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I have 2x old Mistral wall exhaust fans that have a 300 mm x 300 mm shroud going through a 220 x 230mm hole to the outside of my home. I can only find 150mm exhaust fans at bunnings. Both these fans died a long time ago (they just rattle whenever you turn them on) They really need to be replaced.
I'm looking for advice as to what my options are? should I attempt to fill in the internal wall holes to make it a 150mm hole or should I try to source a like-for-like replacement?
It looks like this product wouldn't even cover the massive hole that was made in the wall to accommodate these older fans.
https://www.bunnings.com.au/blauberg-150mm-white-aero-wall-ceiling-exhaust-fan_p4420505
Here are some photos that show the issue I am dealing with.
Hi @MikeTNZ
Thank you very much for that timely safety reminder. I totally agree, gone are the days of just picking up any fan and putting it in your bathroom. It's probably one of the most important things to consider when purchasing fans for your bathroom and that is to make sure that it's rated for bathroom use. I'll be sure to tag @joejoeinc to make sure they see your handy tips for installation.
Eric
So said I would get back to this when I had time, so here we go.
This ended up being a multi-day process as things needed time to dry out and set and also getting over the reluctance of starting the actual project.
I found bunnings sold a drain pipe connector just a little larger than the 150mm hole needed for the fan, perfect! I choked it up with an off-cut in place and went then went to work with the expanding foam. one tip, wear gloves when using expanding foam. the foam is very sticky when it's not set.
The next morning I had a rather solidly held pipe in place
I then hacked the foam back a bit to make enough room to pack some rapid-set concrete in. This meant the concrete would have enough thickness to hold well.
It was easy enough to mix up some rapid-set and wear some gloves while packing it in.
The problem here using concrete it can be hard to get it level with the wall. so I went to another route and pushed the concrete into the wall leaving enough space to add some concrete render later so it would match the wall better.
Concrete rendering around the space is pretty much the same process as how I fill in gyprock holes left by my sparky when he last installed some downlights so it's not hard to do. Just take your time and say oops when you drop a bit clump of it on the ground when you miss the wall 
Next up a test fit.
I then removed the fan to do some prep work for painting. 2 coats of primer. I then had the fan properly installed and wired up by my sparky and we are all done for now. I'll be painting the entire room later so it all matches.
Now I'm not saying I did a perfect job here, far from it. I was learning as I went for most of it. I'd never used concrete render or expanding foam before this.
Side Notes:
- I reckon if I sand down the primer again and add 2 more coats it will be a lot better. For now, I'm stoked, so much better than those old fans (for one thing the new fans actually work!)
- I've added some foil ducting to the rear of the drain pipe connector that reaches the external wall. This should any prevent moist air from getting into the Internal/external wall cavity. It also allows the external wall hole to remain the existing size which is great if any future work or wiring needs to be done.
- I will be replacing the rear vents cover when I can find a 300mm x 300mm rain and bug-proof grill
- These exhaust fans are quite a bit louder than I was expecting, they also move a tonne more air too so I can't really complain that much.
Tagging @EricL as he said he wanted an update when done
Hi @joejoeinc
Thank you very much for posting those photo updates. Your repair job looks fantastic and as I've mentioned before those fans will be more than adequate to ventilate the room.
We look forward to seeing your next project.
Eric
