Bathroom extractors, two fans in one duct?

We have two decent sized bathrooms but they are only fitted with asthmatic fans in the wall on the other side of the room.

I’m thinking of getting the MANROSE MF100T 4 Inch Inline Duct Fan with Timer fitted to each room over the shower. Both showers are in the same gable end so nice short runs.

However, plumber/builder is suggesting common ducting and only one core drill through the gable end. He’s tried telling me that one fan not running works like a none return valve for the other.

I’m tempted to call bs and put it down to him not wanting to core drill. I can’t see a benefit to longer ducting with a tee and a single exhaust over just taking both straight out through the nearest bit of wall.

Your thoughts and comments please.

Can’t resist, no idea on the problem, you could try this site but they’re used tractors not extractors.

http://farm.autotrader.co.uk/used-farm-machinery/category/tractors

Sorry, I’ll get my coat.

There’s no way a fan would act as a non return valve unless it actually has non return flaps fitted. Neither of mine do. You can get non return flaps that sit inline, maybe he’s adding them. The air will simply take the easiest path which will likely be a bit out the wall and a bit through the other fan if you just have a fan. Also if you have the option for a short run go for it. You can use smooth pipe and less of it which will reduce noise

don’t google two fans one duct at work

tick (because if it would work, I can easily do it myself, otherwise I need to get someone in with the right tools/ladders to help make the hole/etc.)

I agree that a fan wouldn’t act as a non return or at least not enough to stop at least some of the humid air going from one bathroom to the other if only one fan is running.

I’ve also seen a suggestion that it’s contrary to building regs although I don’t know how true that is or isn’t.

My company recently refurbished 3 commercial washrooms in a building where they had Male,Female & Disabled toilets that originally had them all being linked into the same ducting with one external outlet.

Each room had it’s own small inline fan but no flaps in place to stop any airflow going back down into the other rooms. The result was that none of the fans had enough power to pump air externally and just filled each room up.

We replaced all of the old ducting and put a larger fan with control flaps in place linked with PIR’s.

For the sake of two small fans with very short outlets I would put a separate fan with it’s own outlet to each room.

If you only wanted one exit visible you could put the outlets close together and then get a louvre/cover made to conceal the outlet points. This is what we did for a big campsite shoer block the other year up in the Peak District. Half a dozen outlets but all concealed.

Your plumber is talking ballcocks. I’ll explain later cause its tea time.

Thanks. To link them would mean about 3m of ducting. Going straight through the wall is as close as you can get a bath to the wall.

MT, colour me intrigued.

Finished your tea yet MT?

I’d be happy to have two fans into one outlet with Non Return flaps added.

Are they non return footflaps?

Are they non return footflaps?

🙂

first thing separate to the main issue, make sure the new fans are not going to be blowing out of the gable on the windy side of the house. These fan types often don’t have the pressure development to over come a breezy day.

As stated by another above, air always goes the easiest way. It will go through the other bathrooms fan, if its windy outside and that stops air leaving the building (this happens often), if the pressure caused by the wind is enough the fan can stall and even go backwards (quite common). Un-powered fans often end up windmilling (going backwards mostly), on a breezy day there’s more than a chance you’ll be send air into the other bathroom.

If the connection from each fan to the hole in gable end is convoluted, like a couple sharp bends in some flexible duct, this will really knacker your fan performance. I’ll not get into the system effect of running fans in parallel and making the single duct section of a larger diameter than the fans as its not going to be that relevant unless your ducts are complicated, though they could work against each and stall or give reduced performance if you run them both together. My advice is keep it simple and make sure that you fit a fan with a damper. The are loads of makes of bathroom fan that are all pretty similar, Manrose are fine however I’d take a look at Airflow fans they have a fan with a damper that only opens when you turn it on, they work best. Back draft dampers are a great thing to have but the rattle on a windy day. Hope all that helps.

Like that ex tractor joke above, my two favorite subjects. Honest.

^^MT is right on all counts.
I’d keep both rooms/fans separate and just go straight out of the wall.
The gable end on our house has 3 fans (two bathrooms and cooker hood extract) cut into it..

Title made me feel queasy for some reason.

That is all.

D.

how about 1 large inline fan and 2 ducts joining in before it?

Well, both showers are close to that wall so it become convoluted but also, doesn’t thatean I’ll need a bigger fan as it will clear both rooms, even when only one is in use?

I’m happy with the idea of two separate fan systems. Just wanted to check if the plumber had a better idea or a lazier idea. Seems from above, my instincts were right and he’s just being lazy.

Rather than two cheap axial fans into one duct outlet, how about two ducts into a more powerful centrifugal fan which is a bit more expensive but has a lot more oomph. It would then clear both rooms equally by sucking from both, rather than two weak fans trying to share an outlet.

I have a longish run of 5 metres from my bathroom, I got the biggest axial fan but it was a false economy as I’ve since added another and it still struggles with clearing steam from the shower, should’ve just stumped up for the centrifugal type in the first place!

Our extractor fans have a fancy ‘star trek door’ type arrangement in that they slowly (creepily) open only when on. Maybe he’ll fit those, so if they are off, they are physically closed?

DrP

Generally, axial fans are good for high flow, but provide low ooomph while centrifugal fans are good for high oomph, but provide lower flow.

We put an Aventa 4″ in-line axial fan in our loft space for our relatively small bathroom & it is OK, but not as quick to clear as I thought it would be. We already had the 4″ ducting in the ceiling so I went with a 4″ fan, but if I was doing it again, a 5″ fan has a much higher flow rate.

As mentioned above, see which side the wind hits the house on. A centrifugal fan might be better if the wind hits the side of the house the outlet will be sited on.

My main issue with a combined ducted system would be the fact that you will always be venting from both bathrooms, even if you only need to be venting from one so your fan would need to cope with the higher extraction rate all the time.

I’d probably go for the simpler option of a separate duct & fan for each room.

Google “whole house extract”. It’s a fairly standard thing to have a single fan unit extracting from multiple rooms – our last flat had one that dealt with kitchen and bathroom.

There is one outlet from the central unit to the outside and ducts to each room. The extracts are “balanced” by altering the relative size of the vent (those screw down types) to allow for different sized rooms/duct resistance.

The fan I’ve bought has non-return flappy bit on it. I’m not doing any ducting though so cant comment on that.

Had a chat to him. Now looking at individual exhausts. Fan he’s talking about is 320m3 /hr so should clear a 15m3 bathroom quite well.

Its fine to have a inline fan extracting from two points.. best option would be a nice sized fan say a 125mm or a 150mm if your overkill use solid duct with a stepdown in size off the fan dependent on the length of the run of duct.

Straight run over the top of area can stick 45s or 90s in install a saddle off the straight run to each extract try and keep flex to a max of 200mm in length if your good solid all the way to the air valve in the ceiling (if your using them)

Sealed on the outside with tecseal and fixed with tec screws.

The smell will filter outwards with the draw of the air flow, stepping down in sizes in duct increases the pull anf more air removed.

Dont worry too much about smell getting back in as the one way dampers or “flaps” in the duct are useless imo. They jam up and become noisy as the duct will fill with skin and dust over time.

Alternative install a square vent axia tx6 with a square to round spiggot on the rear. Has one way mech insalled thaat does work and has a removable grill to service it. But this means you need a square hole.

Ideally you would have a supply of fresh air incomming and the extract taking out, along with a “free air” supply to balance the air in the room.

Gets all complex with air flow and measurements the bigger you get.

And yes ducting and “air management” was my old job.