Hi Helbels, in a roundabout way the answer to your question is within the question!

You wrote "The point at which the fan exits the bathroom ceiling is lower than where the duct exits the building, so there has to be a rise on the duct pipe somewhere in order to reach the exit" - in essence that is the problem! the duct needs to have a high point higher than the wall/grille outlet to the fresh air.

This is the only way to stop condensation running back down the duct line and into the fan.

Whoever installed the ducting/fan orginally has not complied with building regulations as condensation in the duct is a known isuue, hence the need for the outlet to be below the highest point of the ducting. Seems strange that no one has mentioned this before?

The only other potential solution is to fit a condensate trap on the route of the duct at some point?

see; http://www.justfans.co.uk/condensation-trap-p-243.html

If this helps please click on "like"

It sounds like you need a condensation trap. if you cant find one at your local plumbers merchants try (just fans ltd on line). You will need to run the condensation off through an overflow if you go down this route.
Hope this helps-Kevin.

i would suggest maybe a more powerfull fan and change the ducting for insulated flexible ducting, which is also aluminium which also helps in the reduction of condensation build up.

basically your problem is the cold air from outside going into the duct and the warm air from the loft is causing condensation in the pipe and due to the fall in the duct its running into your fan.

There are a few solutions i can think of that will help prevent this.

The first is the outside grill. It needs to be a gravity grill not an open one. you can tell whether you have one of these by looking at the grill if it has flaps that only open when the fan is pushing air out you have got the right one. if no flaps are present this is a likely cause of your problem.

The other solution is a vent you can get that fits inside the duct ( i have only seen these for flexible ducts) and prevents the air returning down the duct from outside.

The last solution is a extractor fan that has a trickle system. this is where the fan constantly runs at a really low speed you cant hear the fan running (so it wont be annoying a night time) as this is constantly pushing air out it should stop the water building up in the duct .

i would try these solutions in order as the first is the most cost effective with the last being quite expensive hope this helps you solve your problem.

Chris,
Whittaker Electrical

Hi, My first answer would be to lag it but as you mentioned that has already been tried and failed.

You didn't really say how long the run was from the bathroom room to the vent, but I'm guessing it is a relatively long run.

My next thought is that the fan you had fitted 5-6 years ago had a higher extraction rate than the bathroom fans you are currently fitting (perhaps a kitchen fan). It may be worth fitting a kitchen fan (providing it has the correct IP rating for its position in the bathroom), I dont know how much you know about bathroom zones but a Part P registered electrician will understand them.

The point is that the airflow of a kitchen fan is much higher than bathroom fans, therefore getting rid of the moisture before it has a chance to condence in the ducting.

Just a thought.

Hope that helps.

Allan.

has the outside grill got a backdraft shutter or cowl fitted to stop rain entering? also the duct if longer than 2m should be insulated to stop condensation (pre insulated flexiduct, foil covered is best) and make sure the run on timer is working, after light turned off the fan should run on for at least 5minutes to get rid of all condensation, have seen this before and its been because new fan had short run on time so worth checking, hope this helps