Problem with thermostat, diverter valve or something else?

[mouse<:3]

Hi all,
Hoping you can help me.
We have today had an issue with our central heating. Both central heating and hot water were working fine yesterday, but today went to turn the central heating on and...nothing. When we turn the hot water on, on the other hand, boiler turns on, heats up and hey presto we have hot water. After reading up on it all night it looks as though it could either be that the thermostat has finally had it and needs replacing, or possibly the diverter valve has gone which an engineer will need to fix.
The termostat does click to turn itself on, but does this mean that it is definitely working? It is quite old.
Or could the diverter valve be the issue?
We have a baxi solo 15 he a, which is about 5 years old.
May be worth noting that when we switch the central heating on downstairs it doesn't trigger the boiler to turn on regardless of thermostat temp, but when we turn the hot water on, it does trigger the boiler to turn on. When hot water has been on for a while and presumably reached temperature the boiler switches itself off until it feels the need to switch back on.
Hoping you can help point us in the right direction!
Thanks

 

[joni os]

Check if pump is running when calling for heat. If running, suspect flow switch on boiler, for which you require gas safe engineer. However, if pump doesn't run then malfunction is external to boiler and you can check why live signal to fire is not reaching boiler/pump.
Programmer, thermostat and motorized valve can be checked at wiring centre, but be aware you are working on live 240 volt system and stay within your competence.

 

[Riley]

Heat only boiler mate

 

[mouse<:3]

Thanks for your replies both.
Yes it is a heat only boiler.
We left the heating and the hot water on last night, on the off chance it might come on (as someone had mentioned elsewhere on the Internet with this same problem that it had come on overnight), and miraculously it did come on, and all radiators were on when we woke up this morning. Which although is good news, leaves us even more baffled as to what the problem was and whether it could be a recurring thing or one off.
If you have any ideas it would be appreciated, just in case it happens again!
Thanks

 

[joni os]

Get to know your wiring centre and identify how each part of your system is connected to it. If, as is often the case, instead of a wiring centre you have a birds nest of wires and connectors stuffed into a standard electrical back box then invest in a 10 way wiring centre and get it wired in recommended manner. If fault reoccurs cause can then be easily established.

 

[KingDoey]

Sounds like you may have a sticky mid position valve.