Inconsistency in logic for extractor condensate

[sxdave]

One of the reasons I have seen for not routing the condensate from an extractor fan outlet into the soil stack (at a point above all sinks/toilets/etc that drain into the stack) is that smells would come from the stack into the room containing the extractor fan. I can see the logic in this but what appears contrary to this is that I have seen AAVs fitted in locations within a room (eg loft conversions etc) but there is no obvious problem with smells. The AAVs probably have a better seal on the one way vent in some way but that, to me, begs the question why there is no better seal available for extractor vents. I have some thoughts on this but would like to get some input that is not biased by my own ideas. Thanks in advance.

 

[snowhead]

AAVs are normally closed and only open when the pressure inside the pipe is lower than the outside.
Airflow will always be from the room through the AAV, when it's open and into the pipe, no risk of smells.

 

[Ric2013]

Why would there be condensate from a extract fan? Is it a heat-recovery type?

If you have a trap or a waterless trap (e.g. HepVo) between the fan and the soil stack, then you shouldn't have a problem with smells. Waterless might be better if the condensate is a seasonal event as a trap relying on water will dry out and stop sealing when unused for an extended period.

 

[sxdave]

AAVs are normally closed and only open when the pressure inside the pipe is lower than the outside.
Airflow will always be from the room through the AAV, when it's open and into the pipe, no risk of smells.

Thanks for taking the time to reply. I think the pressure differential you describe is the same for the extractor fan. The extractor fan pressurises the air to force it to flow. The force/pressure of the flowery the extractor fan opens flaps that are built into the fan. As far as I can see these flaps only open when the fan runs. A pressure differential in the opposite direction would close the flaps in the extractor fan (so air flow in one direction only, out of the room). For some reason I have yet to find out (but have suspicions about) the flaps in the extractor fan do not seal anything like as well as the seals in an AAV.

 

[sxdave]

Why would there be condensate from a extract fan? Is it a heat-recovery type?

If you have a trap or a waterless trap (e.g. HepVo) between the fan and the soil stack, then you shouldn't have a problem with smells. Waterless might be better if the condensate is a seasonal event as a trap relying on water will dry out and stop sealing when unused for an extended period.

Thanks for replying.

There would be condensate from the extract fan because it will be taking warm moist air (from kitchen or bathroom) two a location that will usually be cooler (outdoors) and therefor unable to retain the moisture. For example an extractor fan I am looking at specifies a condensation trap to be fitted when the fan in mounted in the ceiling.

I will look into your mention of the HepVo trap.

 

[Ben-gee]

On any fan I have seen the ‘backdraft flaps’ have extremely weak springs, often they will rattle in high winds - they are open/closing so smell could come in.
They are a plastic disc, or set of slatted flaps and there is no rubber seal - so they don’t form an airtight seal , they just prevent a howling gale coming in.

Run 22mm overflow pipe from the condensate trap to the soil and vent, just form a trap in the overflow by making a downward ‘U’ using four elbows and some pipe. Although this could potentially dry out I have never had any problems with this arrangement in place.

 

[sxdave]

On any fan I have seen the ‘backdraft flaps’ have extremely weak springs, often they will rattle in high winds - they are open/closing so smell could come in.
They are a plastic disc, or set of slatted flaps and there is no rubber seal - so they don’t form an airtight seal , they just prevent a howling gale coming in.

Run 22mm overflow pipe from the condensate trap to the soil and vent, just form a trap in the overflow by making a downward ‘U’ using four elbows and some pipe. Although this could potentially dry out I have never had any problems with this arrangement in place.

Thanks for your reply. As you say, the backdraft flaps are pretty weakly retained in the closed position. I assume that is so that very little fan pressure is needed to open them when the fan operates.
Thanks for your suggestion of the 'U' trap. It is useful to know that in practice you found no problem with it drying out.