"Andrew Gabriel" wrote in message
...
In article ,
"dennis@home" writes:


"Adrian" wrote in message
...
HI All

I've not used float-type auto air vents before - so I may be doing
something wrong - expert advice welcomed !

The float vent I have in my hand has a screw on the top that appears to
close the vent even if the float is not floating.
I believe it is really for fast filling a pressurised system and not
actually an auto vent to release trapped air.
It looks like you open it to let the air out while filling and then close
it
to ensure air can't get drawn in by the circulating water.

The closing is because they don't last long if left open. The ball has
to stay wet to seal, and this means there's moisture exposed. This dries
and eventually leaves deposits behind, and these build up and stop the
seal working, so the valve starts leaking water.

There isn't normally negative pressure anywhere in a system --

Venturii effects say that may not be true, I wouldn't take bets if it were
in a 22x22x15 reducing tee on the main drag in a none pressurised system.

the header
tank is usually higher than the pump by more than the pressure
differential
the pump can achieve, although there are some less well designed systems
which do manage to suck air in the expansion pipe.

That is easy to do, just put the expansion pipe on the other side of the
pump to the fill pipe.