Downdraft! Help please!

[Doc C]

I'm an industrial engineer and not a stove expert by any means. A lot of this can be explained by simple physics. All chimneys may have some downdraft when stove is not in use. This depends chimney location (and height) in relation to the house structure, outside wind and wind direction, any open windows, HVAC system recirculating air inside the house etc. This is normal. What's not normal is that you have smell caused by downdraft.
Adding an external air supply system will not reduce the downdraft but may relocate the smell to where the air intake will be. The air intake should be very tight, with no gaps to keep the smell from inside the house.
I find it a bit strange that you get smell from a stove when it's not in use. Perhaps it only smells when the stove is done burning and you simply let it cool down for a few days while there are some slow burning coals in the stove. This is probably caused by lack of updraft from the cold chimney. This can be fixed by installing a dual-wall chimney liner inside your existing chimney. This will keep the inner chimney pipe a lot warmer thus creating sufficient draft, even at lower exhaust temperature.
Adding cap on the chimney will help to keep the moisture out and possibly reduce downdraft. A properly shaped chimney cap will actually create additional draft (low pressure) in the chimney when wind blows across it. If the cap is missing, wind may create slightly positive pressure in the chimney thus increasing downdraft (Bernoulli's principle from middle school physics).

Smell from a chimney not being used is extremely common especially if it’s a masonry chimney. The creosote forms in the chimney and gets wet either from no cap or from humidity being absorbed into the creosote.

When the air comes down the chimney instead of up the smell is sucked into the house.

There are several ways to approach the problem but it seems the majority of people that try each and every fix still come back and say they can’t get rid of the smell.

 

[Gyprat]

Makes sense. My experience is limited to one woodstove which we currently use. The chimney is double wall insulated stainless pipe going through the attic and all the way up to the roof. I only get about a cup-full of creosote after a full season of burning wood. In my humble opinion, not backed by lots of experience, a proper insulated chimney liner (with other a proper rain cap on top) may help the OP with the smell issue.
I usually plug the chimney at the stove, during off season. We have never experienced a smell issue, even if leave the chimney open.

 

[Doc C]

Makes sense. My experience is limited to one woodstove which we currently use. The chimney is double wall insulated stainless pipe going through the attic and all the way up to the roof. I only get about a cup-full of creosote after a full season of burning wood. In my humble opinion, not backed by lots of experience, a proper insulated chimney liner (with other a proper rain cap on top) may help the OP with the smell issue.
I usually plug the chimney at the stove, during off season. We have never experienced a smell issue, even if leave the chimney open.

I’m sure it would absolutely help to line the chimney. The only caveat being the masonry is cleaned very well. Like beyond what I think most people would do. I have been burning creosote removal product in mine this year trying to get everything to loosen up in order to clean it really good and have it relined.

Also I think some folks use a poultice, at least that’s what they call it in my area, to chemically clean the chimney prior to lining. But the cost was a few thousand dollars for the cleaning in my case hence why I’m trying to do it the best I can.

 

[Speace0181]

Sorry it took so long for an update. I have since been doing really well with the draft. I replaced the 90 with two 45s. I also start with a top down fire which heats the flu and the firebox up quickly. I no longer have to leave a door open in the basement regardless of any appliances running. I still have a little campfire smell occasionally. But only when I leave some unspent coals a few days. Thanks to all who have left ideas and encouragement. Now you insulate the walls so I can run everyone out of the house with heat.

 

[Kevin Weis]

You need an outside air source to the stove air intake. Your basement is too airtight. If outside air is warmer then inside, this will exasperate a reverse draft issue. Kevin

 

[Speace0181]

Possibly, not saying I won't. However for the winter I don't believe it to be necessary. And if it is this summer i believe I would opt to remove and close up the flu for the season before boring through the faceplate of the house. If I had an issue with combustion air I might choose that option.

 

[yooper08]

If it's working great, I wouldn't go the OAK yet. I would focus on the chimney, even just from a safety standpoint as it should be lined. As Gyprat noted, OAK doesn't guarantee fixing downdraft. If you have low pressure near the inlet of an OAK and downdraft conditions exist, it can just downdraft to the outside.