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I get the ashy fireplace smell in the family room when the A/C is running. I haven't had a fire burning in months.

I checked out this question, but it targets a recent fire and a cold air return.

It would seem that I'm getting negative pressure in the house when the A/C is running, being equalized through the fireplace. I have a Buck Stove insert that was there when we bought the house. Consequently, I don't have the standard flue closure mechanism. It's more of a stepped closure to control airflow while the stove is burning.

Is there anything I can do to prevent the negative pressure that comes with running the A/C?

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Are you sure it's negative pressure? A/C doesn't cause negative pressure like exhaust fans or dryers do. Could it just be the cold air return in that room is 'over zealous' ? You might experiment by blocking the nearest return off. – HerrBag Jun 12 '13 at 22:03
@HerrBag, The thing is, the smell is really only in that room, so I don't think the actual ventilation system is actually involved. At least, that's why I surmised it was a pressure thing. – JoshP Jun 13 '13 at 0:41

One way would be a Heat Recovery Ventilator(HRV) that exchanges interior air for outside, which will relieve negative pressure.

Another would be a dampered (and screened) make-up air that connects to the cold air return (a low tech (and low cost) HRV). These can be tied to exhaust fans or automatic via pressure differential. You would want a HVAC firm to discuss options.

A third would be a cable operated chimney cap damper. The stove may complicate this option. I had one on a naturally aspirated fireplace and was good during rain and windy conditions that used to produce that "old fire" smell.

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