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

Well it's apology time...

John tbm... this link supports what you say about the mid position valve sending turn-on power to the boiler...

http://www.diyfaq.org.uk/plumbing/controls/controls.html

It seems that because of the slightly strange wiring of these control valves, when the programmer and thermostats are requiring Heating Only the mid position valve is organised to make the call for heat else the boiler won't fire - I never knew that!

Foston, I'm sorry - I'm unable to imagine a single boiler supplying hot water to two storage tanks and that coupled with your conflicting trouble statements (boiler never firing for water and then just firing for 20 seconds) made me suspicious of your circumstances and imagine that you're a landlord.

If the call for hot water only makes the boiler fire for just a few seconds I can only think that some kind of wiring fault is not making the circulating pump run and the overheat thermostat is shutting everything down - but I'm just guessing. You do not tell what happens next if anything - after the boiler fires for 20 seconds and then shuts down - does it try again on its own some time later?

You do have immersion heaters I suppose?

Good luck...

Peccavi, March 2009

I assure you that the property is not let into flats. It is an owner occupied bungalow and has two hot water tanks only to ensure that there is sufficient hot water when all the family are using the three bathrooms at the same time.
The initial problem of no central heating or water was fixed by installing a new gas valve. However, the system has now developed either an extension of the old problem or a new problem in that we have no hot water via the boiler.
I find it strange that both motorised valves should be in the closed position and therefore I assume that the fault originates other than with the valves.

Foston, March 2009

Peccavi is correct as to it being illegal to service the boiler...but....on many installations the motorised valve actually "tells" the boiler to switch on. Peccavi I tell you no lies..ie. if there is a room stat and a tank stat fitted, the motorised valve for either will send a "signal" (power) to the boiler to make it fire up. If it's defective it may not send the signal. I have came across this and i changed the actuator and everything was fine. ??!!maybe a one off!!??

John the boilerman, March 2009

Foston,

You report two different failure modes in each of your questions - firstly no boiler firing at all for DHW but OK for CH - and now, boiler DOES fire for DHW but for only a few seconds.

You now also add the complexity of two hot water tanks and two motorised valves strongly suggestive of a property let into flats - I think it's illegal (criminal offence) in the UK for a landlord to do his own boiler servicing so I'm reluctant to offer any more help other than to say that motorised valves do not affect boiler firing.

You may wish to call in a certified engineer.

Good luck...

Peccavi, March 2009