Plumbing help – Condensation run off into soil stack

I need some advice on what to do. I’ve installed some rigid ducting for a new extractor fan. This has a vertical section going up to a tile vent and the fan is at the bottom of this. As per the inline fan installation instructions and general online advice, I’ve installed a condensation trap at the bottom of the vertical duct, just before the fan.

Nearby is the top of the soil stack which vents into the roof. This reaches the vent via two flexible pipes. I’d like to connect the condensation run off into this (running to eves is not possible). Ideally I’d just replace one of the flexible pipes with one with a boss, or fit a boss between the two flexible pipes.

Here’s a not very good picture of the two flexible pipes

I bought this
https://www.toolstation.com/viva-900-flexible-pan-connector-inc-pipe-boss/p44790
But now I have it in my hands it seems to small in diameter

Another picture

If you connect the vent from your soil stack into your extractor fan, won’t you get pleasant whiffs descending into the bathroom when the extractor isn’t working?

Even the best fan dampers let air past. I think you’re asking for toxic air coming straight into your bathroom unless you’ve created a mini p-trap. But even then it probably isn’t handling enough water to keep a trap full.

You’re better off running this down to your bathroom sink and into the sink drain between the plug and trap surely.

The easiest solution probably isn’t the best.

How about running the condensate into the guttering? Should be a decent fall and no sewer gas to control.

This is setup I’m working to

The run off would be 21.5mm waste pipe into the soil stack, rather than out of the eves. Air from the soil stack entering the bathroom is a possibility I suppose and something I had in the back of my mind.

Access to the soffits will be tricky as this is a room in the eves/roof space with a small attic above.

However, I might be over engineering the whole thing. I’ve installed the fan and ducting this afternoon. The total ducting used is just over 1.5m. The duct up to the fan runs horizontally and is under the insulation. The duct length after the fan is 0.5m. Essentially this setup would be to catch any condensation in that forms in that vertical 0.5m! I did just think maybe I should do a belt and braces approach with the condensation trap but maybe just insulating the 0.5m duct after the fan will be enough

use insulated ducting to to reduce any condensation

Rigid ducting for the short run was cheaper and less wasteful. (Could only find insulated flexi ducting in 10m lengths locally). Rigid is also better for airflow, which is one of the reasons the fan is being redone, that plus moving it over the shower. I could wrap insulation around the rigid duct above the fan. As I said, the other duct is under the insulation anyway.

The original ducting was insulated and I might have got a good section out of it. It has signs of mould in it and some sections are collaped, so renewing the lot made sense. The duct length has reduced vastly.

Just run the 21mm to the base of the soil stack
Cable tie it to the rear. Chances are the soil stack goes tp a chamber so let it in there and tundish it
Or tundisch it out the roofcand put a drop in then strap boss it into the soil vent

In my old house we had a centrifugal extractor going straight up in the loft space to a roof vent.
Vertical run was at least 2m.

The pipe was wrapped in a decent amount of lagging and we never had a problem with condensation running back down the pipe.

Maybe this condensation trap is overkill?

Maybe this condensation trap is overkill?

It might be. I think the only way is to run it for a bit then check. Or I could just fit one and forget about it.

@singletrackmind, would one of these (placed vertically) before entering the soil stack via a strap boss work?

https://www.screwfix.com/p/mcalpine-tundish-19-23-x-19-23mm/5684V

Yes