Thoughts on chimney hood extractor

[rqy99g]

Hi

I'm in the middle of replacing my kitchen. The previous owners had an extractor which didn't extract to the outside, just used charcoal filters. The reason they didn't vent outside appears to be because a joist is right in the middle of the chimney.

As I have the ceiling down I'm wondering if I can get away with using 4" ducting (this potentially could fit depending on what fittings are at the top of the chimney) down the side of the joist rather than the usual 6" which the manufacturers state should be fitted?

Note the extractor hood will be on the internal wall and will need ducting to cross the kitchen between joists. I'd angle slightly so it went out centrally between the joists on the external wall.

I've attached a few pics hopefully to make it clearer

That way I could get rid of the ugly Vent-Axia!

 

[foxhole]

You could reduce , can you not reconfigure kitchen to use outside wall ?

 

[rqy99g]

You could reduce , can you not reconfigure kitchen to use outside wall ?

No I'm to far down the rabbit hole, the kitchen is being made as we speak.

Reducing would only reduce the efficiency I'm guessing? Conversely I'd use a pipe rather than flexible hose to help keep the negative effect to a minimum.

 

[Deleted member 174758]

Ideally you should match any ductwork to the manufactutrers outlet size. 4in/100mm ductwork (cross sectional area 7854mm2) has only 2/3 of the cross section of 125mm diameter ductwork (12,271mm2) and about 1/2 the cross section area of 150mm diameter ductwork (17,671mm2), so using it will "throttle" your extraction fan leading to reduced air flow and a potential build up of grease and fat inside the ductwork, as well as poor extraction and potential fan motor overheating (so shortened life).

But what about using larger size flat rectangular ductwork in place of the round stuff? It is possible to get a rectangular duct with about the same cross section as the 6in circular cross section (17,671mm2) the manufacturer specifies.

Take a look at Manrose or Kair flat rectangular ductwork (amongst others). They both do a 220 x 90mm system (19,800mm2 cross sectional area) with elbows, bends and transitions to 150mm circular ductwork. Even going to a 204 x 60mm rectangular system (such as the Manrose stuff sold by SFX - 12,240mm2 cross sectional area) would be better than going to 4in diameter ductwork, although still sub optimal

 

[rqy99g]

Many thanks for the reply... food for thought. I'll take a look at the rectangular stuff. Thanks again.

 

[rsgaz]

Reducing would only reduce the efficiency I'm guessing?

It will also increase noise.

 

[Deleted member 174758]

Many thanks for the reply... food for thought. I'll take a look at the rectangular stuff. Thanks again.

If you want some idea about it - on the current (extended) project the ductwork guys have used rectangular plastic ductwork on well over 100 apartments (mill conversion). It is connected into en-suites, bathrooms, kitchens, etc and drops down to a heat recovery unit before being vented out. I've seen it installed; it pushes together and is light enough to be suspended in place by just builders banding. Cuts with a fine tooth hand saw and cleans up with a deburring tool. Looks like a kid of 6 could install it - probably. I've just ordered some 220 x 60 stuff today for installation inside an MF stud wall where the space available is minimal (no more than 80mm front to back)

 

[rqy99g]

I've found a wide chimney extractor which will definitely let me get 125mm ducting in. What size core drill should I be looking at?

Many thanks

Simon

 

[Deleted member 174758]

In theory 127mm should do the trick, but TBH that is liable to be extremely tight and you will probably struggle to get a 125mm o/d pipe through it, so I'd go for the next size up, which is generally something like a 142mm, or failing that a 150mm which should be more readily available. The pipe can be sealed in with expanding foam and in any case it should have some form of gravity-closed shutter on the outside to keep the rain out

 

[rqy99g]

Thanks again. That was my thought that 127mm looks to tight.