Recrtangular ducting.

I intend ducting a cooker hood to the outside using 204mm x 60mm rectangular ducting. Looking at the ducting it has same problem as when I have used smaller size on other jobs the longer length sags a little ie is not a proper rectangle. And to my mind if one uses the standard connection bits this does not solve the sagging which must result in a more turbulent airflow at that point. I know one can get different connectors having seals at either end which will keep any sag to a minimum, has anybody used these?

 

Can't you use clips https://www.tlc-direct.co.uk/Produc...ZN73fGHy6D4TCY47bF0q89JVGLCn2-3saAlJqEALw_wcB

 

Hi

My mistake I was not meaning along the length of the ducting but in this case along the 204mm dimension. As one can see from the attached picture it bows inwards. This is true on 100mm also but more exaggerated on 204mm.

 

They are all like that but I cannot think it will make any difference to air flow as you still have more or less the same cross sectional area, the slight difference will be irrelevant as far as a cooker hood is concerned. If you fit them together using polypipe glue it will square them up anyway. The only problem I think may happen is that perhaps cooking smells might escape if you don't seal the joints. I have used that same size with different fitting types on a tumble drier vent & never had a problem with steam escaping.

 

Am not too keen on glueing as I envisgae some time in the future it parting company. I am thinking of using these
https://www.i-sells.co.uk/kair-syst...ting-to-fitting-quick-connector?ref=ViewTitle
They seem commonly available, Toolstation etc as far as I can see will at least keep the duct ends rectangular.