DIY Doctor UK DIY Forums

I updated this post to try and make it a little clearer. Unfortunately my system doesn't conform to any of the Honeywell 'sundial' plans, and I can't find any schematic that has the same layout (it isn't gravity fed)

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I am struggling to sort out a problem with a single radiator not getting hot. All other radiators in the house work fine and get up to temperature pretty uniformly.
Taking off the rad flushing the rad, as well as the whole system, hasn't helped.
In doing so, I also checked feed/return both had free running water so I don't believe there is any blockage.
I've now closed all radiators except the bad one and it still isn't getting warm.

My system is an old one based on Randall 102 programmer, with a single 2-port valve to control the central heating - so I can have hot water only, or hot water+heating, no independent heating. So I suspect that the water is short-circuiting the radiators by going through the hot water loop. The cylinder has a lock shield valve next to it and I believe this is to control how much water can short circuit the heating loop, and I am tempted to try closing this valve. However, I am concerned that I may cause damage to my system by over-pressurising it when I only want hot water.

I would appreciate any advice.
Thanks in advance.

From the way you describe the system, it does not fit any of the standard designs so we do have to a bit careful how we proceed.

Generally, the cylinder coil has two uses. Firstly, obviously, to heat the hot water, but it also allows the water to circulate the system at all times to prevent the boiler from over heating and the pump from being damaged.

The lockshield in the cylinder circuit should be almost closed. In fact the way to set it is to close it, then open it one or two full turns. Listen or feel the water begin to circulate. As long as it can, is sufficient. If it is open too far then exactly what you describe will happen.

Thanks for the advice plumbbob.

I suppose my biggest concern is that over restricting the flow will lead a high pressure burst of a pipe/joint, or worst still inside the cylinder coil. I have no idea whether a pump is capable of doing this though.

A burst in this way is not an issue. For safety and to prevent damage the water has to be allowed to flow at least to some degree. If this is prevented then the pump may cavitate which may damage it. Also, the flow of water through the boiler heat exchanger may be stopped to such a degree it may reach boiling point with the obvious consequences.