6" fan/filter pushing through existing 4" bathroom extractor fan fixture

Will this work? The room I want to use already has an extractor built into the ceiling and I'd like to use the vent, will the bigger fan simply "push" round the smaller one?

Should I leave the weaker bathroom extractor turned on or off? Do I risk damaging it by forcing air through it?

You really don't want the duct size decreasing in diameter. First of all it takes power away from the fan. Second you'll be creating noise. So someone outside the house might notice the noise. (It will howl like a wind tunnel) Third the original exhaust fan will also block the flow of air. The shorter the distance and straightest route is the best route. Nothing should be in the duct itself without considering the power it will take to pull or push air through it. If you have an filter on your intake and drawing through a air scrubber. Your flow rate will not be what the specs say. In other words a 6 inch vortex won't be moving 449 cfm of air. There's other factors as well. I can't remember the term. But if you're using a tent you'll notice the sides suck in before any air is removed. That also draws energy from the fan.

So you think a 4" fan/filter would be better suited to remove the reduction in diameter? I haven't bought the fan/filter yet, but I chose this size as I'll be getting a 6" cooltube reflector, and also I figured a 6" fan turned down a bit will be quieter than a 4" fan on full wack?

I still think even with the limitations you described, using this outlet is my best option in the space I have to work with. Hopefully if I leave the bathroom extractor on too, it will lower the bottleneck it has on the system?

I can't tell you what's best for your situation. I can only tell you facts. There are as I said above. No matter what I'd try to at least remove the old exhaust from the equation and have as much flow as possible. But if you have a heat problem there will little you can do about. So this may very well be your limiting factor.

I'd remove the bathroom fan completely while u plan on growing then get 6-4" reducer and go. Leaving the fan in, on or off will impede the flow.

Thanks for the feedback guys.

I'm hoping then I can get the existing ducting out the way from this side and just vent into the attic, as getting up into the attack to do a proper job is virtually impossible.

I'm guessing venting into an average insulated loft space, whilst not ideal, will do the job, and the heat should dissipate from there?

GouldyBoy said:

Thanks for the feedback guys.

I'm hoping then I can get the existing ducting out the way from this side and just vent into the attic, as getting up into the attack to do a proper job is virtually impossible.

I'm guessing venting into an average insulated loft space, whilst not ideal, will do the job, and the heat should dissipate from there?

Click to expand...

Yup. Your attic has to have venting in it. Because heat naturally builds up there in summer months.