Would a pinhole leak allow the system preasure to climb above the input valve’s setting? How?
Answer: By allowing AIR in this system and Air can be compressed unlike water. Compressed air equals pressure build up especially when heated Pressure temperature relationship.
Why would the overflow valve weep if the input preasure is regulated at 15 PSI? Does this prove the input regulator is defective?
Answer. Not necessary Check your gage and try the pressure relief valve to make sure it is properly seated Should be tested during the heating season any way.
Not all systems can work with a 15# pressure setting. That is why the factory setting is around 12 PSI.
Even @ 12 PSI if the expansion tank is flooded the relief can discharge as the water is heated it expands and needs a place for this expansion
Can I continue to use the steel expansion tank if I put an Adtrol valve on it to prevent convection? What’s the lifespan of a typical steel tank?
Answer. As long as your expansion can pass a hydrostatic test and not leak it can last literally over 100 Years depending on water quality and condition of it.
I still have the original membrane tank. Will it be usable after nine years in the box?
Answer. I would get a new one PROPERLY Sized for this system. This is not a case of one size fits all
Can it be mounted upside down, or sideways? The pipes are close in to the furnace, so I can’t “hang” it from the air fitting.
Answer: Yes any position is OK BUT you should leave access to the air fill valve in case needed
If I have to mount it “hanging”, how far from the furnace may I place it?
Answer: Close to the BOILER is the best place
If I put a preasure gauge on the furnace, where should it be, and what are the most reliable models?
By the way it sounds like your Dad did a FANTASTIC JOB as I also HATE “Zone valves” GARBAGE