"Gym Sulkinson Fork" wrote in message
news
On Thu, 25 Oct 2018 17:27:20 +0100, Rod Speed
wrote:



"Gym Sulkinson Fork" wrote in message
news
On Thu, 25 Oct 2018 17:04:57 +0100, Rod Speed
wrote:



"Gym Sulkinson Fork" wrote in message
news On Thu, 25 Oct 2018 16:20:54 +0100, Ralph Mowery
wrote:

In article ,
says...

Why are these fitted instead of just a stationary vent like you see
on
caravans?

http://www.flettner.co.uk/van-roof-vents/

I don't see the point in using the wind to power a vent, when the
air
was already moving. Surely that's like fitting a generator to your
bicycle to power a motor to make you go faster?

They are powered by the wind that is blowing, not wind they are
making.

Why not just channel the blowing wind where you want it instead of
converting energy twice?

Because those things are extracting air from the house or van so that
isnt
possible.

Why is it better to move it out instead of in?

Because the hot air in the roofspace is already trying to get
out and it works better to help it do what its trying to do
than to try to stop it doing what it is already trying to do.

It still has to go in somewhere and out somewhere else.

Yes, but its better to help it move faster in the direction
it already wants to go.

Same ventilation takes place overall.

Nope, you move more hot air when you help it to go
in the direction it already wants to go, with a fan.

Ok, I accept your explanation. What surprises me though is just how much
hot air rises. Is there a formula to determine for example, in a
container of certain dimensions, how much hotter the top will be than the
bottom?

Yes, but its complicated.

In the case of the van, they are powered by the wind pressure the van
creates when moving.

The above link claims it works when stationary too (by wind).

Yes it does. You've likely noticed that houses don't actually move very
fast
and do have those vents too.

I've occasionally seen them on old houses on chimneys. I thought that
was
to disperse the smoke.

Nope, its just another example of a wind powered fan which
helps the air in the chimney move the way it wants to go.

I rarely see them here, on any type of boiler/fireplace arrangement.

Yeah, basically because the standard chimney pot works pretty well
when the chimney itself is properly designed so it draws well initially
when you are first lighting the fire and are maintenance free.