Right, I'm going to make a cods of describing this, but here goes.
Two small bathrooms, one above the other, in a dormer bungalow under redevelopment. Both need extractor fans (the lower one has a window that was once external but now opens into an entrance hall).
The upper bathroom will have an extractor in the ceiling, which will duct to a vent chimney thing that replaces one of the tiles in the sloping roof above the ceiling.
It would be possible to extract the lower bathroom by means of a ceiling fan that ducts up into the eaves space, then up inside the sloping roof, joining the same vent chimney thing Above the upper bathroom ceiling by means of some kind of T.
Is this a good idea, or might I end up blowing wet air from one bathroom into the other, or causing water to condense and run down the duct into the other bathroom? Would it be better simply to put in a second vent chimney thing lower down, in the eaves, to vent the lower bathroom independently?
Most importantly, has anyone here actually combined vents like this, or seen it done? Is it an orthodox thing to do, or not?
Right. To start with, your duct pipes don't have to be round (circular). Square or rectangular plastic duct can also be used, as long as it equals roughly the same internal area as a 4 inch tube. A 110 mm (4 inch) pipe has a circumference of 345 mm and internal area of almost 9.5 mtrs, so you need a square duct of the same measurement to allow the same air flow through it. A 50 mm X 195 mm rectangular duct has nearly the same area as the 110 mm pipe, so allows the same air flow through it. A 25 mm X 375 mm rectangular duct also allows the same air flow through it.
Because the shape of the duct has changed, it can allow the venting system to be hidden inside the dry lining or even in the wall cavity. The shape doesn't restrict the air flow through the vent, it just allows the ducting system to be placed in areas where a round pipe would be ugly to look at, or need lots of horrible boxing in.
So, now you know of an alternative to the vent pipe, do you see any other way the duct could to be routed more conveniently if it was a different shape? There are adapters to suit almost all shapes and sizes of pipe, so that's also not a worry.
Thanks - I know this already ;-) At least some of the ducting from the ground floor bathroom will have to be rectangular, to get up between the rafters from the eaves space, to meet up with the other ducting and connect to the vent chimney. My question remains the same - is it a good idea to have two vent ducts meeting like this and sharing the same chimney? Or, am I likely to push wet air from one bathroom into another, or to end up with condensing water running back down one of the ducts?
You could combine ducts and then have one fan sucking from both rooms.
You would need double-pole light switches in each room, one pole switches the room light and the other pole switches the fan. This stops the light in bathroom B coming on when someone switches on the light in bathroom A.
I've recently fitted a bathroom fan that vented to the eaves space. I found two problems:
The fan was not powerful enough to open the one way valve flap thing that I stuck inside the duct. Major drag as I had to dismantle it all and take it out. This means drafts can now blow down the duct and heat can drift out of the bathroom into the eaves space. In principle you could use such valve flap things (I don't know what they are called) to allow you to connect two ducts into one - they should stop blow back down the other duct - provided your fan is powerful enough to open the valve!
Condensation was a major problem. The day after fitting the duct and fan the following morning the floor was wet under the fan and water droplets hanging off the fan. Not good. I wrapped insulation material around the duct in the attic space and this has eliminated the condensation problem.
Hope this helps. Anyone else have problems with fans not being strong enough to open the one way flap valves?
I could, but that's not my plan - I already have a ceil>I've recently fitted a bathroom fan that vented to the eaves space. I
I'm not surprised you had condensation and damp if you were venting into the eaves. I propose to vent to the outside.
Thanks for all the thoughts, but I'm still hoping someone knows something about my specific question, concerning two fans blowing up two ducts that meet at one chimbley ;-)
I dont know what regs say on this, but I do know that if you used one bathroom regularly and dint use the other often, you'd end up with a mouldering ring of water in the unused room, so its not a problem free solution. This is because condensation occurs in the tubing, and runs down to a trap at the fan. Dont opeatye one of the fans, and the trapped condensate just builds up and gets ugly. So... I wouldnt do it.
Sorry about cutting off at the crucial moment, but got called away.
Your proposal to connect two vents into one is difficult to make properly. It has been tried and tested many times, in many ways, by many others in the past. They have all fallen into the trap of having both bathrooms fill with the damp air from the other. If you are thinking of a straight connection between the two systems, then you'll need to allow a drip trap at the very bottom of the chimney stack to catch the water that condenses in the pipe.
To make connection between the two systems work properly, you need to bring the lower system up to near the top of the stack. A bit like having two separate systems, but sharing just the last bit of stack to the outside. That way, the two systems are creating separate chimney effects (drawing warmed air upward) on their own terms. They just join to the last foot or so of the vent to the outside.
One system passing the other on a straight connection, would need valve type systems to prevent them blowing air back into the room. One way flap valves can be created using bits of plastic and things, but they usually fail to stop all the air from being distributed between the two areas. Which is what you need create on venting systems like the one you want.
What you want to create is a natural chimney effect from both rooms, then create a way to force the air up the stacks to remove any moist air more quickly when the rooms are used to shower and things. If created properly, the system should actually draw air through the rooms without the need for fanned assistance. The fans are only used when the air in the rooms needs to be extracted more quickly.
If you can get both system to connect in this way, and a simple test for this is a smoke match used to test flue systems, then you have a properly working ventilation system. If any smoke from the match blows back into the other room, then the venting is going to be worse when the fans are in use.
The systems will need to be tested both without fanned assistance and with fans running, both individually and together. If any smoke testing shows a blow back into either room, the system has failed.
Smoke matches can be got from any good plumbers merchants or Gas Supply showrooms.
On Mon, 17 Nov 2008 05:30:11 -0800 (PST) someone who may be geraldthehamster wrote this:-
It is possible. There are Y connectors which will join two runs of ducting into one. They have a flap valve which blanks off the unused duct if one fan is running or is in a mid position if both are running.
It is better to have a separate vent, provided that this vent is not a means of letting water into the structure of the building due to inadequate installation.
No matter which way you do it, since the ducting is vertical you should take precautions against condensation.
Thanks to Messrs miaow, Wallop and Hansen, and to everyone else who replied. I'll go with the separate vent and chimbley, which will in any case be less fiddly to contrive.
Concerning condensation within a vertical vent that passes through a cold roof space, should I pack insulation round it?
Insulation won't really help. You do have to allow a water trap at the bottom of a vertical stack to catch any condensate that trickles down the vent. Something that you can clean out and dry off when it needs it. We used to put a cap end on the bottom of a 4'' pipe stack being used as a vent. It lets you unscrew the bottom off and dry it.
On Tue, 18 Nov 2008 15:16:23 GMT someone who may be "BigWallop" wrote this:-
Manufacturers offer water traps which fit into a vertical duct just above the fan. These have a water outlet which is intended to be connected to a suitable drain.
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