Why is the pressure on my boiler fluctuating so much?

[meghanp10]

Apologies in advance for the long explanation..
This morning, my boiler's pressure had dropped to 2 PSI, so I added water until it was sitting at 12 PSI (which it should be at). I checked on it again around 9 pm and it was sitting at 35 PSI!!! The relief valve had not blown. I drained a bunch of water out until it was at 15 PSI and not 20 minutes later, it was already up to 18 PSI.
What could be causing it to fluctuate so much? Water can't be entering the system since it will randomly lose a bunch of pressure and I then have to manually add water.
My next step is obviously to call a professional.. but I thought I would see if anyone has any idea as to what's going on.
Could it be caused by my expansion tank being full of water?
Some background info..
Boiler & house are about 100 years old. Very old system. Expansion tank is strapped to the ceiling and is an older style tank.
I'm afraid to leave the heat on over night at this point in case the pressure skyrockets again but it's -25°C tonight so not sure that's a good idea either.

 

[John.g]

If you check the air end of the E.vessel and water comes out then the diaphragm has failed.
If not, you need the system drained or at the very least no pressure at the boiler (if the E.vessel is located above it) and then check the pressure at the air end, it should be ~ 10/12 psi, then re pressurise the water side to 14/16 psi and shut off the filling valve. If the pressure still rises to 35 psi then the the E.vessel is borderline/too small for the system. The relief valve may have a set pressure of ~ 43/44 psi (3.0 bar). Also check the air end pressure when the system is hot as this should then read the same as the boiler pressure after making any allowance for any difference in the elevations of the pressure gauges. This will ensure that there is no blockage in the pipe from the E.vessel to the system, (ideally this pipe should be removed to check for any restrictions)

 

[ShaunCorbs]

Expansion vessel has gone / needs checking the old type that’s fixed to the ceiling isn’t the best maybe better off getting it replaced