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Community and Q&A

Use old furnace exhaust as makeup air?

CaplanAmerica | Posted in Green Building Techniques on

Climate Zone 5B (Boise, ID)

We remodeled a 1950s 2 story, daylight/walkout basement home. Full gut, new insulation, sheetrock, open floor plan, new slab on grade hydronic radiant floors downstairs, mini split upstairs (and upstairs master bath with radiant floors). The central HVAC system was removed. The radiant system provides 100% of the heat downstairs and 95% of the heat upstairs.

The home is ventilated via exhaust fans. There are 3 Panasonic fans in the lower level (2 bath, 1 laundry), a range hood kitchen fan in the lower level, and 3 fans upstairs (bathrooms). The fans are on wall switches, but they do run often and several of the fans are the variable speed type with occupancy sensors that we keep on all the time.

The boiler is in the lower level in the laundry room. It feeds the radiant flooring (closed loop) as well as an indirect hot water tank. This room used to have the furnace but everything was removed except for the exhaust duct that runs up the chimney. The laundry room runs warm all the time since much of the plumbing and radiant manifolds give off residual heat.

My question is, what should I do about the exhaust duct from the old furnace? Right now it is open and it acts like a makeup supply vent. When all the fans are off in the home, there is a slight exhaust effect (I assume due to stack effect with the warm air exhausting out to exterior via the chimney). When the fans are on in the house, the duct supplies air. This is somewhat convenient, since that room runs warm anyway.

Should I put a backdraft damper on the duct to prevent exhaust, but allow supply air? Close it up completely?

I should note that the exhaust for the boiler is separate, and the fireplace/chimney is not used for any other purpose.

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  1. GBA Editor
    Martin Holladay | | #1

    What you are calling "the exhaust duct from the old furnace" is more properly called a flue or a chimney.

    Is this a brick chimney? Or are you talking about a stainless-steel chimney?

    The main reasons that you don't want to leave this wide-open hole in your thermal envelope are these: (1) When makeup air isn't required, it's allowing a great deal of conditioned air to escape. (2) When makeup air is required, the incoming air may smell sooty, since the outdoor air is being pulled through an old flue.

  2. walta100 | | #2

    I say cap the flue pipe both top and bottom. Having this hole in your house make as much since leaving a window open 2 inches all winter.

    Most 1950s home will have more than enough leak to allow your fans to work.


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