Weird issue: Automatic hot water only switches on if system powered off and back on

[tastyfish]

Hi,
Odd issue here, not sure if it's a plumbing one or an electrical one. Apologies if posted in wrong forum.

Background
Our central heating and hot water system operates off a pair of 2-port valves controlled by a HW/CH digital programmer.
The HW is programmed to switch on at certain times during the day. The CH is set to 'ON' permanently on the programmer and heating is controlled by a separate digital thermostat.
The system is a closed system (unvented).

The issue
The HW coming on at the programmed time does not trigger the boiler (or valve?). Nor does it trigger the boiler (or valve) when BOOST is pressed.
The only way I have found to make the boiler power come on (or for the HW valve to operate if the boiler is already on for CH) is to briefly switch off all power to the system, and then to switch it back on again after a few seconds. Doing this results in the HW valve opening (you can hear the surge of water going through and feel the hot water in the pipe through the valve).

So, in short, HW only triggers the boiler (or HW valve) if I switch off and on the power to the whole system. Where is the issue? In the valve? In the programmer? Somewhere else?! I asked a plumber mate and he has no idea. As I said, weird (the issue, not him).

 

[Echo the husky]

Most probably a fault with the programmer, what model is it? If you switch CH and HW off at the programmer and then switch only HW on does the valve open then? If it always works on its own but not when CH is also selected the programmer is faulty. The easiest way is to test if power is getting to the cylinder thermostat when HW is selected.

 

[CountryFan]

Sounds more like a sticking motorised valve imo. Do you know the make/model? If not, can you post a photo of them?

 

[dilalio]

You need to be able to check the MV.
Which is best done with a multimeter and access to the wiring centre.

Can you hear the valve motoring across and the click of the microswitch as the armature contacts with it... You can hear the click if you've got some silence in the house!

Usually better to listen for the motor winding back when the demand is ended, but a click is heard when powered on... After about 10-20secs