How to replace square exhaust fan with a round one?

[johnwetherby312]

Hi I have a heller 250mm square exhaust fan that doesn't work well so I wish to replace it. When I removed it from the roof, I saw that the cutout was square in shape while most of the Bunnings exhaust fans have round ones. How do I convert the square cutout to a round one? Also I noticed that most 250mm exhaust fans at Bunnings have cutout sizes that are 290mm. Why is the cutout bigger than the fan? Thanks

[MitchellMc]

Welcome to the Bunnings Workshop community @johnwetherby312. It's to have you join us, and many thanks for your question.

Could you measure the square cutout in your ceiling for us? That way, we can advise on whether a repair to the ceiling will be required before fitting your new circular exhaust fan. In some instances, it might be easier to patch the square cutout and move the new exhaust fan's location.

The size of the fans is described as the diameter across the blades. The cutout is larger than the blades, as the exhaust frame is also inserted inside the cutout. The fan's faceplate cover is larger again.

Please let me know if you have any questions.

Mitchell
 

[johnwetherby312]

Hi Mitchell, the faceplate of the current Heller fan is 250mm x 250mm and the cutout underneath is probably 240mm x 240mm. Please let me know if you need other details

[EricL]

Hello @johnwetherby312 

In order to help us further with your project, have you decided which exhaust fan you'll be using to replace the old one? Can you please tell us the exact brand and model. As Mitchell has mentioned, it might be easier to patch the square cutout and move the new exhaust fan's location.

If you need further assistance, please let us know.

Eric

[johnwetherby312]

Hi Eric

I was ideally hoping to go with the fan linked below. The current fan is ducted but there is no duct actually connected and it vents straight into the roof cavity so I imagine the HPM non-ducted fan that I have linked should work fine. I haven't gone into the store to check the box and see what the cutout size will be but it looks like it will be round and approx 290mm. The current cutout is 240mmx240mm so the distance diagonally across might be about 340mm. I'm assuming this will be too big for any current fan if I was to try and make a 340mm circular cut out. Could I just make the 290mm circle cutout over the top of the current square, leaving the corners unoccupied by the new fan?

Thanks again for all the help

[MitchellMc]

Unfortunately, you'll need to repair the ceiling and re-cut your new circular hole, @johnwetherby312. I wouldn't advise trying to mount a circular fan in a square hole. Although you might get it to sit in there, I doubt whether you'll be able to attach the mounting clips securely.

As mentioned, repairing the cutout and moving locations would be best. This gives you a fresh piece of plaster to cut your new hole into instead of one that was compromised due to the repairs.

Mitchell

[johnwetherby312]

No problems, thanks for all your help. 

I'll get a plasterer to help me move the location