Trying to isolate a leaking radiator. I've closed the locksheild valve by turning it clockwise until it tightened and set the thermostatic valve to 0.

I've just tried bleeding the radiator to check that it's isolated (and what is dripping is just the left over contents of the radiator) and water is coming out at a pace as I bleed it.

How long should water come out of a radiator whilst I bleed it, if the radiator is isolated?

  • 1
    I'd give you a +1 just for "whilst"...
    – keshlam
    Sep 3, 2015 at 13:19

2 Answers 2


It could be that the 'stat valve or the lockshield valve isn't closing properly (it's not uncommon).

As to how long water will come out, that will depend on the size of the radiator and how fast the water is draining.

If the radiator doesn't have a drain point, I'd put a bowl or tray (whatever will fit under the valve) and remove one of the valves from the radiator. You'll soon see if that valve is letting water past when it's closed - you'll have water coming from the valve as well as the radiator. If water slows to a trickle, it's rpobably coming from the otehr valve. You could then take it off and check.

Note though, the water might be a sludgy black colour, so you might want to avoid getting it on the carpet.

If one of the valves is faulty, the options left are to either drain the whole system down or to freeze the pipes leading to the valves.


I am no plumber, but I have been in the trade long enough to see this type of need arise.

Most hot water radiator systems are in a "closed loop", that is the pipe that feeds the radiators goes from the boiler, to a pump, then to the various radiators on that loop, then return back to the boiler. There can be more than one loop. These loops are called zones. Each zone may have one to 4 or more depending on the heat loss that each radiator emits doing it's job before the water returns to the boiler to be heated again. There is a formula/calculation that plumbers use to determine how many radiators a zone can handle.

To bleed the system there is a faucet to drain the system (zone) completely, the other zones need not be affected. This part I am rusty on, Knowing what type of system you have and how old it is would help... for there are other valves that need to be closed (main water supply feeding the boiler, NOT the whole house) if the system is tied into your domestic hot water, as some will have that feature.

This may generate enough info to you to get you started, and have another member chime in...

And... to answer your question, it will be gallons, the amount that is in the line to the radiator and the other ones on the same zone.

The volume/quantity really should not matter since there is typically a garden hose attached to the drain valve or faucet, and that is run to a floor drain, sink or toilet, lower in height than the drain valve. You will hear the water passing, and after a time, say a half hour or less the water should have drained out, unless there is a supply still feeding it that needs to be cut off.

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