Advice on awkward kitchen extract

[duotwr]

Hi - I'm looking for some advice on an awkward kitchen extract problem. We've moved into a Victorian Terraced house, and the gas hob in the kitchen is located on the internal wall with no easy access for an extract duct. For various reasons we generate a lot of steam from our cooking (including making homebrew on a hob).

Options for ducting route are:
- long duct (6m) to the rear of the house, probably with a couple of 45 degree bends to account for obstacle - and running through the utility room at the back of the house
- shorter duct (5m) but with 2 90 degree bends through roof void and out side wall
- through the roof - approx 2m run then vertical for 2m, needing a terminal installed in existing low pitch slate roof

None of these options sounds particularly easy, and the current no-brand extract hood we've got is probably going to struggle.

Would I be better having an inline fan installed to deal with the longer duct lengths? How hard is this to set up for competent DIY'er.

Any suggestions would be very helpful.

Keith

 

[stem]

Extractor hoods have a centrifugal fan (as opposed to an axial fan) which is designed to generate a high enough pressure to overcome pressure losses in ductwork. I would avoid the flat duct designed to run along the top of kitchen units, because it has a higher resistance to air flow.

If you were to use plastic soil pipe instead of corrugated flexible duct for long straight runs, because it doesn't have ribs, it has less air resistance and is also neater / easier to support over a long run.

Maintain the diameter of the duct throughout the system at the same size as the hood outlet connection, don't reduce it.

I would estimate as a rule of thumb, that a 90 degree bend has the same resistance as 3 metres of straight duct, and a 45 degree bend the same resistance as 2 metre of straight duct, so taking this into account both options have an equivalent duct length of 10 or 11 metres. Based on that, I would expect the ductwork to give no more than an 8% to 10% loss of hood efficiency.

If you are concerned about your "no-brand" model of hood, the fan will usually still have a reasonable performance. You might be able to check the manufacturers data.

 

[motorbiking]

There are some fairly powerful inline ducts on the market, the other option is to back it up with a 6 inch wall extractor as these are usually designed to meet the current building regs of 60l/s You can run it off the lighting radial or as a fused spur off the ring. with a fan switch. Its probably going to be the easiest option and no ducting.

 

[stem]

A hood would be a better solution as it captures the homebrew steam from the hob at source, rather than dragging it across the kitchen.

There are some fairly powerful inline ducts on the market

Eh? are you confusing a duct [a tube or passageway in a building or machine for air, liquid, cables, etc.] with a fan?

 

[motorbiking]

I made two suggestions in one post, which may have been confusing.

option 1. I'm suggesting the addition of a wall mount extractor not adjacent to the hob as required by current building regs in the absence of an extracting hood. no need for any ducting.

option 2 an inline duct extractor which will over come the resistance of the bends.

 

[duotwr]

Hi,

thanks for the replies.

I've seen some info online about the inefficiencies of bends - stem's estimate of 11m equivalent duct length is useful. But hood manufacturers seem to be cagey about duct lengths - anecdotally people seem to suggest a max 4 or 5 metres. But if stem's correct that increasing this to 11m is only around a 10% loss in efficiency then that's not too bad. It's a 90cm wide hood - I'll try to identify what the fan spec inside is.

I think I'll probably proceed with the slightly more convoluted but shorter run through the roof void as a first pass and see what difference that makes. It might not be enough - we often cook with the door open into the dining room, and the condensation appears on these dining room windows. But then I can keep a separate wall extractor as a backup additional option.

thanks again

Keith

 

[stem]

I've seen this question pop up a time or two, so after you have done it, a quick report back on here, may be useful to those that follow......you should be able to feel a good flow of air at the final discharge point.