I have a Glow-Worm Ultimate 120FF central heating boiler that stopped working last week. Under my Direct Line Emergency cover a plumber visited and put in a new Gas Valve. Both the Central Heating and the hot water worked for 36 hours. However, whilst the boiler continues to heat the radiators it is not heating the hot water. If I turn off the hot water at the Potterton timing switch and set the wall thermostat to a higher temperature than the current room temperature the boiler starts to operate. However, if I turn off the Central heating at the Potterton Timer and turn the hot water setting to 24 hours the boiler remains off even though the water from the hot water taps runs cold. No controls within the boiler were touched since the installation of the new gas valve. Direct Line say that this is a new problem and is not an Emergency therefore they cannot help.

Can any one advise what the likely cause of the problem is?

Foston, March 2009

apologies, apologies....If I have misled. It's just that i attended a callout recently with exactly the symptoms and it did turn out to be the motorised valve on the supply to the hot water cyl that was defective. maybe I missed some info on other threads that was relevant.

John the boilerman, March 2009

I originally posted the following as a new question, however, to maintain the thread I think that I should have posted it as an answer.

Thank you for your helpful responses. I have made further observations and these are as follows:

I have two hot water tanks each serving parts of the property. Each tank has a motorised valve in line. The central heating system also has a separate motorised valve in line. Both of the hot water valves seem to be in the off position – the leaver on each is in the right hand position. If I move the leaver to the left I meet resistance at the half way point. I have moved on of the valve leavers fully to the left and locked it in this position. The pipe either side of the valve is cold. With the time control set to “24 hours” and the central heating set to “off” the boiler still refuses to stay on – it comes on for say 20 – 30 seconds and then cuts out. If I set the central heating to “24 hours” the boiler burner ignites and the radiators get hot.

I cannot visually follow the pipes from the hot water tanks back to the boiler [as there are numerous pipes all of which are lagged] therefore I am unsure whether there are any additional valves/pumps in line.

If I am correct that both motorised valves are off it would suggest that there is an electrical fault somewhere in the system. However, I cannot understand why with one of the valves locked in the “open” position this does not result in the boiler burner coming on. I also have no idea where the electrical switch to the valves is.

Does anyone have any further thoughts?

Foston, March 2009

For John the Boilerman...

If the motorised valve were in the wrong position - i.e. set to drive water around the radiators - the boiler would still recognise a call for hot water in the storage tank and attempt to staisfy it and fire up - the radiators would get warmer and warmer irrespective of the central heating controls.

There is no report of radiators being too warm but only the boiler not firing when the programmer is calling for hot water. It's unlikely, therefore, that your diagnosis is correct.

Peccavi's friend, March 2009

the motorised valve controlling the on/off to the hot water tank is broken. If there is a thermostat strapped to your hot water tank make sure it is set to around 60 degrees. I would be 99.99% sure that it is the motorised valve though...You'll probably need a plumber and an electrician as the plumber won't touch the wires and the electrician won't touch the valve. Hopefully you will just need a new activator and this will cut out the need for the plumber.Call the electrician first...or a good handyman if you know one. You can check this for yourself first by locating the motorised valve which controls the hot water supply to the hot water tank. It will be on one of the two pipes going into the side of the tank. You will see either a red cog wheel or a metal bar on the end of the valve. get someone to turn on the hot water control at the heating switch, if the valve is working you will either see the red wheel turn to a point where you will see a flat part on it or you will here a buzzing/whirring sound as the valve opens..no movement no sound means no work..call spark to replace.

John the boilerman, March 2009

Just as you have a room thermostat that controls the central heating and by adjusting it you can make the boiler come on or go off - there needs to be a similar arrangement for the hot water to tell the boiler to start and stop - although people seldom need to adjust the hot water thermostat it's a user control of sorts.

On my hot water tank in the airing cupboard there's a hot water thermostat clamped to the copper tank - a cut out in the insulation lets the "stat" rest against the metal of the tank.

If the hot water stat is set to a really cold setting or if it's defective (open circuit) the boiler will believe the need for hot water is satisfied and consequently not come on even though the programmer is asking for hot water.

Yo may wish to investigate the thermostat on your tank but take care because it carries mains voltages.

I do not believe the two faults are connected in any way and having the second fault just 36 hours after the first was repaired is just unhappy coinsidence. In a system such as yours you may well have (should ideally have) the benefit of an immersion heater so I can sympathise with Direct Line in declaring it a non-emergency.

Peccavi, March 2009