The strong ash smell may be from an inversion in temperature where the air outside is warmer than the air in the chimney so that air drops pushing the smell into the house if the damper is not sealed shut.

I think you need to explore a little more the correlation between the fireplace and the sewage smell. Why would it stop if you ran water down the drain? I think there may be a couple of different things going on. Perhaps others will share their opinions.

Somehow when you light the fireplace the increased draft created seems to be pulling the sewage gas down into the house. How this could be happening is very hard to say. The only possibility I can think of is a partially clogged vent pipe - leaves, even birds' nests can do this. Because of a clog the vent pipe draft would be very weak. It's sufficient most of the time, but when the fireplace is lit, it's not enough to overcome the draft created by the fire. Not a very satisfactory explanation, because it does not account for how the gas is bypassing the trap unless it (the gas) is seeping out of the vent pipe above the trap. I hope somone else will come up with an explanation, but cleaning out the vent pipe would be something to try.

I agree, there's a sewage vent problem somewhere.
The fireplace sucks air from inside the house, and moves it up the chimney. So when you smell sewage, there's a leak or break in the vent pipe somewhere inside the house.

Time to go hunting with the fireplace lit and follow the smell.

Yes, unless outside air is provided, a fireplace will suck the air from inside the house when a fire is burning. That is why open fireplaces are so inefficient. You are sending your heated air up the chimney. You literally can not create enough heat from the fire to overcome the amount of heated air going up the chimney.

Regardless of how your particular problem works out, If you use your fireplace regularly, you should add an outside air source for your fireplace.

To temporarily solve your problem you can try cracking a window open slightly close by to your fireplace.

Have you recently replaced the toilet in that bathroom with one of the new "power flush" versions? They void the water really quickly and may suck water out of an adjacent p trap if the venting is not sufficient. This would leave it open for the sewage smell to come into the house. It would also explain why the smell would stop when you ran water down the drain filling the p trap again.

It may be possible that the extremely hot temperature of ash is making the sewage stuff evaporate causing the foul smell. I think a plumber or someone who is expert in sewage treatment may help you.

Hello. We are dealing with the same issue in our house with the sewer smell When using out fire place. Are there any recent updates on this?

We are dealing with the same thing right now. I just reached out to our plumber to see if they can help troubleshoot.

We have a wood stove and are finding the same issue! Going to check the vent.

Wondering if anyone has an update on this. We have the same issue, and the smell is coming from the bathroom sink…thx!

After going nearly crazy last winter/early spring with trying to figure out why this was happening (our plumber was involved too), we ended up installing odor hogs on the vent pipes on our roof and knock on wood— we haven’t had an issue since!

We have the same issue from our kitchen sink….will be following! it stopped smelling bad for sure by the morning and after the fire was out. i might’ve notices it another time, but didn’t put 2 and 2 together until this most recent time.

Sarah we have the same problem. Who indtalled the odor hogs on the vent pipes? Thank you! Maria

Hi Maria,

We had our plumber install them. We just replaced the charcoal bags a few months ago for the first time after I started to smell it again slightly in the front yard. So I'd say we are on the once every year or two program to replace the charcoal bags--but it's totally resolved the issue!

Thank you very much!