Custom ducting from GRP

[AdamSee]

Long story short. I have to squeeze a duct over a RSJ through a 50mm gap while mainingtaining 8,500mm2 of ducting area. This is to be used to extract stale air from the downstairs bathroom and utility and send it 16m up a 125mm duct to the loft mounted MVHR unit. 

 

So my question is. Has anyone made custom ducting from GRP? And can anyone forsee any problems with this in a inaccessible ceiling void?

 

I've seen on Jeremy's blog that he's made a exhaust silencer out of MDF and others have made custom distribution manifolds out of timber. But I've not see anything made from GRP. I've thought about various other methods but due to time constrains my options are limited. I've attached some very quick sketches of the difficult location that I need to get the pipe in to. Part of the issue is that I have to squeeze the duct in-between a new RSJ and the existing floor joists which run parallel to it. The RSJ is supporting the floor joists which are perpendicular to the forementioned. This is actually helpful for duct running as it provides a void to run the duct through over my kitchen. But the new RSJ is too close to the parallel joists to get off the shelf fittings in and even it I could buy something I still cant get a square ducting coupler to join on to the side of a round duct (which is a spiral galvanized duct)

 

My plan for making these fittings........is still a working progress, leaning towards making it as two halves using a MDF mould to ensure the inside wall is nice and smooth. Can readily join the two together when the things made. Thought about 3d printing but concerned about the strength, air tightness and would need to be made out of many small sections. 

 

Any thoughts, or ideas would be great. 

 

I've attached a sketch. Top drawing is a cross section view looking at the end of the spiral duct and a side view of the gap the custom ducting needs to squeeze through. Lower drawing is a plan view showing how the joists change direction. 

 

[Temp]

Instead of making split female molds you might try foam male molds and cover with tape. After the fibreglass is set dissolve out the foam and pull out the tape.

[Temp]

Can you fit 54mm through? Perhaps by taking the corner off the wood joist?..

https://www.ducting-online.co.uk/PBSCCatalog.asp?ActionID=67174912&PBCATID=2828999

 

Need two parallel paths as 54x110 is only about 6000mm^2.

 

They also have a 60mm range.

https://www.ducting-online.co.uk/PBSCCatalog.asp?ActionID=67174912&PBCATID=2829003

 

Edited August 1, 2022 by Temp

[Temp]

Actually if you are already using 60x204 duct can't you cut a bevel on the joist to get 60mm through and reinforce the side of the joist if necessary.

[SteamyTea]

1 hour ago, AdamSee said:

Any thoughts, or ideas would be great. 

With looking too hard at your sketch, if you can make the shape you want out of expanded polystyrene, minus about 10mm on any critical dimensions, paint loads of emulsion paint on it until it is totally sealed, the apply normal wax release agent, then PVA release agent, then sheath it in GRP.

To get the polystyrene buck out, melt it with a heat gun or acetone.

 

You do run a risk of it smelling of polyester resin for a while, especially in hot weather.

There is also a fire risk that needs to be addressed, though you can get fire retardant resins.

 

Just seen.

34 minutes ago, Temp said:

Instead of making split female molds you might try foam male molds and cover with tape. After the fibreglass is set dissolve out the foam and pull out the tape.

Similar idea.

[PeterW]

26 minutes ago, Temp said:

Actually if you are already using 60x204 duct can't you cut a bevel on the joist to get 60mm through and reinforce the side of the joist if necessary.

 +1 to this - sister up the joist on the other side with D4 and screws. 

[AdamSee]

I did consider using PIR or polystryene and covering it in foil tape. Could then dig it or dissolve it out as suggested, but wouldn't have the an ideal internal finish.

 

I had written off the idea of squeezing in a 54 or 60mm duct as it would mean shaving the joist down which I was under the assumption was a big no no. Granted the joist is supported along it's length by the perpendicular joists, so its got the least amount of load on it. And it would only need to be the corner removed. 

 

Unfortunately I can't access it well enough to sister it up. Will post some photos of the current situation tomorrow. 

[SteamyTea]

9 minutes ago, AdamSee said:

I did consider using PIR or polystryene and covering it in foil tape. Could then dig it or dissolve it out as suggested, but wouldn't have the an ideal internal finish.

That is down to how well you make the model.

This was made with polystyrene and emulsion paint.  Don't use PIR, not so easy to disolve.

(I made it)

Edited August 1, 2022 by SteamyTea

[Temp]

14 minutes ago, AdamSee said:

did consider using PIR or polystryene and covering it in foil tape. Could then dig it or dissolve it out as suggested, but wouldn't have the an ideal internal finish.

 

 

Don't use foil tape as it breaks into bits too easily. You want something like a reinforced plastic tape so it comes out in one piece.

[Temp]

17 minutes ago, AdamSee said:

I did consider using PIR or polystryene and covering it in foil tape. Could then dig it or dissolve it out as suggested, but wouldn't have the an ideal internal finish.

 

I had written off the idea of squeezing in a 54 or 60mm duct as it would mean shaving the joist down which I was under the assumption was a big no no. Granted the joist is supported along it's length by the perpendicular joists, so its got the least amount of load on it. And it would only need to be the corner removed. 

 

Unfortunately I can't access it well enough to sister it up. Will post some photos of the current situation tomorrow. 

 

To fit 60mm you only need remove a tiny triangle with sides roughly 15mm x 15mm. 

[JohnMo]

Just chamfer the timber to get an off the shelf duct piece through - job done.  10mm or so chamfer for 200-300mm done.

[AdamSee]

Thanks for all the input fold. Had yesterday evening off so didn't get a chance to work on this. So my aim is to ensure that I don't constrict the airflow at it has a further 3 to 5m to go via flexible radial ducting (x3).

my joists are 54mm wide, I would have to shave off 25mm  (max) to be able to squeeze through a 60mm duct at 45 Degrees. The joist is supported every 40cm by the perpendicular joists which sit on that wall plate. So load on the joist is pretty well distributed. Which is fortunate, because there's no way of sister it for reinforcement. 

Making this adaptor is now pretty straight forwards. Going to mould it straight over a well lubricated bit of spiral duct, which has a piece of 60 x 200 ducting affixed to the side at roughly 12mm off the centreline. Can pop it off when its set, clean it up, then glue it back on with something abit more flexible and a few rivets.