If HW is put on; CH rads go cold and don't recover?

19 May 2014
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United Kingdom
Hi folks, I have been trying to suss my system fault: I have inherited a conventional old-school vented gas system with 2 zone valves, on for CH one for HW, a thermostat for each service and an old-school programmer.

If I only switch the CH on, and the HW off at the programmer, the CH rads heat up as normal.

If I simultaneously, or later on; switch on the HW at the programmer, (leaving the CH on), the CH rads all go cold. They stay cold even if I then switch the HW off again at the programmer, or turn the tank stat down.

Both CH and HW zone valves are working and moving in response to the programmer and thermostat settings.

So if the HW is or has been on at all, the CH rads do not heat up - even 2 hours after I have reverted to CH only.

After extensive testing, including checking all the wiring and confirming with a multimeter, and viewing the zone valve heads moving, I am a bit stumped. (I am an engineer, so I understand how the system works and what each component does).

Could this be a duff HW valve internally - i.e. not the electric motor head, (which I can see working), but the actual valve element inside the brass housing failing and allowing water to leak past?

Thanks in advance.
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Could this be a duff HW valve internally - i.e. not the electric motor head, (which I can see working), but the actual valve element inside the brass housing failing and allowing water to leak past?
Feel the outlet pipe from the HW valve and see if it is hot if it is CH should still work, post a pic of the HW cylinder setup, do you have a restrictor on the outlet of the coil ?
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Have you tested everything, including the room thermostat?
Have you verified that the valves are moving as they should be under different command scenarios from the controller? One very common fail when using motorised valves is to borrow the motor live to feed the microswitch. This works fine if only 1 zone is active. Trouble comes when zone 2 (in your case dhw) also calls for heat. The borrowed link holds both valves open until BOTH thermostats are satisfied- which in your case will not happen cos there's no restrictor on the primary cylinder feed so the entire boiler output goes there, none to the rads, stat still calling for heat so BOTH valves stay open until programmer shuts BOTH circuits off.
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Under what conditions/by what process are you able to get the CH working again?
Thank you for your time so far: I have inherited this old system, and did not install any of it myself.

Both thermostats work and drive their respective motorised zone valves correctly. The wiring is all correct - wired to LWC 3 and verified with a multimeter, but I will double check as oldbutnotdead asks.

There is no restrictor fitted to the HW coil output, which is just plain 22mm from the HW zone valve to return - I suspect this is part of the problem.

When the whole system is cold, the CH on its own works perfectly, but if the HW is selected on at any time, the CH then stops working - even if the HW is selected off again. The boiler still runs. Hot water is present through the pump and towards the radiators, but the rads stop getting warm, and the boiler short cycles: (on for only about 2 mins at a time) even though the rads are all cold and the room thermostat is calling for heat, and the CH valve is open. I think the HW valve is not sealing when it is hot, even though it mechanically moves to off.

The only way to restore the CH rads getting warm is to let the whole system cool down, then select HW off and CH only on.

I am wondering if the ?rubber element inside the HW valve is breaking down and softening under heat, and then does not seal properly? With the system cold; the HW valve shut and no hot water flowing past the HW rubber element; perhaps it does not get hot enough to soften it, so the system works fine on CH only?
I say again, I did NOT install any of this ! Some covers removed for investigation, and TRV's on all radiators, by the way.

The CH zone valve is just visible feeding the 28mm pipe going vertically through the floor. Both valves move correctly.

I will be tidying all this up considerably when winter is over, and adding a magnetic cyclone filter etc.


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