Boiler doesn't fire at all if hot water selected

[SimonWestmoor]

I have a Worcester Danesmoor (circa 1990) oil fired boiler with Honeywell Y plan control system. All been working fine for many years, but I now have a strange problem. As of this weekend we are having an issue with Heat vs Hot Water.

With CH On and HW Off from the programmer then CH works fine. When room stat demands heat, boiler and pump run; when room stat has no demand then boiler/pump stop. HW doesn't heat (as expected).

With HW On from the programmer boiler never fires and pump doesn't run. Makes no difference if cylinder stat is demand/no demand or if room stat is demand/no demand. Also makes no difference whether CH is On or Off on programmer, the boiler won't run so long as HW is On.

Have tried swapping Mid Position valve for another one, no change in behavior, valve makes all expected movements.

Any help appreciated.

 

[Hugh Jaleak]

Are you confident you've a Y plan system? If memory serves me correctly then the valve doesn't get involved when the call is for HW only, as valve should default to rest position, i.e. Open to Hot Water.

If you have a Multi Meter and are confident testing Mains Voltages, there is a series of excellent diagrams showing the electrical flowpaths here: Honeywell Y-Plan Explanation (octaveblue.co.uk)

 

[SimonWestmoor]

Thank you.

Yes, definitely Y plan. Will take a look and see if I can get some pointers.

 

[jackthom]

Sounds like the HW ON signal isn’t coming out of the programmer, or it is somehow lost on the way to the cylinder thermostat.

 

[fixitflav]

I have a Worcester Danesmoor (circa 1990) oil fired boiler with Honeywell Y plan control system. All been working fine for many years, but I now have a strange problem. As of this weekend we are having an issue with Heat vs Hot Water.

With CH On and HW Off from the programmer then CH works fine. When room stat demands heat, boiler and pump run; when room stat has no demand then boiler/pump stop. HW doesn't heat (as expected).

With HW On from the programmer boiler never fires and pump doesn't run. Makes no difference if cylinder stat is demand/no demand or if room stat is demand/no demand. Also makes no difference whether CH is On or Off on programmer, the boiler won't run so long as HW is On.

Have tried swapping Mid Position valve for another one, no change in behavior, valve makes all expected movements.

Any help appreciated.

First guess is a problem with the programmer. HW has changeover contacts, HW on live when HW calling, HW off live when HW not calling. If it's not giving 240 volt at HW on terminal when HW calling that might explain the symptoms, so I'd check that first.

 

[fixitflav]

Sounds like the HW ON signal isn’t coming out of the programmer, or it is somehow lost on the way to the cylinder thermostat.

You beat me to it as I was typing!

 

[oilhead]

Are you sure your programmer is actually switching when it says it is? When you select both, does the water heat up along with the heating? If you have a Y plan, then the boiler fires up off the cylinder stat with both selected, and only relies on the Y plan head in the central heating mode. If the boiler fires when both are selected, if the programmer is not actually switching, then the Y plan will works as if only CH is selected. Time to get the multimeter out.

If I'd typed less, then I might have been first!!!

 

[SimonWestmoor]

Thanks everyone, will check with voltmeter.

Oilhead: If both heat and hot water selected boiler doesn't run - it will only run when hot water is switched off (and heating on of course)

 

[fixitflav]

Sounds like the HW ON signal isn’t coming out of the programmer, or it is somehow lost on the way to the cylinder thermostat.

Good point, needs checking at the cylinder stat, and maybe points in between.

 

[CBW]

Definitely needs some testing of voltages.

 

[SimonWestmoor]

Fixed

Thanks to the superb animated diagrams and the suggestions to check/trace the HW signal from the programmer to the HW stat, I’ve found the problem.

Programmer was giving correct signal, which was making it as far as the Honeywell wiring centre upstairs, but not to C terminal for the HW stat in the wiring centre.

I suspect circuit board in the wiring centre is starting fail; running a fly lead from the HW On terminal to the C terminal got it all working correctly.

Will purchase new connection centre to resolve properly. The existing one is stamped 1989 so it’s pretty old.

Thanks once again.