HW works but CH intermittent

29 Nov 2019
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United Kingdom
I have a older vented fully pumped system with a trusty old Glow-worm Spacesaver 75 boiler, new Drayton LP722 2 channel programmer, new Grundfos pump, 2 x 2 port Sunvic MOMO valves, and Honeywell thermostats for the CH and the HW cylinder.

Been working fine for years, with CH and HW working independently. HW still works fine, but CH has been increasingly intermittent. The odd thing is that I can sometimes kick start the CH valve by turning the HW cylinder thermostat up, which makes no sense to me. CH valve will then start the pump and call for heat, and stay on as long as required, even if HW is turned off at the programmer.

I replaced the Grundfos pump (which was getting a bit noisy), the HW zone valve (which was leaking), and checked that the CH valve turns freely. I became convinced that it must be the old Microgyr programmer, so I replaced that with a new Drayton LP722. Still have the same symptoms, i.e. HW fine, but CH rarely works.

If I manually open the CH valve by the red wheel on the actuator (with programmer on and room thermostat up) the motor returns it to the off position. However, if it does start from a signal, the microswitches trigger the pump and boiler as expected. When CH and/or HW are working, both thermostats behave as expected.

I'm beginning to think it might be a wiring issue. I have visually inspected, but wouldn't feel confident with a multimeter. I'm not sure how to test a 4 wire room thermostat by bypassing it. The wiring in the junction box looks very amateur, and I didn't have a lot of confidence in the electrician that did it, but to be fair it's worked for 10+ years.

Anything else I can check before calling in an electrician? Thanks in advance.
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Anything else I can check before calling in an electrician? Thanks in advance.

If you don’t feel confident with a multimeter or safe isolation i would call in a heating engineer/electrician who deals with these systems.

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