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Bogart

Bogart

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"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.
 
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...
 
Bogart said:
"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.
Click to expand...

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.
 
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.
 
Chuck said:
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.
Click to expand...
Sounds much the easier way:)
 
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.
 

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