boiler turning off in HW only mode

[2db]

Evening experts!

Grateful for advice.

Gas CH and HW system, connected via motorised mid position valve to pump then to boiler (conventional setup I presume) .

When we have CH running alone, or both CH and HW running, all is fine - boiler runs, heats up radiators and hot water - nice cosy house and mrs 2db gets lots of hot water for a bath.

However, when in HW only mode, the boiler turns off after a few minutes, but the cylinder stat on the side of the hot water tank still calls for heat, and the pump keeps running (presumably never turning off because the boiler isn't heating the water up).

Any ideas what the cause of this could be? My guess is that there's something wrong with the motorised valve perhaps? (the lever on it can however be seen moving between all three positions correctly) - but not really sure.

Grateful for any thoughts.

 

[Agile]

My thought is that you forgot to tell us the boiler make and model !

 

[D_Hailsham]

On a system with a mid-position valve the boiler is controlled by the HW thermostat when in HW only and HW with CH. As the system works OK when in HW and CH mode, but not in HW only mode, it sounds as if there is something causing the boiler temperature to rise too quickly when in HW only mode, so the boiler thermostat turns the boiler off for a few minutes.

It could just be that the boiler's output is more than the HW cylinder can dissipate, or there could be a blockage in the cylinder circuit.

What happens if you turn up the thermostat on the side of the HW cylinder?

 

[2db]

In hw only mode, increasing the hw stat on the side of the cylinder has no effect as the hw never reaches the set temperature. Turning it down, the stat turns off the pump as it should.

 

[D_Hailsham]

There is a lever in a slot at one end of the motorized valve. Turn the power to the system off and back on. This resets the valve to HW only position - lever at Auto end of slot.

Now move the lever to the MAN end of the slot. You should feel resistance as you are opening the valve to CH against a spring. When you release the lever it should move back to the Auto end and you should hear the gears running.

If the lever feels floppy the valve is sticking. You may be able to correct this, but it all depends on which valve you have and how old it is. Can you post details of your valve?

 

[2db]

all seems fine playing with lever - defaults to HW position, motor moves to mid point or CH only position, can feel movement.

Drayton lifestyle mid position valve - one of the older variants with non-removeable control box. (boiler is an ideal standard, (going from memory, RS?) ~15 years old)

I've also stripped the valve controller down, verified microswitches work and motor works. Looking at the gearing it seems to move along the full rotation of the teeth on the coupling attached to the valve - valve also seems to rotate (I guess I'm not too sure if it's rotating fully internally). I'm presuming it might not be returning fully to the HW only position to reduce the water flow? Don't know if that's plausible or not.

thanks

 

[D_Hailsham]

In your first post you said:

when in HW only mode, the boiler turns off after a few minutes, but the cylinder stat on the side of the hot water tank still calls for heat, and the pump keeps running

On most boilers as old as yours the pump and boiler switched live are connected together, so they go on and off at the same time. The only way the pump will keep going without the boiler running is if the boiler's internal thermostat has turned the boiler off. Have you tried turning the boiler thermostat down?

 

[2db]

thanks - no, that's not something I've tried. Will take a look.