Hot water stays on, even when programmer says it's off

[Julian7]

I have a conventional heating system. Gas boiler with radiators and a hot cylinder. Heating has a single thermostat in the hall and the water, a thermostat on the cylinder. There are two zone valves (Drayton ZA5), one for the hot water and one for the radiators.

The problem is that the hot water is now heating all the time. The boiler cycles on and off (I assume according to its own thermostat) and the pump in the boiler is running all the time. The programmer (Hive) has the hot water and the heating OFF.

I can move the spring-loaded levers on both the zone valves. i.e. They aren't stuck. I have replaced both zone valve actuators as this has happened before. Everything has been ok for a while, but now the problem has returned.

What else can I check? Thanks for any suggestions or troubleshooting tips.

Julian

 

[steadyon]

1. Could be that the valve itself has stuck in the open position. Remove the head and try turning the spindle. Should turn easily with firm finger pressure, but only about 20 to 30 degrees from open to closed. If not, replace the whole valve. This is a "wet" job, so you will need to isolate or drain down the system (CH and hot water).
2. If you do replace the valve I'd suggest replacing with a Honeywell V4043H, which in my experience last longer. Its a little more work as you have to remove the nuts and olives from the pipes as the Honeywell has difference threads from the Drayton. You can't just reuse the old ones.
3. You could test it first by swapping the CH and DHW valve heads and juggling with the thermostats.

 

[Murdoch]

Has it worked properly since the Hive was fitted?

 

[Deleted member 120897]

Could be the microswitch in the zone valve sticking/stuck.

 

[Julian7]

Has it worked properly since the Hive was fitted?

@Murdoch Yes. The Hive has been there for years and years. It's the Mk1 version. I had it installed pretty much when it was first announced.

 

[Julian7]

Could be the microswitch in the zone valve sticking/stuck.

@Undertrained. That's what I thought but I've replaced the whole Zone Valve actuator, which ought to preclude a switch sticking.

 

[Julian7]

1. Could be that the valve itself has stuck in the open position. Remove the head and try turning the spindle. Should turn easily with firm finger pressure, but only about 20 to 30 degrees from open to closed. If not, replace the whole valve. This is a "wet" job, so you will need to isolate or drain down the system (CH and hot water).
2. If you do replace the valve I'd suggest replacing with a Honeywell V4043H, which in my experience last longer. Its a little more work as you have to remove the nuts and olives from the pipes as the Honeywell has difference threads from the Drayton. You can't just reuse the old ones.
3. You could test it first by swapping the CH and DHW valve heads and juggling with the thermostats.

Thanks very much for the suggestions. I unclipped the actuators and both valves turn very easily. I pushed them to their limits in case there was something stopping them opening or closing completely. I'll see how that goes.

Do you know where I can find a circuit diagram of how this is connected, or is every setup too different? I get the impression that the cylinder stat opens the zone valve and the zone valve then tells the boiler to turn on, which causes the boiler pump to turn on, but I'd like to see that set out.

 

[Chester]

Google 'S' plan wiring diagram

 

[Julian7]

Google 'S' plan wiring diagram

That's fantastic. Thank you so much.

 

[nicam49]

Hi can I suggest it might be a faulty tank stat that's stuck on 'demand'.. had this problem with a friend's system that the British Gas engineer failed to suss. I removed the tank stat and it fell to bits on me.

 

[Julian7]

Hi can I suggest it might be a faulty tank stat that's stuck on 'demand'.. had this problem with a friend's system that the British Gas engineer failed to suss. I removed the tank stat and it fell to bits on me.

Thanks for the suggestion. I'll check it. I had ruled it out because the heating is on even when the programme is off, but maybe I don't understand how it's connected up.

 

[Lou]

I hope you get it fixed soon

 

[Chester]

Thanks for the suggestion. I'll check it. I had ruled it out because the heating is on even when the programme is off, but maybe I don't understand how it's connected up.

Check if there is 240v on the brown wire on the hot water valve with the programmer not calling

 

[ShaunCorbs]

Pull the Programmer off the backplate and re test

If it’s gone prog at fault

 

[Julian7]

This what I found out. Thanks very much to all those who offered suggestions.

Although the hot water was staying on, it was the central heating zone valve switch that was faulty. Because in an 'S' plan system (thanks @Chester ), the two zone valve switches are connected together to start the boiler and pump, the faulty c/h zone valve switch was causing the water to be heated and pumped. Even though both zone valves were closed, I guess the pump was still able to pump water through the cylinder. The cylinder stat was indicating temperature had been reached, and the hot water zone value was therefore closed, but this made no difference to the boiler.

I have replaced the Zone Valve actuator on the c/h valve, and everything now works as it should. C/h and water thermostats operate their own zone valves and, if either valve is open, the boiler fires and the water is pumped.

Julian