Extractor fan backdraught prevention

[Theo Markettos]

Our somewhat jerrybuilt extension toilet/showerroom has a wall-mounted extractor fan that has no prevention for draughts getting in. The extension is perpendicular to the row of terraces, so the wind whips along the terrace and into the duct. Given the room also has four exterior surfaces (out of six) it's a trifle chilly.

I'm trying to source a grille that'll stop the incoming draught. I think it's a 9cm pipe, but haven't taken off the existing grille so it might be

10cm (extension was built in 2003).

Apart from fitting an inline trap (messy, as it probably requires sealing inside), the options seem to be gravity flaps:

a right-angled bend, so airflow comes from below: understand that the flaps tend to rattle in the wind, which would be annoying. How would these two compare for effectiveness? Does the bend actually prevent that much air ingress? How much more efficient are the flaps?

Thanks Theo

[fred]

In article , Theo Markettos writes

I suppose the ideal would be a cowl with a (backdraught) damper:

about half way down. Haven't used these but they sound like they would do the job.

[David Hansen]

On 07 Mar 2007 17:09:53 +0000 (GMT) someone who may be Theo Markettos wrote this:-

The one with a cowl will probably rattle less. However, it depends on the local wind conditions and what is actually causing the rattle. If rattles are caused by sudden pressure changes outside then the one with a cowl will rattle just about as much.

Alternatively leave the terminal alone and fit a fan with an integral back draught shutter such as

[Huge]

We have exactly the same problem in our en-suite. I've bought a Vent-Axia fan with power operated shutters. This one, to be precise;

replace the existing fan which only has "back-draught" ("draughty") shutters.

I'll let you know how well it works after I install it this weekend.

[Theo Markettos]

Interesting... what does the damper part consist of?

Theo

[fred]

Sorry "Haven't used these . . . ", you'll need to buy one & see :-)

Unlikely to be horiz flaps like your first example, more likely an oval plate mounted at an angle and pivoted slightly off centre to let gravity close the opening when flow stops. Let us know what you find.

[Theo Markettos]

I visited Screwfix on Friday (opening of our new trade counter, yay!) and acquired one of these:

does have a single large unsprung flap inside hanging vertically with the pivot at the top, intended to stop a backdraught. Haven't fitted it yet, but it's only two quid so I won't have lost very much if it doesn't work.

Theo