Worcester 30 CDI system boiler

Hi I have a Worcester Bosch 30 CDI system boiler the radiators get warm, not hot, when there is hot water demand but when there is no demand for heating. I have replaced the CH zone valve but the radiators are still warm.
Is there anything else that can cause this or have I fitted a faulty Zone valve

 

Hi itchyspanner.
I think you are correct and it is a problem with the pipe work. Last winter I called out a plumber as there was no heat to the radiators in the extension (3 radiators) after some investigation he found that the flow pipe 15mm plastic pipe was totally blocked this feed was taken by the plubmer that piped the extension from the old bathroom radiator. The option to replace this section of pipe was not possible due to having the bathroom floor tiled. The new plumber decided that it would be a better option to tee into the flow and return at a different point.
Now where he teed in is on the main 22mm flow and return pipes, the tee on the flow is now the first take off just after the boiler and the return is the last tee just before the pipe returns to the boiler.
Now what I think could be happening is that when return water from the hot water circuit reaches this new tee just before returning to the boiler it takes the new route to the extension radiators and sets up a reverse flow in these three new radiators and then returns to the flow pipe of the main heating circuit heating the rest of the radiators in the property,could this be possible as I'm no expert I'm only guessing here. Anyway I turned off the three radiator valves on the return side of the three radiators in the extension and this seems to have stopped the other rads getting warm. Can you confirm that this is possible and if there is a none return valve that could be fitted in the return pipe work for the extension that will prevent reverse flow through these radiators. Appologies if my expalation is a little difficult to follow but as I say I'm no expert.

 

The system was flushed by the guy that repiped the extension

 

The guy removed a radiator and flushed across the two fittings with the heating on, the system worked fine when the boiler was first installed four years ago, there was no problem for first two years even when the pipe to the extension got blocked the rest of the system worked fine. The blocked pipe was blanked off and bypassed last winter so the unwanted heating of radiators wasn't noticed unill the heating was turned off when the warmer weather started.

 

Magna clean filter fitted when boiler was installed cleanser used when flushed and inhibitor used after. The boiler is serviced annually. Would there be enough pressure on the system to operate a none return valve if I fitted One in the pipe work to the extension

 

There is a bypass valve fitted under the boiler, before the zone valves, between heating flow and return pipes.
Would fitting a NRV in the return pipe of the extension pipe work prevent water entering the extension through the return. thus ruling out my theory above. This is looking more and more like the cause of the rads heating up when there is no heating demand. As when all three rads in the extension are shut down on the rad valves, so preventing flow through the circuit, there is no unwanted heating of the other radiators.

 

Hi Plumberboy
Thanks for confirming my suspicion, would fitting a none return valve in the extension return pipe prevent the reverse circulation stoping rads getting warm when there is no demand for heating and allow the system to work correctly.

 

Re-pipeing is not an option as too much work involved in lifting the tiled floors. I will go for the NRV and look to re-pipe when ever we renew the bathroom.
Thanks very much for your help & advise.