Got a baxi back boiler about 20 years old works fine upto the other day when the heating is on the thermosate dos'ent seem to be working the boiler is just running on full all the time and not coming on and off as it should.

sounds like the hot water sensor is shutting down early .also check flame sensor is clean .

needs tune up change filter and nozzle and check electrodes are clean with no build up and clean out chambers

Seems that you may have bad circulation pump.

Try the baxi potterton wedsite you can down load manuals there or they have at tech line

is the boiler a combi if so have you checked the pressure. it needs to be at 1.5 bar 2 bar at the most . also check that the heat the boiler is producing is completeing the circuit on the heating if you have access to the pipe work there should be 2 or 3 22mm copper pipes. now turn the heating on and check which one is getting warm. the one that is is your flow and in about 2 to 3 min there should be one more pipe getting hot let me no if this is the case

there should be a switch which toggles between hot water only /// hot water + central heating /// & possible off on
Either on the boiler or on a separate timer control

pilot has to be on all the time on this baxi ,piolt might be a bit large might need adjusting down

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you could have a flow problem due to sludge or scale in the boiler, it could be a pump problem as it may not be circulating enough water around the system. or it could be a faulty thermostat if it is not cutting out soon enough.
1st try turning the boiler stat down to its minimum, if it still over heats then it is probably the stat.
if you have a switch on your pump to increase the speed make sure it is on setting 3. if the pump is old then it may be time for a replacement.

first check the pilot is alight, it sounds like it is cause you can hear the boiler click. that click is the gas vale solenoid trying to open, if there is 240-230volts at gas valve when you here the click and pilot is alight the gas valve is broken. if you test for voltage look on the gas valve and see if its 240 or 24v obviously if it 24 v check for 24 volts, let me know