I'm about to knock a hole through the end wall of the house, in the roofspace above the bathroom, to take the output of a 5" extractor fan.
After the job's done and tested, I suspect I may find myself needing to fit a second such fan close by. However, I would prefer not to have to make a second hole through the wall to fit a second grille on the outside (mainly for aesthetic reasons) and wonder if there's a way of ducting the two together so they both exit via the same grille? I'm imagining a large Y-connector with built-in one-way valves... does anything like this exist?
I've seen 4" Y connectors as you describe in my local plumbers merchants but never 5". Have a chat with yours to see if there are 5" ones on special order.
Yes, a Tee would be really restricting on the flow.
I've just been designing and installing dust extraction for my workshop using steel ducting. The application is more onerous than room extraction because there is a lot more air as well as dust and chippings to move, however, the principles are the same.
While there are tees available, they are strongly discouraged because they have an enormous effect on air flow. In my set-up I have Y branches (at 60 degrees between the legs, 30 degrees each to the common output) and branches at 30 degrees, then where I need to make a right angle, a further 60 degree branch is added some distance away.
The other factor, is if you are going to combine two fan outputs at
125mm each, you need to raise the size of the outlet to the outside so that you don't create even more flow resistance and back pressure.
If you work out the area of two 125mm ducts, to achieve equivalent area, the duct needs to be 177mm. The nearest standard size to that is 180mm, so really, the combined output should be that size. On a short length, you could probably get away with 160mm for the combined bit if it's short.
I used ducting made by Nordfab
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sold here by DISA Airmaster
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in metric sizes. There are certainly Y branches, and they will make up whatever you want. However, this is galvanised steel in quite heavy gauge, so over-engineered for what you want and probably even galvanised steel is not a good idea with moist air in the long term. Not cheap either
- a y branch like this would probably cost around £80.
However, I think that this is the right industry area, and you could probably find something from a supplier of air conditioning ducting. These people
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were helpful when I called them although I didn't buy anything there. They seem to have duct and fittings in PVC, which is what you want, I think.
Another option could be to use 160mm soil pipe and fittings, although looking at the prices, that is an expensive hobby also, even to buy just the pipe and fabricate something.
You could also make up a plenum chamber from WBP ply with connectors joining at shallow angles, and the interior painted with a sealer. This would be cheap to make, albeit a little time consuming.
This site has a bunch of ideas and references where you might find something useful
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It's also important to insulate any of this and ductwork in the loft so that you don't get condensation.
Thanks a lot for all the really useful info Andy. Must admit I hadn't thought about the need for a larger outlet to take 2 inputs, but TBH it's more than likely that if I fit the second fan, it will be not be in use at the same time, so I should be OK. Think I've satisfied myself that I only need to knock through one hole in the brickwork anyway, which is what I really needed to know at this stage! I'll certainly revisit your post if it turns out I need (or more plausibly, SWMBO decrees it) the 2nd extractor.
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