Ducting length for a cooker hood extractor fan?

[HappyHippyDad]

Good evening all.

This job applies to a new, or rather replacement kitchen. Walls have been taken down, extensions added etc etc. It's a big job. The new kitchen is in the same area as the last one but has been made bigger.

The cooker hood fan has been installed with a carbon filter (as it was before, i.e existing). The customer has now changed there mind and wishes to have it vented out of the house.

This would involve a run of approx 6m and 2 x 90degree bends. Rigid ducting.

I'm a bit uncertain how to determine if this is acceptable. If it was just the customer asking then I may be inclined to just do it, as I'm sure it would still meet the 30L/s minimum requirement, even with such a horrible ducting run. I dont have the MI to hand but the cooker hood will do approx 100L/s vented directly out of the wall.

Part F even says you don't need extraction if the existing didn't have it, as long as its in no worse condition (3.31). Meaning the long ducting run will still be better than existing.

It's a little confusing. Any ideas/suggestions?

I have asked the builder to clearly ask the building inspector what he would be happy with.

PS.. I don't think the customer would be happy with a separate fan elsewhere, venting to the outside (even though that would be much easier).

Thanks.

 

[binky]

I suspect it will work fine, especially if using a smooth walled pipe. The airflow part of F is really aimed at new builds
, which are far more airtight than existing buildings. My own house was built in 1886, airflow is not an issue as there's ruddy draughts everywhere

 

[Murdoch]

It’ll be fine as long as you use rigid round ducting.

Part F applies to new builds AND extensions

 

[Richard-the-Ninth]

..........just saying Click me

 

[Chewbacca]

..........just saying Click me

Interesting, that's been kept surprisingly quiet.

 

[Murdoch]

Thankfully the retired are the lucky ones.

What is alarming is that many “builders” don’t understand the current version of part F

 

[kerching]

Part F is an effing joke NICEIC want £400 fora 2 day course……who would have thought they would be promoting an Only Fans revenue stream. BC won’t sign off a job unless it has the Part F sign off, flow rates, design, pressures etc

 

[Marvo]

If your target air volume is 30 l/sec you need to look at the manufacturer published fan curve to see what pressure drop is going to equate to the fan delivering that volume. Then you then need to calculate what the static pressure of your proposed duct design will be at 30 l/sec air volume and make sure it's lower than that figure.

For example if 30 l/sec coresponds to 160 pascals on the published fan curve. Your duct design has 2x 90 degree bends which are 35 pascals each at 30l/sec and a 5m straight duct that's 50 pascals, add the discharge louvre at 15 pascals and another 20 pascals across the fan itself gives a total static pressure of 155 pascals which is less than the 160 pascals the fan is capable of at that volume so everything looks good.

If however your straight duct needed to be 6 meters long that would mean it will be 60 pascals which puts the total static pressure higher than the pressure the fan can deliver your target volume. Also obviously these figures are just fairytale, without knowing the type, dimensions and shape of the duct I've no idea what the actual figures are.

You also need to be careful with pressure/volume curves for manufactured appliances. You need to know if the curve is already compensated for the filters and structure of the appliance as it was supplied of whether the performance curve is for just the fan in open air in which case you must add pressure figures for the built-in filters etc yourself.

There ends my Part F training course, that will be 400 quid please. Off you pop, see you in 6 months for a refresher.