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

 

[tastyfish]

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.

Thanks for taking the time to reply. Prog is a Horstmann Centaurplau C27. So I tried your suggestion with the CH Off and then turned the hot water in three ways: manually, on timer and with Boost. In all three cases the hot water came on (boiler on, valves on). Which is possibly mimicking in some way my action of turning all HW/CH power off and back on again.

Any thoughts on what it may be? The programmer?

Come to think of it, I only noticed this issue at the start of winter when the CH was kicking in.

 

[tastyfish]

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

Hi. They are both Honeywell - see pic. They look the same but seem to have different model numbers - the CH is V4043H1056 and the HW is 272848

 

[tastyfish]

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

Thanks for the reply. I don't have a multimeter but I can ask someone. I can't hear the click of what must be the microswitch when the HW is meant to come on, only when I force the HW with the CH off or the system switched off/back on. If I switch the HW off I then hear the whirring of the valve closing.

 

[Echo the husky]

If the HW valve opens fine on its own and fires the boiler on it will be the programmer at fault. The internals use a series capacitor to provide the supply to the electronics, this ages over time and can no longer provide the current required to power both CH and HW relays at the same time, but it is still able to power one at a time.

If you buy the same model you can just fit the new one to the old back plate without altering the wiring in the back plate. Just switch the mains off before taking the old one off.

 

[tastyfish]

Thank you Echo, yes, I think you're right, although once the HW is on both do work at the same time, so it may just be that initial connection?

I'll have a look at replacing it, hopefully straightforward. Thanks once again.