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Question DetailsAsked on 10/3/2017

what is the proper venting for water softner, dryer, tankless water heater

in an old home we have a water softner, tankless water heater and dryer all venting out an old chimney we arew getting a toxic detergent smell , can that be causing the odor

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1 Answer


You did not say if these all have individual vent flues/ducts up through the chimney, but if not, then Whoa - RED ALERT. You should never have a dryer venting through a combustible fuel exhaust flue - MAJOR fire and carbon monoxide hazard. If that is the case I would guess the lint buildup in the flue has been catching fire ort at least scorching - or clumps may have fallen right down into the flue segment from the water heater. Get a Heating and A/C contractor in there immediately to check it out and get your venting right - and I would turn the water heater off till fixed, as the next time it fires up could be the time you get a flue fire (like a chimney fire).

By combining these in one old chimney (if directly vented into the chimney) you have created the ideal conditions for buildup of lint (getting damp from the water vapor in the exhaust gases) either molding (if staying wet) or clumping up with the moisture and now exhausting, which would eventually start collapsing and falling down in and blocking the flue, causing a fire hazard.

Now down to brass tacks -

1) I have never heard of a water softener needing a vent (other than the backflush drain pipe, which is carrying predominately liquid, not gas) - unless this is the rare distillation type purification unit, in which case if gas fired would have a flue.

2) Dryer should have an independent metal dryer vent, running the shortest distance feasible to an outside vent - and if at all avoidable, NOT up through the roof - should run as horizontal or downhill as possible to minimize lint buildup in the ducting.

3) water heater, since tankless, is most likely high-efficiency so it would normally have quite a small diameter (typically 2 or 3") flue, which if a condensing unit would normally be PVC pipe. If it is venting directly into the chimney (not through a flue pipe) that is against code, and is (unless maybe if steel flue lined of proper diameter) typically way too large for the flue regardless of whether high-efficiency or not.


Now- IF these have independent vent ducts/pipes up through the old chimney, not combined, then less serious in general, though as I read the code still illegal to run other ducting/pipes up alongside the flue in the chimney, but that is open to interpretation because the code does not really fully address low-temp exhaust flues and allows plastic pipe for them.

If these are independent exhaust flues, then likely the lint going out with the dryer exhaust has clumped up in the duct because it failed to make it to the roof (which is commonly too great a run length anyway if more than 1 floor below roof) and is probably falling back down to the dryer, and partly blocking the duct and likely scorching - causing the smell. If backing the airflow enough, then you could well be getting scorching in the dryer itself - in the lint screen or in the air pasaage through the dryer - a major fire hazard. If this is the case, might just need Dryer Duct Cleaning - but given your situation and likely code violations I would get a Heating and A/C contractor in there to assess your situation - and I STRONGLY recommend that if all possible you reroute the through-the-roof dryer vent to one out the side of the house (usually run in between the floor joists).

Either way I would not use the dryer till checked out at a minimum, and if all running together in the chimney (not in separate pipes/ducts) I would turn off the water heater too till fixed.

Answered 2 years ago by LCD

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