Bathroom Fan exhausts

We’re getting our bathroom done next week. We have a separate toilet and bathroom and we’re having a fan outlet in each.

Originally I was planing to use a single inline fan (Manrose MF100) sucking from bothrooms. I was advised this wasn’t a good idea and having done a little googling I think I agree.

So now we have two fans, one for each room. Again I was planning on joining the exhausts and having a single hole in the wall, e.g.

. . . . . . . . . . . . . .|<—- Backdraft Shutter <—- Fan1 <—- Bathroom
External Wall <—
. . . . . . . . . . . . . .|<—- Backdraft Shutter <—- Fan2 <—- Toilet

I’ve been told this also isn’t the best idea and better to core 2 holes.

Any thoughts – is there any advantage in practice to having 2 holes in the wall?

With one hole the bathroom grill would be < 1m from the external wall and the toilet grill around 3m.

With two holes each grill would be < 1m from the external wall.

I’d be asking whoever told you it wasn’t a good idea :p Only thing I can think of is maybe if both fans are running at the same time the exhaust of the fan with the longer ducting run might be compromised

In principle it’s only air flow* so as long as the pipes are large enough to evacuate the joint volume if the are going together (assuming two fans) and swept T or elbow joints to prevent swirling or weird are flows not going where you want, then I’d look to two into one.

I’d probably use one larger fan for the pair with the relevant diameter ducts.

*taking into account any condensate and the effects of it etc but, that’s the same no matter how many ducts etc.

Shorter pipe runs means more flow for a given pipe size. I’d go for 2 holes. If you can’t do that go for 125mm pipe rather than 100mm – bigger pipe means more flow.

If the exhaust hoses join, the you might end up pushing air from one bathroom in to the other – none of my bathroom extractors have shutters (butterfly valves? non-return valves?) at the bathroom end, so this is very possible.

While we’re on the subject, how do you intend to control them? I had a humidistat one once, but the bathroom was cold and poorly insulated so we were never able to find a suitable humidity setting. It was often always on in winter and never on in summer, so I drilled out the faceplate and would end up perpetually adjusting it to suit the weather.

There was a similar thread a few weeks ago – someone on here mentioned a type where if it was only one a minute or two, the fan went off straight away after the light. If it was on for longer, it would stay on for a time after the light was off. They sound good.

Otherwise I’d be tempted to wire them in separately – like with a separate switch – the family bathroom almost never needs the extractor on. The ensuite needs it on for much longer after a shower.

I have recently installed the fan the op mentions ( or a very similar one) with a separate humidistat and I am impressed with it.

I also installed that fan a while ago, venting out of a fake roof tile – it’s a proper beast (and quiet with it).

This is the reason why it’s better to have two holes in your walls…

Shorter pipe runs means more flow for a given pipe size

…and more flow is beneficial because it empties the room faster and clears humidity (bathroom) or smells (toilet) quicker!

Resulting in greater customer satisfaction I suspect :o)

i would be too worried about air flow issues causing backdraft, or overworking the fans. Im no fluid engineer.

Both people i got to quote for fans said they would have to core a hole through the wall and get a electrician to fit the fan because its in a special area under Part P.

Local roofing company put a tile vent in, and i put an inline in the loft, which has significantly better flow, is quieter, i could fit myself and didnt mess up my new drylining and tile job.
The roofing guys did it off a ladder, which saved a on scaffold or tower hire. Only took him an hour, and saved me falling off a ladder.
100 quid i think it cost.

I would get two tile vents fitted. (assuming your rooms are upstairs)

Yes + plus one for the tile vent. I did it myself with a ladder, was pretty easy (assuming you don’t mind heights!) and you can’t tell from the outside.

Somewhat conflicting advice I see :)

I was expecting that the backdraft shutters would eliminate backflow.

The fans will be individually switched (not on the lighting circuit or controlled by humidistats or timers).

I expect it will be rare that both fans will be active concurrently but no doubt it will happen occasionally.

I’m leaning towards a single exhaust for now and see how it goes, if it’s an issue then I’ll get a 2nd exhaust cored.

Intresting post.
I have fan ducts to put in for a new build we are doing.
The option of putting in tile vent sounds good but will there be a risk of condensation running back down into the fan?

I expect it will be rare that both fans will be active concurrently but no doubt it will happen occasionally.

I’m leaning towards a single exhaust for now and see how it goes, if it’s an issue then I’ll get a 2nd exhaust cored

You’ve allowed for backdraft shutters to select which fan is pushing to the outside (I missed that in my first reply, above) so I think you’re good. Crack on, and like you say, you’d consider going back and improving if you need to, so you’ll probably notice.

I am not a plumber/sparkie :-)

Coming back to this just got the quote in for the electrics, I think it’s taking the p***, am I being unreasonable?

Job is to do the following:

core external wall for the fan exhaust
wire up 2 fans as described and connect ducting, I guess core the ceiling for the grills
install 4 spots (core the ceiling again)
wire up a digital shower
4 switches, shower+fan, spots, 2nd fan, second light (switches are moving out of the bathroom).
We’re using wireless switches (quinitec) to save having to re-plaster the hall walls
– he needs to raggle out space for a single 16mm backbox + install the switch (no channel needed for the wire as they’re wireless)
second switch is in plasterboard so cut a hole for that

We’re supplying everything except the wire used to connect the wireless receivers to the mains ring and wires to shower, lights fans, i.e. we supply the fans, spots, wireless receivers, switches, back boxes and ducting.

Quote we have been given is £430 – it’s a lot more than I was expecting. We’re in Glasgow. Thoughts?

It’s less than I was expecting.  Coring an external wall can be expensive, even more so if it is at any height.  Does the shower not need to wire all the way back to the fuse box which will also make it pricey or is the wiring for that already in place?

The shower is off the combi, it’s a digital shower so needs a mains fuse for that, only 3A fuse.

What sort of wall is being cored? It can take hours and be very expensive in wear on the core drill.

The other big variable is running the wiring. How good is access? How long are the runs? What material is the ceiling?

1900s house, walls are rendered brick, 25cm is thick.

Access is good, hatch into the roofspace.

Wiring I think is easy. There is a light circuit which I’m assuming the spots will be added to. There’s currently an electric heater in the bathroom on a false wall. The fan is being removed, my assumption is that that circuit can be used for the shower and fans. The ceiling is being lowered by the bathroom fitter so will be whatever way he fits it. Natively I don’t see any issues here either.

Probably a bit more than I’d charge but having not seen the site couldn’t be sure. Is the existing wiring ok to extend, has it already got RCD protection or does that need factoring in also?

With regard to the two fans into one, whenever you introduce an obstacle to airflow like non return shutters or even flexiduct not pulled tight you reduce the fans ability to move air. Manufacturers quote the airflow in M3/h so maybe look at getting higher rated ones if it’s a longish run.

Old bricks are very variable in hardness. I cored a wall for a pal in a brick building. 3 hrs on the core drill and £50 worth of wear to it.

Assuming you can tap into an existing circuit is dodgy

Snaking wiring thru a old ceiling after removing insulation then thru a false ceiling has the potential to be a complete pain indeed when I did similar I did a first fit / second fit ie ran the cabling before the false ceiling went in and then fitted the lighting after

From the description I would think that job is going to be an utter pain. Just my veiw

I would expect a skilled tradesman to be charging £250 a day plus at least £100 for his labourer

The existing wiring is good, the house was rewired by the previous owner. RCDs are in place already.