Cold Radiator

[BobbyOllox]

Newb - please bear with me - I use this a lot for reference and advice, but first question. I have one radiator that refuses to get anything more than luke warm. I just spent most of my Sunday replacing it with a new one and new TRV and lockshield - with only marginal difference.

Let me try and explain....

The boiler is in the roofspace. There are 12 radiators in the house, on 2 floors. The problem one is in the living room (ground floor) which is an extension. The flow and return are buried. I cant work out the order of the rads, but it isnt the most physically distant from the boiler, but it is the only one to have buried pipes - the other downstairs rads are all fed with pipes that come down from the ceiling,
It is the last to get heat, the flow pipe gets hot. and after a while, the radiator gets warm at the top, then lukewarm all over. I assume this is just conduction from the flow pipe, rather than water movement. The return pipe stays resolutely cold.

There is a drain valve on the return pipe. and if I open that (after closing the lockshield) , within a few seconds piping hot water will flow out of it. So return is not blocked, and no sign of an airlock

Its slightly better after changing the rad today, but not good. If I turn all the other rads off in the house, it gets almost hot, but not too hot to touch. The other rads will get hotter when they are on.

I dont know where to look next - changed all components, no blockage, no airlock. Im guessing that it could be bad layout of pipes due to the long spur and the return flow is inhibited somewhere, or possibly the pump doesnt provide enough force to lift the return water from under the ground to the roofspace. I dont know - strictly amateur.

Any thoughts or suggestions gratefully received.

 

[TCCHeating]

If it doesnt get hot enough with the other rads turned off then its prob a blockage or piping problem.

From reading your post it seems that you have proved the return pipe is not blocked, but have you checked the flow pipe to the radiator?

 

[BobbyOllox]

Yes I confident that there is good flow to it - it feels burning hot, and due to a plumbing accident earlier today, there was quite a spectacular spray from the flow pipe - now fixed).

In a moment of inspiration, I have tracked down where both flow and return rise from under the floor and run up the wall to the 1st floor. With all other rads still, off, the pipe can be described as noisy. Its not banging, and its not leaking, you can just hear the water flowing through the pipe It sounds disturbed or interrupted - do you know what I mean ? - Half way up the rise there are two isolating valves. The valve screw on the return is not quite vertical - I tried but it wont entirely straighten - so that my be impacting the return flow. To be honest though the noise is louder on the first flow under the floor boards where the pipes must join onto the main circuit I will take a look in the morning, and see if there is anything amiss, or if I can access under the floor.
update - closing either of the valves stops all pipe noise referred to above - so i am convinced the flow into or out of the spur is disturbed in some way - it may not be the valves themselves of course

Must be worth changing the isolating valve though dont you think ?

 

[BobbyOllox]

Today after running the system for a few hours, I got all the proof I need.

The return flow pipe, below the isolating valve ( the rad side) was lukewarm, and the temperature of the flow pipe above the valve ( the boiler side) was significantly hotter.

The valve, even when open, is providing a barrier, preventing flow back from the radiator.
... and now I found lots of comments saying isolating valves shouldnt be on a ch pipe. This seems to support that view.

So I shall take it off, and while the pipe is open, take the opportunity to flush out the spur with mains water - just in case.

Any advice on what to put in its place - I guess take a little more of the existing pipe out, and replace with two couplings and a short section of new pipe.