Cooker Hood Ducting (Kitchen Extractor Fan)

[MortarThePoint]

Our kitchen design has two side by side cooker hoods. Is there such a thing as an Inline Rectangular to Round 90 degree bend to put in the left hand hood as in the first image below or do I have to have them both bend/tee onto a straight run as in the second image. The blue circle is a soil pipe I have to get round and the dark brown lines are the wall surfaces of the kitchen.

 

 

 

 

[NOTE: I accidentally started this thread in the wrong section so am stopping it there restarting it here]

[MortarThePoint]

@Nickfromwales Posted:

Any of this get you out of the 💩?

 

https://www.tlc-direct.co.uk/Main_Index/Ventilation_Index/Ducting_Flat_6/index.html

[MortarThePoint]

1 minute ago, MortarThePoint said:

@Nickfromwales Posted:

Any of this get you out of the 💩?

 

https://www.tlc-direct.co.uk/Main_Index/Ventilation_Index/Ducting_Flat_6/index.html

 

Thanks @Nickfromwales, I've seen those parts from other suppliers and I can't see an Inline Rectangule to Round Tee. I was think along the lines of a straight section of rectangular duct with a spigot in the bottom for a 150mm Round to join. I guess I join the two extractors in 150mm Round and then adapt to Rectangular instead.

 

Any reason Rectangular can't be used on its side, so the 220 or 204 dimension is vertical?

[MortarThePoint]

Here's what it would look like using Rectangular ducting on its side:

 

Passing from one of the extractors:

• 150mm Round Straight 600mm
• 150mm Round Bend
• 150mm Round Straight 200mm
• 150mm Round Tee
• 150mm Round to 204x60 Rectangular Spigot
• 204x60 Rectangular Straight 1450mm
• 204x60 Rectangular to 125mm Round Adaptor
• 125mm Round Straight 450mm
• 125mm Round Bend
• 125mm Round Straight 350mm
• 125mm Round Vent/Grating

I read bends count as 1.2m equivalent. This has 4 bends so overall is equivalent to 8m straight length of 204x60 I'd guess. I've read it should be kept under 5m equivalent, but is there a building reg that needs satisfying?

[MortarThePoint]

Here's a comparison of ducting resistance per metre for different types. At 288m3/hr it works out as:

                            144m3/hr  288m3/hr

150mm Round     0.4Pa      1.9Pa

125mm Round     1.4Pa      3.9Pa

100mm Round     4.1Pa      14.3Pa

220x90 Rect       0.4Pa      2.0Pa

204x60 Rect       2.1Pa       7.8Pa

121x60 Rect         6.0Pa        -

110x54 Rect       13.5Pa        -

 

 

http://www.nuaire.info/catalogue/Nuaire_Ducting_Specification_Guide.pdf

[MortarThePoint]

Using the values in that Nuaire PDF, the total resistance becomes:

• 150mm Round Straight 600mm   1.1
• 150mm Round Bend   16.6
• 150mm Round Straight 200mm   0.4
• 150mm Round Tee   14.5
• 150mm Round to 204x60 Rectangular Spigot   90est. <<<<
• 204x60 Rectangular Straight 1450mm   11.3
• 204x60 Rectangular to 125mm Round Adaptor   14.3
• 125mm Round Straight 450mm    1.8
• 125mm Round Bend   26.4
• 125mm Round Straight 350mm   1.4
• 125mm Round Vent/Grating   ??
• TOTAL: 177Pa @ 80l/s (288m3/h)

Half of that is from the plenum (aka "150mm Round to 204x60 Rectangular Spigot") which you'd almost always have in a system using rectangular ducting.

 

[MortarThePoint]

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From these, I'd guess 150mm Round to 204x60 Rectangular using a 150mm Round Bend and then Adapter would be about 30Pa @80l/s so much improved over the Plenum (~90Pa @ 80l/s) if space allows.

 

Edited February 3, 2023 by MortarThePoint

[MortarThePoint]

Do Building Regs set out a minimum distance between an extractor fan exhaust and an openable window? I'd be surprised if there is as I see bathroom ones near windows all the time.

Edited September 2, 2023 by MortarThePoint