CENTRAL HEATING NOT COMING ON WHEN PROGRAMMED TO

[lordsainsbury]

Hi,

I have a Glow Worm Ultimate 40FF boiler controlled by a Honeywell C27 unit and I have an intermittent fault.

The hot water is programmed to come on from 0530-0600, followed by the central heating from 0600-0630. However, a couple of days a week the central heating will not come on at all for the programmed 30 minutes but the hot water always comes on as programmed. Other days both the hot water and central heating come on as programmed just fine.

The temperature on a morning in the hallway where the thermostat is sited is definitely lower than the temperature it is set to, so I'm confident the heating doesn't come on simply because the house is already warm enough.

The central heating works without fault when I manually put the heating on in the evenings selecting the 24-hours setting.

Any guidance will be very appreciated.

Thanks.

 

[stem]

I assume your system isn't a combi, but a heat only boiler with 1 or 2 motorised valves. If so, it could be the microswitch in the motorised valve, quite a common failure, the valve opens to the central heating only position but doesn't turn on the boiler every time.

It is possible to test for this with a multimeter. when the heating doesn't come on when it should, check for a live on the orange wire from the motorised valve. It is this live that operates the boiler.

 

[lordsainsbury]

Stem,

My system isn't a combi, it has 1 honeywell Y-plan 3 position diverter valve.

I have oberved the movement of the diverter valve motor under different demands last night. (couldn't check voltages as i don't own a multimeter)

When CH is on the arm of the motor moved slowly anti-clockwise until it made the microswtiches which triggered the boiler to kick in. Then when i switched CH off and HW on the arm moved clockwise much faster to it's original position (approx 90 degrees movement). I then put the CH back on and the arm moved slowly back to make the microswitches again before i knocked the heating off for the night.


The HW came on again as programmed this morning but the CH did not, i looked at the position of the motor arm and it was in the same position it was in for HW only last night.

Would love to know your thoughts.

thanks.

 

[stem]

As it's the microswitch that switches on the boiler when central heating only is required, it may be faulty, the contact inside may be pitted, sprained or wornout. So visually the microswitch may be working, but electrically it may not be. I have known several to operate intermittently.

Unfortunately the only way to find out is by checking the orange wire for a live output (or lack of) when the fault is present, or by replacing the valve, or valve head to see if that fixes it.

The first microswitch in the valve is used to hold the valve in the centre position when hot water and central heating are required, the second microswitch at the far end of the valve's travel is the one that turns the boiler on when only central heating is required.

When hot water is required, the boiler is controlled by the cylinder thermostat and as this seems to be working it would suggest all is well with the boiler.