Plumbing – How to get this pump casing off the motor

1. Turn off the power to the pump.
2. Turn off the feed to the house (there should be a valve between the pressure tank and rest of house). This is simply to make the next step easier.
3. Drain any water from the pipes where the pressure switch is screwed in. Usually there is a sed valve.
4. Unscrew the existing gauge.
5. Check the T where the gauge was screwed in for build-up/blockage. This may be what messed up the existing gauge in the first place. If there's any, clear it out as much as possible.
6. Use teflon tape or pipe dope on the threads, and screw in the new gauge.
7. Turn everything back on.

You've bought a programmable thermostat so you can lower the temperature of the house at night, however by doing that you are making it much harder work for the boiler to bring the house back up to temperature. Before your house was at a constant temperature so you boiler would turn on for a bit, off for a bit, on a bit, off a bit over the whole day. Now it goes off for a long time and your house gets cold, then it has to run a very long time to get it back up to temperature again, leading it to overheat and shut down.

It may be your boiler simply doesn't have the grunt to heat your house for that long. Try setting your night-time temperature a bit higher and see if that improves the situation.

It could also be that your system is gunked up, leading to poor water circulation in your system. Did you have your heating engineer put a cleaner in the system? If not, have him try it.