TRVs just read this info

[Bogart]

"The only point to remember is if radiators with lockshields in the flow require bleeding, the heating may need to be turned off first. If the system is a vented low pressure type with header tanks, opening the bleed valves may result in air being drawn in rather than expelled." Is this correct? I cannot see what difference it makes as to if the trv or the lockshield is on the feed side. I just always put the bleed valve at the return end.

 

[king of pipes]

Yes it is possible to draw air in if you vent with the pump running on a low head open vented system, so fill and vent with system off . Kop

 

[Ric2013]

I suppose if there is a restiction on the flow side, then cavitation could occur? I'd be more concerned about a negative pressure system, the old pumped return type. Yet to actually have it happen to me though...

 

[Chuck]

"The only point to remember is if radiators with lockshields in the flow require bleeding, the heating may need to be turned off first. If the system is a vented low pressure type with header tanks, opening the bleed valves may result in air being drawn in rather than expelled." Is this correct? I cannot see what difference it makes as to if the trv or the lockshield is on the feed side. I just always put the bleed valve at the return end.

If you open a bleed valve at a point of negative (gauge) pressure air will be drawn in. Such points can occur in some low-pressure systems depending on the pump head and hydraulic impedance of the pipework, state of the valves, etc. So you can either do a complicated and difficult calculation for each bleed point to ensure it is at positive pressure or just turn the system off while you bleed it.

 

[Bogart]

Yes it is possible to draw air in if you vent with the pump running on a low head open vented system, so fill and vent with system off . Kop

What would you class as a low head? Am still bemused as to how the water knows the difference between a lockshield and a trv?

 

[Welder]

Personally, I always bleed with system off and having been off for a while. That way any micro bubbles can coalesce out into removable air.

 

[Bogart]

If you open a bleed valve at a point of negative (gauge) pressure air will be drawn in. Such points can occur in some low-pressure systems depending on the pump head and hydraulic impedance of the pipework, state of the valves, etc. So you can either do a complicated and difficult calculation for each bleed point to ensure it is at positive pressure or just turn the system off while you bleed it.

Sounds much the easier way

 

[Best]

I do know I have came across rads at a low head position that sucked the air in if heating running when I tried to bleed them.
Pump I think was on the returns.
I used to turn one radiator valve off (I presume it was the return side) to stop the suction.
Or I just made sure the heating was off.

I would have thought lockshields on the flow would mean if you turned TRV off it would actually be an advantage and prevent suction?
So that statement is wrong.