Combi worcester 240. Plenty of hot water when heating running bot only luke warm when heating off. The pin in diverter valve working fine and ive tested the switch and that is ok !! Anybody got any ideas. Thanks.

I would say you are not far off with your opinion

Combi-boiler-turn hot water tap on/off and the heating stops for ...

singletrackworld.com/.../combi-boiler-turn-hot-water-tap-onoff-and-the-...

Combi-boiler-turn hot water tap on/off and the heating stops for 20secs ... hot watertakes priority over central heating so when you run the hot tap, heating is ... Yeah onmy worcester combi there is a diverter valve - it feeds either the heating or ...

My central heating works but I have no hot water - 24'7 ...

https://www.247homerescue.co.uk/my-central-heating-works-but-i-have-...

Diverter valves switch heat between the hot water and the central heating. When youturn your hot tap on, the diverter valve temporarily shuts off the water supply ...

Does your boiler keep switching itself off? Here's why ...

www.dhs-uk.com/does-your-boiler-keep-switching-itself-off-heres-why/

Jul 7, 2015 - ... Bathroom Suites, - Showers, - Wet Rooms, - Taps and Accessories, Information and ... Low water pressure can cause your boiler to turn itself off. ... boiler is not turning off, you aren't getting the amount of hot water you require. ... If there is air in your central heating system, then your boiler may shut down.

check the diverter valve. look to see if when hot water turned on the small electrical switch at the end operates. If not then a ned valve or diaphragm

This boiler, from memory, has an electric diverter valve.
If the system has dirty (sludge/scale laden) water then the diverter valve may not return under spring pressure, and therefore not divert all of the hot boiler water to the hot water heat exchanger, leaving some going to the rads.
If I am right then the rads will not get fully hot.
If the valve has failed completely, or electrically, then the rads will get fully hot and there will be no hot water output (hot water will be cold).

check your expansion vessel is not flat.if so recharge..simple way to check press in valve on vessel if water comes out or little air recharge..if water still in vessel the diaphram in vessel has split so replace vessel?hot water relys on expansion??

Check to see if diverter valve is in domestic mode or heating mode.

Most likey problem is the diverter valve, not energising fully. This in turn will not move the little plastic lever arm that actuates the microswitch on the side, sending the signal to ignite.

You can buy a diaghram kit from most local plumb centres, you need to replace the diaghram and seals, don't forget the silicon grease to ensure the plunger doesnt stick. Kits less than a tenner.

You should have noticed before it stopped working completely the water had reduced in temperature before the weeks it failed.

To make sure it is a diverter valve fault, depress the microswitch if you can, when the hot water is flowing and hey presto the demand for hot water light should shine.

vailant boilers are one of the best on the market and should last you more than 6 years depending on installation and servicing.
the engineer you had should not have replaced the diverter valve if the boiler was firing up when the taps are opened. if you was getting luke warm water with the boiler alight it is a classic symptom of a blocked plate heat exchanger on the central heating side.
a new heat exchanger can cost around £100 depending on the age and model of the boiler.alteranatively it could be descaled but may clogg again. it would be a good ideal to fit a magna clean filter on the system to help prevent further debris if you have a dirty system .if you don't trust your engineer i suggest you ask another. find out if he is registered first and don't pay until you are satisfied with the work. if you are asking the same engineer back try and get a discount off the work as he has failed to rectifiy the original fault and fitted the wrong part due to wrong diagnostics. if he is registered you can complain to the governing body ie corgi or gas safe register.
good luck.

replace pressure release valve

35 cdi worcester combi about 3 years old,heating not working,hot water works fine.It's been serviced not long ago.Room stat turned up and trv's ,but boiler want fire up,there's power to the boiler and the room stat the built in time clock is switched to on, but still nothing. do you think it could be the diverter valve, when i got there it was flashing SE which mean's it needs a service,cleaned the heat exchanger and burner with the proper worceter brush etc then re set boiler with technical, they also said it could be the built in clock and told me to disconect it, and just use the heating stat to control it.It still want fire.