I want to install rigid extraction ducting for 41.4 m3 living/kitchen area. Duct will run about 4.5m from proposed extractor to outside. Duct will go along wall from kitchen through bedroom to outside. Its a straight run other than a short length of flexible ducting needed (say less up to 0.75m) to link from extractor to this rigid ducting. Worked out I need extraction rate of 165.6 m3/hr (46 l/s) for building regs. What min size rectangular ducting is required? I could use 100mm round but think that for this 4.5m run and loss of pressure, a diameter 150mm would be needed. Obviously rectangular would make a neater job. So I'm thinking something like Kair system 150 rectangular (180 x 90mm) or would 100 (110 x 54mm) rectangular do? Thanks.
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Extraction ducting size
- Thread starter Wavy Davy
- Start date
I'd use the bigger rectangular. It should be pretty much invisible on top of wall units.
I used 150mm round at my last house. It did the job, very well, but was definitely visible. I wouldn't recommend it.
Whatever you use, if you can avoid using any awful flexi stuff you'll definitely get much better airflow. Even a short length will slow the flow lots.
You should be able to get a round to rectangular adaptor, which is also a right-angle turn. So you then just need a straight length of 150mm rigid round to get from your extractor to it. If you need to move in or out relative to the wall then a pair of angle connectors would still be much better than flexible.
Put sealant on all joints. A small amount on the male half only before assembly, then more outside and smoothed out. If you put it on the female half then it can get pushed in, obstructing the airflow and gathering muck.
"Duct Tape" is not for ducts. It's a really stupid name for gooey botch tape.
I used 150mm round at my last house. It did the job, very well, but was definitely visible. I wouldn't recommend it.
Whatever you use, if you can avoid using any awful flexi stuff you'll definitely get much better airflow. Even a short length will slow the flow lots.
You should be able to get a round to rectangular adaptor, which is also a right-angle turn. So you then just need a straight length of 150mm rigid round to get from your extractor to it. If you need to move in or out relative to the wall then a pair of angle connectors would still be much better than flexible.
Put sealant on all joints. A small amount on the male half only before assembly, then more outside and smoothed out. If you put it on the female half then it can get pushed in, obstructing the airflow and gathering muck.
"Duct Tape" is not for ducts. It's a really stupid name for gooey botch tape.
Sorry yes, that's what I meant. The minimum angle you can get away with, use a pair to make a kink towards whatever side you need to go to.
The other parameter is what fan you will use. Good fans might have data describing how their extraction rate depends on the ducting. For example, this Vent-Axia fan has graphs on page 2:
That could achieve your ~ 50 l/s against a static pressure of ~ 85 Pa at 100mm diameter, or against ~ 150 Pa at 150mm diameter.
Now you just need to determine what length of ducting will have that much resistance at your speed and length. I bet there’s an online calculator for that somewhere…
There’s another factor to consider - how much do you care about noise? For a given extraction rate, a larger fan (rotating slowly) will be quieter than a smaller one (rotating fast).
That could achieve your ~ 50 l/s against a static pressure of ~ 85 Pa at 100mm diameter, or against ~ 150 Pa at 150mm diameter.
Now you just need to determine what length of ducting will have that much resistance at your speed and length. I bet there’s an online calculator for that somewhere…
There’s another factor to consider - how much do you care about noise? For a given extraction rate, a larger fan (rotating slowly) will be quieter than a smaller one (rotating fast).
It's a kitchen extractor, so I'm assuming it's a cooker hood with its own fan built-in.
Mine had a 150mm outlet and a 150-100mm adaptor was included, which I binned and used 150mm all the way. But the one at this house is 100mm, so I used 100mm ducting. There's no point oversizing beyond the size of the outlet, as it's already constrained by that anyway.
My 100mm one was already fitted and I didn't want to change it. It was recirculating, I just added the ducting to turn it from a cowboy kitchen fitter job into something that actually works. It's OK, better than the 0% extraction it had before but nothing like as good as the one I fitted at the previous house. If I was starting from scratch I'd only choose a hood with a 150mm outlet and decent fan.
Mine had a 150mm outlet and a 150-100mm adaptor was included, which I binned and used 150mm all the way. But the one at this house is 100mm, so I used 100mm ducting. There's no point oversizing beyond the size of the outlet, as it's already constrained by that anyway.
My 100mm one was already fitted and I didn't want to change it. It was recirculating, I just added the ducting to turn it from a cowboy kitchen fitter job into something that actually works. It's OK, better than the 0% extraction it had before but nothing like as good as the one I fitted at the previous house. If I was starting from scratch I'd only choose a hood with a 150mm outlet and decent fan.
So regarding the power of the extractor, what you are saying is that I need to calculate the static pressure of the proposed ducting and the fan needs to at least overcome that pressure for the flow rate required?
Something like that. Not an expert.
Buy a decent one that you like the look of with a 150mm outlet.
Be careful if you have a fireplace, ours used to pull air down the chimney at the opposite end of the house. Modern houses are too well sealed in a lot of cases, you need an inlet somewhere, e.g. trickle vents on windows.
Be careful if you have a fireplace, ours used to pull air down the chimney at the opposite end of the house. Modern houses are too well sealed in a lot of cases, you need an inlet somewhere, e.g. trickle vents on windows.
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