Reduce back draft along exhaust vent

[ADarkDividedGem]

I have a fan that blows 93,4 m³/h of air with a static pressure of 2,61 mm H₂O and I am wanting to ensure that air gets blown along the duct and out the vent. My current grid vent means my fan competes with the wind when it's blowing parallel to the vent.


After some quick Googling I found some possible solutions that will help prevent or reduce the amount of back draft:

1) Add a 90-degree turn to the vent to prevent wind from blowing straight into the duct but I am not sure how successful this will be on windy days
http://www.builderdepot.co.uk/manrose-100mm-4-brown-external-hooded-weatherproof-cowl.html

2) Maybe an exhaustion valve but I am not exactly sure what “The valves provide high initial resistance with wide throttling capability which is necessary to offset the relatively high system resistances.” means in practice
http://www.waterloo.co.uk/product-models/vb-exhaust-valves/

3) Air Volume Control Dampers might be another solution that have “been designed to provide positive control of airflow” but I am not exactly sure how it achieves that
http://www.waterloo.co.uk/product-models/wdd-control-dampers/

Have any of you used any of those products before or have any advice on the best way I could reduce the back draft?

Thanks

 

[Nozzle]

The fan is gutless, that's about the sum of it.

Nozzle

 

[^woody^]

I'm not sure what you want to achieve with those exotic solutions.

Anything you do to the duct or terminal is going to reduce the fan airflow. So the normal solution is a simple anti back draught shutter either in the duct (where the wind won't rattle it) or at the terminal. Along with a suitable cowl or hood similar to that in your first link, but that Manrose thing has a flap that sounds like Morse Code on windy days.

 

[ADarkDividedGem]

The fan is gutless, that's about the sum of it.

At what point would the fan not be considered gutless? I can replace the fan for a stronger version that can go all the way up to an airflow of 186,7 m³/h and static pressure of 7,63 mm H₂O

I'm not sure what you want to achieve with those exotic solutions.

Ideally something that wont simply stop all airflow like a shutter as I need to ensure hot air is constantly being expelled. I am assuming that a reduction in airflow is better than a complete stop. Maybe such a solution doesn't exist and with that in mind the best bet is getting a simple terminal with a hood. Thanks for pointing out that Manrose product has a flap.

 

[Nozzle]

There's the thing, when the non-return flap closes because of strong wind it's closing because the DIFFERENCE in pressure forces it shut. Imagine if the flow along your pipe was the same and the wind was strong enough to shut the flap, only the flap wasn't there, then the flow along the pipe has stopped anyway. The very same difference in pressure than forces the flap shut is also stopping flow... even if the fan keeps on whiring.

What is this hot air, which must not be stopped, all about? If it's truely hot, you need something with metal blades and a motor engineered for working in heat.

Nozzle

 

[endecotp]

Up to what wind speed do you want the fan to continue to work?
Say 20 m/s (about 45 mph).
Wind pressure = 1/2 rho v^2 where rho is the density of air, about 1 kg/m^3
= 200 Pa
Now look at the flow rate vs. pressure curves for your fan, and see what flow rate you get against a pressure of 200 Pa. If it's off the curve, you'll get back flow. If it's on the curve you'll get forward flow, but less than the curve shows because you also need to allow for the dynamic pressure. But it's a start.

I think that cowls and flaps are mainly to stop rain being blown in and to prevent back drafts when the fan is off; I don't think anything except a more powerful fan will help the fan to overcome wind pressure. In most cases wind is variable (gusty) and it's not required for the fan to work perfectly continuously. What are you venting?

 

[Nozzle]

^^^Listen to this guy,

Nozzle

 

[Nige F]

A Mushroom Vent Cowl from Toolstation ( for example) may be worth a try @ £12. No valves to start with.