I've been having continued problems with the pressure in my home oil boiler rising to 30psi over time (and then leaking out the pressure relief valve). Technicians have examined it multiple times but have not addressed the issue.
The last technician indicated there were only two ways the pressure in the boiler could increase: through the water intake valve or a faulty expansion tank, and through backpressure from the external hot water heater (heated via a boiler zone).
To validate it wasn't the water intake valve/expansion tank, I shut down the system, closed all incoming and circulating water, then lowered the pressure to zero manually off the pressure release valve. Once it was at 0, I opened the water intake to restore pressure (it climbed to 15), then shut that valve before turning the system back on.
After a few days, the pressure has again climbed to 30psi. So, my questions are: was the technician correct that the leak must be coming from the water heater? Is the only solution replacing the water heater in that case? Or is there anything else that can cause a pressure build-up?