Worcester 24cdi combi - two radiators dead

[spiffytheseal]

Hi DIYnot,

We've just moved into an older property (1700s building) with a Worcester 24cdi combi gas boiler. We have 6 radiators total - two upstairs, a towel rail in the bathroom, one in the hall, one in the kitchen and one in the living room. It is a closed system.

The latter two (kitchen/living room) do not get hot at all. They are "drop" radiators (as in, the main flow/return from the boiler is upstairs and these two radiators are fed by plastic pipes that "drop" through the ceiling and T-off to each one).

We have bled all radiators, balanced them, drained the entire system five times from three different drain-offs with all valves open, flushed the two problem radiators by taking them off the wall and putting a hosepipe through them and confirmed all lockshields/TRVs work and function as expected. When we bled the radiators there was a large amount of black/grey water, but none of it was thick or runny. We are confident there is no airlock in the system, as we have been able to put a hosepipe on one drain off and have it push water through the whole system, out of another drain off through another hosepipe.

But there is no sign at all of the "dropped" plastic pipes getting hot - they remain stone cold, whereas the radiators that are before the "drop" in the system get lovely and hot.

The boiler is set to 1.8bar cold, it rises to 2bar after operating for a while.

However, what we have managed to do is get hot water flowing through these two radiators by draining one of them through the drain-off whilst the boiler was on (not advised, I know). This did lead to hot water going to the inlets on each radiator, but as soon as we stopped draining they started getting cold again.

It's a long shot, and my job for next week will be speaking to various CH engineers in my area, but does anyone have any ideas?

 

[easyflow]

Have you balanced the radiators ?

 

[jackthom]

Your drain off test shows that at least one of the drop pipes is letting hot water pass to the radiators but there's a possibility that the other pipe is partially blocked.
Could you try draining one of the offending radiators with both valves closed then disconnect one valve from the radiator at a time and try opening the valve into a bucket or other suitable container.
Don't lose too much of your system water (it'll need a top up anyway after or during this test), just check that you get a good strong squirt of water through each in turn when the valve is opened.
This may even clear out any sludge in the down-pipes if that is the problem.

 

[spiffytheseal]

Thank you both for your very speedy replies.

Have you balanced the radiators ?

Yes we have, multiple times!

Your drain off test shows that at least one of the drop pipes is letting hot water pass to the radiators but there's a possibility that the other pipe is partially blocked.
Could you try draining one of the offending radiators with both valves closed then disconnect one valve at a time and try opening it into a bucket or other suitable container.
Don't lose too much of your system water (it'll need a top up anyway after or during this test), just check that you get a good strong squirt of water through each in turn when the valve is opened.
This may even clear out any sludge in the down-pipes if that is the problem.

There are two drop pipes - a flow and return, which T-off from the main copper flow/return upstairs. The two problem radiators each T-off from the drop flow and rejoin at the drop return - it's almost a sub-system of the main flow/return system upstairs.

We did actually try that - we drained the kitchen radiator with both valves closed. Opening the lock shield valve, water came gushing out. Opening the TRV, water came gushing out too! So in my mind, there is pressure both sides of the radiator - only it's cold water and not hot.. Same with the living room radiator - there is positive pressure and water on both valves there too but again, only cold water.

It seems as though hot water is not getting down the drop pipes by pressure in the system alone. Is there any possibility the pump may be clogged or furred up so much that it's not outputting the volume of water needed to push it down into the drop pipes?

 

[easyflow]

If you have just moved in to that property I would first look to see if they have been piped properly,
As in you two flows or two returns rather than one of each,
I that checks out ok then look for a blockage normally in a bend or T.

 

[spiffytheseal]

If you have just moved in to that property I would first look to see if they have been piped properly,
As in you two flows or two returns rather than one of each,
I that checks out ok then look for a blockage normally in a bend or T.

Thanks again. Yes, moved in two weeks ago and been trying to battle this ever since.

There is one "main" flow and one "main" return - this is copper and feeds all working radiators. There then T's off to another flow and another subsequent return - these are plastic and drop down to the ground floor of the house, which then T's off to two radiators (the two trouble ones). Cold water can get through these if we flush/drain the system using drain-offs and hosepipes - even hot water can get through if we drain a troubled radiator whilst the boiler is running (this heats up both radiators!) but the boiler doesn't seem to want to push hot water around them by itself in normal operation..

Annoyingly, the junction where the main flow/return and the plastic flow/return meet is under tongue/groove floorboards which are not easy to get to. Short of taking these up, I'm trying every trick I can get my hands on to try and remedy it!

 

[easyflow]

If all the other radiators are getting hot Its unlikely the pump is at fault.
So would follow the two pipes back to flow and return and make sure they are connected as they should be.
Or a blockage is present.
Have you shut all other radiators off to force heat to the offending rads.

 

[spiffytheseal]

If all the other radiators are getting hot Its unlikely the pump is at fault.
So would follow the two pipes back to flow and return and make sure they are connected as they should be.
Or a blockage is present.
Have you shut all other radiators off to force heat to the offending rads.

Yes I have - all radiators off and one/both of the trouble radiators on. No change - still stone cold. The pipes aren't even warming up.

Would a blockage still be the case if we can pass water through the system using drain-offs?

These are the farthest two radiators from the boiler in terms of the circuit, if that matters at all...

 

[easyflow]

Still a blockage or two returns or two flows.