50 or less, mechanical ventilation is required. This should be of interest: Adding Mechanical Ventilation to Old Houses, and here is a BS* + Beer Show episode you should check out: Playing With Fire: Indoor Combustion.

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