Semi-rigid ducting tight bend

[jayc89]

A couple of my duct runs cross the loft between the original part of the house and an extension which is approx 600mm lower, which means they'll be above the insulation level at this point. Options I've thought of;

 

- Wrap them with insulation at this point, between where they protrude the insulation in the "original loft" and where I can get them back below the insulation in the extension loft. 

- stick a couple of 90 degree bends on them so the ducts follow the insulation level

 

Bends would be easier, but I haven't factored them into my velocity calcs. Should I go with a couple of lengths of duct insulation instead? 

Ducting is 90mm, and the longest runs are approx. 16m. 

[JohnMo]

Bends do not affect velocity calculation, but they do change the pressure drop calculation.

 

90 deg elbows and follow the insulation would get my vote

[Conor]

Just need to allow for the pressure drop, iicr about the same as an extra 2m of duct, if talking about 80mm semi rigid.

[jayc89]

Pressure drop is indeed what I meant!

Anywhere I can get 90mm elbows online? All the normal places don't seem to list them on their sites. I've dropped BPC a message and even they're having to check their stock for them. 

[Conor]

Lindab.