If the rad valves are both working, a single cold rad can be caused by
the circuit being 'unbalanced' ie, the other rads are 'stealing' all
the flow. If you reduce the flow through some of the other rads by
partially closing their lockshield valves, hot water will now pass
through the cold rad.
No! Don't do that, shut the other non lockshield valves off on all other
rads, then all the available flow goes through the cold rad. My system
is like that, even though it is fed from a normal boiler. My system is a
pig to get up and running again after a drain down because of all the
loops in it and this is what I do to get it up and running again.
For lots of fun it is possible (with experience) to have a replacement
valve ready, enclose the whole thing in a stout polythene bag and a
seriously decent cotton soak sheet. With both hands inside the bag
working VERY quickly you hold the old valve in place, remove the nuts,
whip the old valve out of the way and have the new one in place before
more than a cupfull of water is lost.
Takes practice, confidence and even more confidence, but it does avoid
draining down and creating all sorts of airlocks afterwards. Do make
sure before you try this that the threads on the old and the new
valves are identical. Some faint hearts plug the outlet and the vent
at the feed and expansion tank before doing this. A kit called a
plumbers mate consisting of two tapered rubber plugs is available for
such a task
Explained this to a mate once, when he needed to introduce a service
valve and a tee into an existing set of rad tails. Vented system, not
too much head etc. He said he understood what to do. Anyway, I was
working on something elsewhere in his house and heard a "scream", went
back to find a new valve assembly in place, but black iron stained water
all him, and all over the place including the ceiling! I said I would do
the next one. Pipe cutter until almost through - wiggle of the tail to
snap it off and thumb in over the pipe end - then quick swap for the
waiting valve - push down with one hand while doing up the compression
nut a bit to quench most of the flow, before giving it is final tighten.
Total water lost; about a cup full.
While watching me he commented that I had pre-assembled the compression
joint first! (he had uncovered and recovered the pipe to place first the
backnut, then the olive, and then the valve over the pipe, and also
forgot to turn the valve off first!
Well worth having, but a carrot can work if you don't have those!
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