On a back boiler central heating system my hot water works fine but cant get my central heating working i have changed the pump there are no motorised valves on the system i have checked the the expansion tank in the attic and thats fine what could be wrong with the central heating

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

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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.

you should have a programmer that will let you have the hot water and heating on or off . You could have a motor rised valve on the system which controls both . depends on what system you have . Fully pumped of just pumped on the heating side .

Seems that you may have bad circulation pump.

Have you bleed the air from system and run hot water and heating at the same time , bleed the pump . you could have air in the pilot after turning it off

not an engineer but maybe a heating compant,, it sounds like you have a relay that has gone bad, you have what we call a winter summer system your domestic water comes from a tank inside the boiler, you might want to think about splitting the system to save you some money in the long run, seperate the water heater from the boiler so you only heat up in the winter, right now you are heating the system that heats your house to get 30 or 40 gals of domestic hot water, it would cost you less by seperating them

back boiler run from a gas fire? check the instructions to operate the gas heat to the back boiler not just the fire
I presume that this a fully pumped system - air lock doubtful There maybe on/off valves on the pipes to and from the back boiler [situate close to cylinder] make sure they are on - anticlockwise turn
or there could be a blockage in the outlet from header tank usually in the loft. or If there is no water in it maybe the mains cold feed it is off.
Note if no water in it DO NOT USE the back boiler

what size of gas line is going into boiler and water heater? is it black iron ? natural gas or propane? have you checked gas pressure at water heater and furnace ? if they work fine individually then you have a problem with incoming gas pressure. Call gas company and ask them if they would come out and check gas pressure.

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.

if the boiler fires when the tap is opened then the diverter valve is working ok it must be a blocked plate heat exchanger which is the stainless steel rectangular block inside the boiler connected to the pump and the mains cold water.
it will be blocked on the central heating part probably with black scale. it is possible to clean out but sometimes it is better to just change the plate. but remember to flush the system as well or the same thing will happen.