We have a pressurised central heating/hot water system run by a Worcester Bosch Greenstar 24i boiler coupled to a large 210l cylinder and the central heating side has been set to 'OFF' for a few months now so it's only hot water that is operational. For a few months now our hot water which we set to come on every day automatically hasn't always been getting hot as it should and has quite frequently required the use of the HW boost facility on the programmer. We didn't think too much about it and put it down to extra hot water usage by Osem Jnr until more recently when I discovered that the timed HW on/off periods don't seem to be getting the boiler to fire at all and the BOOST function is also intermittent. This is the weird bit - when this first started happening some months ago, in order to see if the boiler had conked out altogether I tried switching on the CH briefly via its boost function and the boiler immediately fired up as it should. Having confirmed it was still working I then cancelled the CH boost setting, waited about a minute and then tried the HW boost once again. This time the boiler fired up again perfectly and within an hour we had a tankful of very hot water. Last night we'd run a bath and I set the programmer for a 2 hour boost to replenish the tank. The light on the programmer came on as it should but I later found out that the boiler hadn't fired up and the water was stone cold. However, once again, by switching on the CH the boiler ignited and after cancelling that the normal process of controlling the HW via the boost function worked once again and we had hot water. Obviously if the hot water tank is still hot then the boiler won't ignite regardless of any programmer settings but I'd have thought that as we used up all the water it should have come on at some point when the water temperature dropped below a certain level.
I'm not intending to fiddle around with anything on the boiler but would like to have a clue what the problem is so has anyone got any idea what might be wrong? I changed the programmer for a new one of the same make which simply plugs into the same backplate to rule out programmer issues by the way. The problem appears to be the same with either programmer.
TIA as always.
