So, just moved into a place which has Economy 7 electricity and the immersion heater was making some strange noises in the night so I got up and turned the heating elements off.

Anyway, I take a shower this morning and the water is boiling hot…too hot to shower even when on the coldest setting.

Looked at the system and the cold water/expansion tank (black plastic tank above immersion tank) is very very warm…as though it contains boiling water too.

So, my question is…have the element thermostats gone causing the water to boil too much and overflow into the cold water/expansion tank (the settings on the front of the thermostats is set to 60 when I checked quickly this morning).

Can I just check these with a multimeter?

Cheers.

So, my question is…have the element thermostats gone causing the water to boil too much and overflow into the cold water/expansion tank

Sounds like it – from very recent similar experience 8)

Can I just check these with a multimeter?

Unless you know what temperature the water in the tank is, how are you going to do it?
Also, you’d need to wait for E7 to kick in, or power it from another source to check it.

Was thinking that the ohms reading would change across the thermostat if I adjusted the temp setting so I could check the thermostat opening/closing?

I wouldn’t need to know the temp of the water per-say…just adjust so that I’m above/below current temp and see if the reading changes as the stat opens/closes?

What can happen IIRC, is that the water in the cold water tank can continue to heat until the tank fails catastrophically. There was a recent case where a kid sleeping underneath died.
Rather than piss about and pay a plumber a fortune, id just change the thermostat and see what happened. New thermostats are set with a much lower max tho so you might find you have the opposite problem

Edit, thermostats start at around a tenner and are dead easy to fit with good access

Modern immersion elements are now fitted with a safety thermostat that is housed within a separate prong of the element itself (a thermowell). This ‘stat is in addition to the ‘stat you see on the front of a cylinder.

The reason that the ‘stat was added to immersion elements is to try and stop the situation you describe. IIRC it all came about from the case sweepy mentioned when boiling water from a failed tank come through the ceiling and killed people in bed below.

Get a plumber/heating bod to fix it.

The control stat is protected from failing by the high limit stat. Unlucky if they’ve both failed, try the turning them down and you’ll might them hear them click as they switch, if not you could put your meter across them to see if they are closed or open on ohms or beeper if it has one.

Check they are not shorted out against a wiring diagram of the layout.