I tried venting the Schrader valve on the expansion tank and nothing came out---no water and no air. The system is not running right now....does that make a difference?
When the service guy was out here last he suggested the regulator on the incoming water was bad. He said the type of regulator on the system is obsolete.
I included a schematic of how the expansion tank and incoming regulator are configured. He wants $400 to replace both the expansion tank and the regulator. Seems expensive to me.
You ask for a Manual J Heat loss be done by any person bidding on the job. It may cost you a couple of bucks without it you can't properly sized the boiler. There are other aspects that go with this as well. In conjunction with the heat loss, the existing radiation should be measured. Most homes are 35% over radiated. This allows the option of calculating out door reset curves (cruise control for your boiler) that can provide you additional savings via system efficiency.
The boiler or AFUE efficiency is only how efficiency that piece of equipment is. It is not a reflection of the enitre heating system.