Making a flap valve

[Matty F]

My range hood has a horizontal flap in it that is shut when the fan is not going, and opens upwards when the fan starts. The flap is of course to stop the wind coming in when the fan is not in use.

I've just made another range hood for a very confined space. Nobody makes o ne small enough, and I don't even want to discuss that. But I can't make a flap valve that opens and shuts properly. I have a 100mm ID pipe that goes from the range hood to outside, and I have cut a long rectangular hole in it and have a hinged curved flap inside. Bu t a 40 watt fan won't open an aluminium flap. I've just made a very lightwe ight plastic flap, and that gets hingebound. There's just too much friction . Perhaps I could have a circular flap inside the pipe, but unless it's much smaller than 100mm it will foul on the pipe when it opens. Any other ideas?

[harryagain]

I've just made another range hood for a very confined space. Nobody makes one small enough, and I don't even want to discuss that. But I can't make a flap valve that opens and shuts properly. I have a 100mm ID pipe that goes from the range hood to outside, and I have cut a long rectangular hole in it and have a hinged curved flap inside. But a 40 watt fan won't open an aluminium flap. I've just made a very lightweight plastic flap, and that gets hingebound. There's just too much friction. Perhaps I could have a circular flap inside the pipe, but unless it's much smaller than 100mm it will foul on the pipe when it opens. Any other ideas?

A lot of hoods don't have a flap valve. They get gummed up with deposits anyway. They rely on the fan and filter to limit back draughts.

[Matty F]

Well my existing range hood has a perfectly good flap valve, so I know it's possible. I have seen a horrible fan mounted in a window that has a manually operated shutter. It's horrible and I don't want that.

[dennis]

You need to weight one side of the flap so it only just closes under gravity and then the fan will open it. Well it will until the grease and stuff sticks to it.

You could fit the spring return mechanism off a zone valve if you have one lying about. Powered opening, spring closing.

[Matty F]

not going, and opens upwards when the fan starts. The flap is of course to stop the wind coming in when the fan is not in use.

es one small enough, and I don't even want to discuss that.

have cut a long rectangular hole in it and have a hinged curved flap inside . But a 40 watt fan won't open an aluminium flap. I've just made a very lig htweight plastic flap, and that gets hingebound. There's just too much fric tion.

uch smaller than 100mm it will foul on the pipe when it opens.

You'd think so, but that doesn't work. It is impossible to make the curved flap fit the pipe completely, so air goes through the gaps and the flap doe sn't open.

The air pressure will not open against a spring. I need some fresh ideas.

[Richard]

Maybe

or ?

[Matty F]

Yes the last one might do. I had not thought of hinging it in the middle.

[dennis]

I think you misunderstand.. the power opens the flap using the motor in the zone valve mechanism, the spring closes it when the power is removed. You just connect the motor in parallel with the fan.

[John Rumm]

Could you not use a flap vent on the exit?

Like:

[John Rumm]

Its a variation on this theme:

slight variation in the areas of each side meaning it blows open from one side and shut from the other, but is nearly balanced.

[harryagain]

I've just made another range hood for a very confined space. Nobody makes one small enough, and I don't even want to discuss that. But I can't make a flap valve that opens and shuts properly. I have a 100mm ID pipe that goes from the range hood to outside, and I have cut a long rectangular hole in it and have a hinged curved flap inside. But a 40 watt fan won't open an aluminium flap. I've just made a very lightweight plastic flap, and that gets hingebound. There's just too much friction. Perhaps I could have a circular flap inside the pipe, but unless it's much smaller than 100mm it will foul on the pipe when it opens. Any other ideas?

Is the hood in position? If so you have to let air into the room to replace what's sucked out. If not there will be no air flow through your hood.

[Matty F]

OK, I've installed a flap vent with a single flap and it works perfectly. But the wind causes it to bang shut every few seconds, and that makes an an noying noise. I have put some foam for it to bang against and it's quieter but not quiet enough. It needs some kind of damper, but not one that stops it opening when the fan is going.

[dennis]

Put a cowl over the end to stop the wind blowing into the pipe.

Use the motor driven valve I suggested earlier.

[Matty F]

There is a cowl over the end. I'll think of something.

[Dave Liquorice]

How much shove has the fan got? Would adding a bit of weight to the bottom of the flap stop the wind opening it but still allow the fan to? I expect that to be an AOT exercise.

If that doesn't work I think you'll have to look at a solenoid something to force the flap open against a weight or spring that is strong enough to stop the wind opening it.

[Matty F]

I've already made two flaps that the fan can't open. I think I put put some foam at the top of the flap by the hinge. I am sure that will work.