Instead of a solenoid have a look at 'duct damper actuators'.

These come in a range of prices, you can even get small ones for car vent ducts. They are designed to motor open or closed so only require power to change state.

You can also get complete duct dampers with actuators attached.

There are some gravity type flaps set at a slight angle that only open when there is internal pressure. Often fitted to cooker hoods, no leccie iused at all.

Generic car door lock actuators are pretty powerful and can be found quite cheaply in various designs and sizes. You need the 5 wire "central" one for stand alone jobs. About a fiver off E-Bay.

I've used them as a powerful solenoid substitute. Around a "long inch" travel I think.

Clive

Gravity flap with adjustable weight?

Regards Ian.

Our cooker ventilator, also rattles if the wind is in the right direction.

When we lived near the sea, many years ago, to prevent excess draught burning all the sold fuel overnight, I made up a flap with a weight on an arm, and used this to bleed air into the flue, and limit the draught. Worked a treat, pure guesswork, of course!

Howard

While some (most) ventilation can be positively avoided, do not cover any ventilation designed to provide a draught for fuel burning appliances. As you say you only want to close those with flaps, you may well be aware of the risk of carbon monoxide poisoning.

Try googling "belimo actuator" for a wide range of damper actuators – essentially a motor driven crank that turns trough 90 degrees. Other variants are available but looking at ebay at the moment there seem to be quite a few on offer around the £25 to 30 sort of mark.

HTH Simon