thanks for the reply johnd. i am not sure at all that this is needed and I hope not. the system is about 18 years old and i have one large double radiator in the lounge that is cold. i read in the forum that the likely cause could be a rad valve that is sticking. is this easy to rectify?
if it has a thermostatic valve, it could be stuck closed. if so, take the head off and see if the pin pops up (you may have to wiggle it)
if no TRV open and shut the valves at each end
close the other rads and see if this one warms up.
bleed it
see if it is cold at the top, bottom or middle when (if) it gets warm.
what size are the radiator pipes?
if it doesn't get warm, close the valves at each end tight. Open one of them and see if water squirts out when you bleed it. then shut that valve and open the other, and see if water squirts out when you bleed it.
do you know where the drain cock is?
does it have a feed and expansion tank in the loft, and does it have mud at the bottom? If so bale it out and sponge it clean.
Are you able to take that radiator off? you can then squirt the valves into a bucket (if they have flow) and hose the radiator through in the garden.
If you have a sediment problem you will probably need at least a chemical clean. Can you do basic DIY plumbing?
If you can get any flow into that rad (e.g. by closing all the others) then you can use a chemical cleaner such as Sentinel X400. You leave it in for 4 weeks and it loosens dirt and sediment so it washes round in the water, then you drain and rinse it out. It costs £15. This often saves you the cost of a powerflush, and will only help, not harm. If you have a sediment problem and can afford £100, you can additionally fit a Magnaclean on a vertical 22mm return pipe to the boiler, and you will be amazed and delighted at how much black sediment it traps out of the water once it has been loosened by the chemical. It continues working permanently to prevent future blockages.
if there is no flow at all a chemical will not reach it.
if you can't free off the TRV by wiggling and pressing the pin you can fit a new one. Post a pic and someone may suggest an easy fit replacement.
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