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I assume losing pressure through the PR means Pressure Relief Valve (PRV) or temperature pressure (TPR). Open in TPR may mean boiler is overheating/overpressurized. Open in PRV means the Pressure Reducer/Fill Valve in the fill line of the boiler is broken.
House Water Pressure in a home is 30/40 PSI min to 60-80+PSI max.
Boiler PRV/TPR are set at 30 PSI open (SYSTEM PRESSURE) so too high a house water pressure will open the PRV/TPR. Replace the reduction regulator on/in the fill valve assembly
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)
A boiler is a water heater? Sound to me like a thermostat problem and these are often obtained at W. W. Grainger at a price that is much less than if you call a service man in and he will just make some checks and change the thermostat. The hard part is finding out which one you need.
Switch off the boiler at the mains, remove the front panel and 'lower' the display unit/pcb. Locate the (blue) sensor in the condensate trap (rear left hand corner of the boiler). Remove the sensor and dry the area around the probes which are on the end of the sensor which enters the trap. Restart the boiler - if it runs correctly you have a problem with your condensate overflow (as did I). Good luck.
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.