Downdraft-1 out of 3

[lotsalogs]

My Setup:
I have 3 separate flues in my chimney. The chimney runs through the center of our home, in open space for 3 floors. All 3 flues exit at the same level, which is approx 4' above the highest point of the roof, and have individual chimney caps (screened). 1 flue is for my LP water heater and gas boiler heater vents located on the basement level. The center flue is for the wood stove located on the second level. The 3rd flue is unused, but was used for a wood stove located on the basement level. The masonry thimble for the unused flue is capped and sealed. All 3 cleanouts are located at the basement level.

My issue:
With no appliances running, I have a positive updraft on the 1st flue and the second flue. I have no issues with draft on either on these. The unused 3rd flue, however has a constant downdraft regardless of what the other appliances are doing. This becomes an issue when the other appliances are running, because it draws exhaust gasses from the other outlets down the third flue and into my basement.

If I have good draft on 2 flues, but downdraft on the third with nothing venting, what on earth is going on? The flue is clean and intact.

Thanks!

[HayZee518]

natural flow through a chimney or "flue" is afforded due to the fact that heat rises. burnt gases are lighter than air so naturally float upward. because of the fact that you have sealed off the wall thimble, there is still a backwards effect of air flow which is make-up air being drawn into your basement through whatever leaks you have in your trash [ash door] if you put a roof cap on the unused flue and seal around it and gasket your ash door then it would be as if it wasn't there to begin with and you shouldn't get a reverse flow

[lotsalogs]

Thanks HAYZEE. That makes sense...and it would stand to reason that, even when not burning, my middle flue has a positive draft because it can allow sufficient warm air to flow into the flue from the woodstove on the second floor.

Do I need to worry about degredation of the masonry flue due to trapped moisture after I seal it? We may make that room a den in the future, at which point I'd like to install another stove.

Thanks Again,
Bill

[lotsalogs]

Also, are the roof caps a standard, buy online item? Or, should I just bend some sheeting at the shop?

[HayZee518]

they should fit a standard "tile"