Help with calculating kitchen extractor vent size / flow rate (& location?)

[Martin-W]

Shortly going to be fitting an in-ceiling cooker hood to our extension, which is currently planned to be vented through the roof.

Roofers have been in and fitted a simple tile vent and a 100/110mm flexi-tube coming down from it.

 

BUT...reading the specs for the cooker hood, it needs a minimum 150mm diameter tube.  No problem - the ceiling's not in yet, we can change that.
Another BUT...the roof tile they've fitted.  Looks pretty restrictive.  No point swapping the flexi-tube for a bigger one if I'm just moving the choke-point downstream.

 

So, much googling later, I CANNOT translate roof-tile-vent "10,000mm2" into either a flow-rate or an equivalent tube diameter.  It looks like it comes from Part-F and relates to passive, not active extraction.  But this tile has a c.100mm x 100mm canted grille on it, which doesn't look very free-flowing.

 

Can anyone on here offer any advice, please?  In case it helps:-

 - Hood flow rates are "normal max" of just under 600m3 per hour, and "boost" of nearly 800m3 per hour

 - We probably can still duct sideways and out of the wall if that would be better than sending through the roof.  Builders will probably moan a little, but we can negotiate on that.  Roof would probably be better (cheaper/quicker) if we can make it work though?

 - ...and we can re-use the roof vent and 100mm tube in the loft as we need to duct out a bathroom extractor fan next month.

 

Thanks,

Martin.

[saveasteading]

3 minutes ago, Martin-W said:

100mm x 100mm canted grille on it, which doesn't look very free-flowing.

got a picture of that? It certainly sounds restrictive.

The step down to 100mm isn't ideal either, but if it is at the outlet end then the constraint on flow is not so extreme.

Worse would be a corrugated flexi pipe all the way or sharp bends.
I won't be doing any sums on this though as it is totally forgotten.

[Martin-W]

 

Thanks SAS, and know what you mean on the calcs...my head is hurting already!

 

Above is an equivalent image to what we've got fitted.  Reading around this feels more suited to a bathroom vent or passive venting than a kitchen, but I'm happy to be told I'm wrong.

 

And guess what profile the 100mm tube is...yep, corrugated flexi! 😂

[nod]

Most of the more powerful extractors state 150 6 inch 

[saveasteading]

4 hours ago, Martin-W said:

what we've got fitted. 

I haven't seen one of those before. I'd say that is like 3/4 free air of 100 dia when all worked out. 

So about half of what you need if the fan is going to work efficiently.

[Martin-W]

Quick update (for SAS as much as anyone) - builder has agreed to remove and refit an Ubbink UB47 6" chimney-style vent, with 6" ducting from the unit.  If we can minimise the use of (corrugated) flexi, I think we're there - there's only about a 3-4' drop from the vent to the rafters at that point, and less than that horizontally, so it'll be short tubing anyway.

 

The vent tile soudns well weatherproofed so I THINK we may get away without a non-return valve, and I'm just trying to work out if I need a condensation trap or not.

 

Thanks for all the comments confirming the need for more ventilation - that really helped, and I'm now confident we're not going to muck it ALL up! ;o)

[Martin-W]

Another update.  The UB47 has arrived, and it looks like some sort of Ukrainian anti-tank weapon - it's bloody huge!  (and black)  It's going to look very obvious sticking out of the roof, although thankfully it's a couple of tiles down in a valley-roof, so it's only going to be glaringly obvious from next door's garden and looking diagonally down the side of the house from the front, but may now be visible from the back garden

 

Any other bright ideas?

Venting through the wall (which now feels like it would have been the right solution if we'd talked it through with the builders 6 weeks ago) is going to be very difficult - the roof-space above the rafters (see image) is only about 750mm tall at the peak (which has a lintel supported on blue bricks, so no vent-bricks going there), and ~2m from side to side, meaning it'd HAVE to go diagonally right below the blue bricks and nestled between roof joists and rafters, so probably too close to soffits and structurally-critical stuff.

(Only way we can do this is to drop a box-section down from the ceiling to house the extractor and the ducting, but that will spoil the look internally...which feels like a silly compromise not to spoil the look externally - internal ceiling line is currently a very neat trapezoid - pitched either side / flat across the central 50%+)

 

I've even double-checked with extractor fan suppliers and roof vent suppliers, and between them they're adamant that the flow rate requires 6", non-reducing ducting and a mushroom roof vent.

Am I missing something?