Pressure in boiler getting too high

I will give this a try.

Physics says that water can not make steam, at sea level pressure under 212 degrees F.

It is also why steam pipes and automobile systems are under pressure, you can hold more heat, when water is under pressure and not have it boil. Pressure cookers cook food faster for the same reason.

The only way to make steam, from water, at 180 F, is to do it under partial vacuum.

To get water to boil at 180 F, you need to reduce the air pressure to about 7.5 psi.

7.5 psi is a column of water about 15' tall.

If a boiler tank, had a 15' tall pipe, filled with water, and caught between a drain trap and the return to the boiler, it would make steam at 180 F.

I have a physics background, and I am not a stationary engineer, well not a train engineer either, to be honest.

Hopefully someone who has actually worked on a boiler system can take this and run with it.

• Note, never pay someone to work on a boiler that thinks the temperature of the water should be below the boiling point of water.

You are better off hiring a competent plumber to work on a storage water heater. You get the same result with fewer parts, faster, and for less money.

Your diagnostics so far point to the indirect hot water heater coil leaking potable water into the furnace loop as the most likely issue. You could perhaps verify if you can shut your hot water down for a few days (not convenient, I know.)

Depending on the overall condition of the indirect water heater tank, the coil can often be replaced as a part, if the indirect hot water tank is otherwise in good shape. Some are not designed this way, however.

While your test might leave room for a failed expansion tank, if you did not cool the boiler down too much (so the cool water then expands as it's heated; which with a failed expansion tank would raise the pressure significantly, and with a functioning one will raise the pressure slightly) it's probably valid, especially if the pressure rise took days, rather than shooting up the first time the boiler fired to temperature (a sure sign of expansion tank troubles.)

For a typical "modern" bladder-type expansion tank, there's an easy, quick test of the tank integrity - drop the pressure to zero on the system again and check the pressure on the air fill valve for the expansion tank. If it's also zero, the tank (diaphragm/bladder) has failed. The even quicker (if crude) test is to "burp" the air fill valve very slightly (don't want to release significant air or you need to drop the system pressure and refill it) to see if air or water comes out - if it's water, the tank has failed. I would expect your service technicians to have already done one or both of these tests, given your symptoms.