Take two pipes close together on a roof chimney. One comes from the

[qim]

Take two pipes close together on a roof chimney. One comes from the fireplace; the other is a vent from a toilet far away from the fireplace. The vent works fine and draws air when the fireplace is not burning wood. When it does there is a downdraught carrying smells (gases: carbon monoxide) from the burning wood.

What causes thsi? Is the hot air, possibly combined with wind, that causes the downdraught in the other pipe?

Help!

qim

 

[JaredJames]

The fireplace is heating the air. That warm air will rise. Some will rise out of the chimney as exhaust from the fire.

This air leaving the building needs to be replaced and so new air must be drawn from somewhere. In this case it's the toilet vent (there will be other places too, but the toilet vent is the easiest route and so draws most in).

As the hot air is escaping up the chimney, the cold air is drawn in through the toilet vent. The problem is, because they are close together on the oulet end, the suction of the toilet vent then draws in the exhaust of the fire, back into the house.

If you moved the toilet vent outlet end away from the chimney, I think you'd notice the problem disappears.

 

[qim]

Thanks. It makes sense.
However, there is another toilet with another vent and in that one I do not get the problem. I wonder why.

Besides, it is a large apartment and I would have thought that air could be replenished from other areas (i.e. opening, closing doors)

qim

 

[JaredJames]

Thanks. It makes sense.
However, there is another toilet with another vent and in that one I do not get the problem. I wonder why.

Besides, it is a large apartment and I would have thought that air could be replenished from other areas (i.e. opening, closing doors)

qim

You'd be surprised.

And it will generally follow the easiest route. As per most things in life.

The one vent may offer less resistance than the other.

 

[rcgldr]

So you're saying air is bubbling through the gas trap and water that's in the toilet? That seems unlikely. It's more likely there's some leak between the sewer and that vent pipe for the toilet.

 

[JaredJames]

So you're saying air is bubbling through the gas trap and water that's in the toilet? That seems unlikely. It's more likely there's some leak between the sewer and that vent pipe for the toilet.

I took it to mean an air vent in the toilet room itself.

If it is from the actual toilet for venting sewer fumes then I'm not so sure.

 

[rcgldr]

I took it to mean an air vent in the toilet room itself.

OK, that makes more sense, I gather the "smells" are the smells from the fireplace, not the toilet. Usually the chimney extends well above any other pipes to avoid this issue, and relies on the hot air rising to avoid this problem. Also there shouldn't be any vents that close to the chimney. You could try extending the chimney, which would be easier than trying to relocate the vent.

 

[JaredJames]

Again, that's how I envisioned it.

It depends on the building. My house for example, has a fixed brick chimney so altering that isn't that easy.

 

[qim]

Thank you all. I have a builder coming in today to have a look at it, but any more scientific explanations/solutions would be welcome.

qim