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Don't know your specific unit, but in general, heating systems are two-stage devices:
1: Its too cold: the motorised HW/CH valve moved to the required position, and on achieving it, the circulaion pump starts and the boiler is enabled. (The circ pump may start immediately - its not usually important.) The hot water from the boiler circulates through the heat exchanger and/or radiators until the thermostat thinks its target is warm enough, whereupon it disables the boiler, stops the pump and closes/relaxes the valve(s).
2: The boiler is off while disabled. When enabled, it heats the circulating water to a 'high' temperature and seeks to keep it there. If it reaches that temperature, the boiler will cycle on and off as necessary to maintain the set 'high' temperature. (The circulation pump must run all the time the boiler is enabled.) This may be what you're noticing. Additionally, some boilers will run the circulation pump for a little while after being disabled to reduce the temperature if the water in the pipes and boiler.
Your question doesn't say how the pump running corresponds to thee thermostat settings, but if it were stuck on, I think you'd know!
It could be dyslexia, but I think you want to know what may be wrong with your boiler in this case! This is a VERY old design. The diaphragm is on the water section, this operates a switch to turn ON the pump, and a further device causes the diverter valve to divert boiler water from CH duty to HW duty. From your description it appears that the boiler is shutting down on internal boiler water temperature (too high), your 'briefly too hot' HW symptom may be caused either by the boiler side of the secondary heat exchanger being blocked/caked in sludge, OR the DHW side of it being scaled. Either way a new or replacement (descaled) secondary heat exchanger MAY inprove things. It's not easy on these old clunk-pots, and EVERY disturbed seal will subsequently leak, so get a good supply of o-rings and fibre seals before you start ("you" being the competant fitter you employ)
Check the flow switch. Small black box lower left hand side as you face the from of the unit. Try to jump the connection and then see if it fires up. If it does, replace your flow switch. The flow switch prevent the boiler from firing because it is not sensing the water moving through the pipes. The boiler won't fire if water is not moving through it.
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
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.
hi sounds as though the system is sludged up & needs power flushing.the reason is that the water cannot circulate so over heats & pumps into the header tank.
expect to pay approx 350 400 pounds to have it properly cleansed, it will restore the system efficiency.
The other area worth considering is that it could be a faulty pump os that when th eboiler fires it cannot circulate so in turn expands & vents into the tank.
My advise is to arrange for a corgi engineer to sort it out for you.