bathroom extractor

I am fitting a bathroom extractor fan the bathroom has an existing vent via an outside airbrick if I remove the vent can I put my new 4inch ducting upto the old airbrick or will this restrict the airflow.
Would it be better to cut a hole in the airbrick and put the ducting through it with an outside backdraft shutter.

 

It will restrict the airflow, but perhaps not by too much. Personally I'd try it and see.

 

You are putting 4" out thru 10" vent? will not restrict flow.

 

Yes but the 10 inch brick is not a straight through outlet it's made up of a series of holes which in total might not
Add up to the volume of the 4 inch pipe.

 

If it was a toilet, there wouldn't be an issue, but being a bathroom, you're extracting moisture laden air that is going to condense on to the cold airbrick, and eventually cause a problem. Use tubing and an outside shutter, and remember to have a slight downward slope towards the outside for the water to drip away.

 

Had the same thing on my bathroom for 20 years , never a problem.

 

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