Quiet the Vent

Our kitchen exhaust fan dates from the 1950’s and there are probably millions of similar ones still in use across the world. If you examine how this thing works, there’s a aluminum grate that you can see that screws into the fan in the ceiling. Inside the grate there’s a round disk of metal fibers that acts as a filter. Above the fan but before the pipe exits to the roof, there’s a butterfly valve of sorts that is supposed be blown open when the fan is on, then gravity pulls it closed when not in use to keep drafts out. However there are two problems here. 1) the plastic of the valve element is pretty lightweight — it has to be in order to be blown open by the fan — but this means that outdoor noise is not blocked and comes in past the fan. 2) Even worse, over time the weight of dust, oil and dirt on top of the plastic valve wings means they get heavier and heaver therefore opening less thus restricting the airflow and usefulness of the exhaust fan.
Rather than cleaning or repairing the thing, I created a better solution: bypass the butterfly valve, and add a manual sound dampener .

Bypassing the valve:
The most difficult part of this alteration is removing the fan itself to get at the valve above. If you’re clever you might be able to do this through the fan blades, but for me it was easier to unscrew the two bolts to afford full access. A simple paper clip is all that’s required — just clip the two flaps above the fan so they stay together in the open position.

The dampener:
The blocker itself (version 1) is pretty simple. A circle of cardboard cut to a diameter just slightly larger than the hole in the ceiling. On the upward side of the cardboard, some Dynamat is affixed and cut to match the diameter of the cardboard. https://www.amazon.com/Dynamat-10612-Self-Adhesive-Deadener-Superlite/dp/B002SG0OK0?ref_=ast_sto_dp Dynamat is a thick rubber adhesive layer that can be put on on anything — from car doors or clothes dryers — to attenuate noise.

Lastly, some foam pads are needed on the grate to hold the blocker snug against the ceiling.

I also added a nut to the gnarled bolt, which keeps it attached to the grate and makes putting it back up much easier.

Its true that on occasions that we need the fan, we’be got to unscrew the grate and remove the dampener. But the grate just unscrews and can be taken off in about 15 seconds. Otherwise our kitchen is quiet, non drafty, and when the fan is in use, the amount of smoke getting sucked out out the exhaust fan is maximized.