Negative Pressure - Any ideas?

[deebs]

Hello everyone! In our house in northern New England, we are dealing with some negative pressure issues that cause smoke to enter our full basement while running our fireplace on the first floor. Here are some pertinent details:

• The fireplace is on the first floor and has an ash catch running down to the basement.
• The fireplace utilizes an outside air kit that comes in through the ash chute. We only run the fireplace with this open.
• The masonry chimney contains two separate flues, one for the fireplace, the other for an oil-fired boiler in the basement.
• Both flues have their own ash catch in the basement, on either side of the chimney. They appear to be separated by masonry of some kind.
• Both flues exit the chimney at the same height, under a large stone cap that sits maybe 2-3 feet above their exit supported by four corner posts. I attached an example photo (this is not ours, just something I found that is comparable)

Sometimes while the fire is going, smoke trickles into the basement through some place I cannot distinguish. It gets bad if we let it go unwatched or if no windows are open and would certainly set off our CO detectors/smoke alarms if we let it get to that point. It is especially bad during shoulder seasons, when the boiler isn't running enough to keep its flue hot all the time.

I understand the basics of what is happening here (please correct me if I'm wrong) - the fireplace needs a ton of air, and is grabbing it from the only place it can in our tightly sealed home: the boiler's flue. Obviously a previous owner had this issue as well and installed the outside air kit. We have also gone crazy with silicone sealer on the boiler trying to fill any gaps we can find in the ducting elbows & junction with the chimney. Why isn't this solving the issue? What else can we do?

Thanks all!

 

[begreen]

The issue is not that the fireplace is grabbing air from the boiler flue, it's that both flues terminate at the same height and negative pressure in the basement is siphoning smoke from the adjacent flue. The common remedy it to raise the upstairs flue termination by 12-18" with a flue extension.

Is this a boiler or a furnace system? (boilers are hot water piped and furnaces are warm air ducted). Another option is to change the boiler or furnace to a high efficiency unit that can vent out the side wall with PVC piping. Then the basement flue can be capped at the chimney top to stop the siphoning of smoke.

 

[deebs]

Thanks begreen! I had read a bit about this on other posts and it makes sense - wanted to see what the community recommended for this specific case regardless. My only question is how is a flue extension accomplished on a masonry chimney with a stone cap like this? Seems like it could get pricey.

To answer your question about the oil-burner, it is a boiler, not a furnace.

 

[begreen]

Doing it is relatively simple, but it would change the aesthetic of the cap.

 

[masemase]

Can you tell me more about the outside air kit that connects to the ash chute? I think we could use one of those. Incidentally, your post is otherwise very similar to what I would post. Having same issues with makeup air in our place (in Vermont).