Honeywell ST9420C Issues

[Rattlymatt]

I have recently had issues with my central heating and water system. When the hot water is on, the radiators are hot. I thought this was the Honeywell Valve head, so I replaced it. Problem still there.

I then spotted that the ST9420C programmer wouldn't fire the heating (the water was off) when trying +1 hour or when set to continuous (it doesn't work on auto or once either). The hot water does fire. I then remembered that the +1 stopped working earlier in the winter, but forgot because I relied on the auto program to heat the radiators.

Could this be a duff programmer?

 

[DP]

Get the user manual out and read it.
Programmer does not fire anything, it just gets things moving to result in boiler operation

 

[jj4091]

If you have a sealed combi boiler system the it sounds more like a problem with the boiler diverter valve.

 

[Rattlymatt]

Thank you for your replies. It is a conventional boiler. Leading to an immersion tank.
The thermostat programmer light will come on and there is a click to signal the hot water is on, but it heats the water and the radiators like the diverter is open for both.

There is no response from the thermostat programmer for the heating. No light, no click and the boiler does not respond.

 

[ericmark]

In the main, the programmer and thermostat works the motorised valve, and once the valve is fully open a micro-switch in the valve fires the boiler. So at the valve when it works the bleed lever becomes slack, and there is normally an indicator to show it has opened. And often you can hear the valve working.

However, with a conventional boiler there are three methods used, I have said how the S system works, there is also Y and C, plus some odd ones like W, but question one is what motorised valves if any you have? No valve likely C Plan, a three port valve and like Y Plan, and two x two port valves like S Plan.

I have seen with Y Plan how it will only work central heating if the DHW was calling for heat when it fired up, the Y Plan three port motorised valves is an odd system.

The valve is designed to hold at mid position when both CH and DHW are required, at rest is DHW only, and all the way across CH only, this

may help you understand if you do have Y Plan.

 

[CountryFan]

If the programmer doesn’t light up then it’s not sending a switch live signal to the heating. However, if the radiators are coming on with the hot water then it’s either going to be the motorised valve or the programmer switch live may be stuck. Voltage testing would confirm this.

 

[Rattlymatt]

It is a Y-plan system with a 3 port valve. I assumed the problem was the valve and I replaced the head yesterday, but it did not resolve the issue.

I then noticed the programmer responded for the hot water but nothing from the heating controls. I am swapping the programmer with a like for like in the hope it resolves it.

Ideally I would rely on our local plumber but he is unable to help at the moment, so I am tinkering until he can.

 

[ericmark]

The DHW is selected with the motorised valve, but the electrical connection is direct. I do find programmers and thermostats where to get access to the terminals the device needs removing a pain, the instructions show the wiring diagram but the connections in the wiring centre are not standardised, and the heating will not work with the programmer removed. As a temporary quick fix, on the side of the motorised valve there is a bleed lever, sometimes latching this lever will allow central heating to work.

What test equipment do you have? I struggled with daughter's house as those diodes give some odd results.

 

[fixitflav]

I thought this was the Honeywell Valve head, so I replaced it. Problem still there.

If you just replaced the head, perhaps the valve itself is the problem. The rubber ball can come adrift, either because the circlip holding it on the shaft has failed, or the shaft has unscrewed itself from the arm. I've had both of those in the past.