Questions on near-boiler piping

Ken C.

… that was piped in a way I hadn't seen before, but then again, I'm a plumber new to the heating field. Perhaps you folks can enlighten me. Here goes:
1. The boiler had an air scoop that was plugged on the bottom tap. On top of the air scoop was a coin vent and, if I remember correctly, 1/2" copper tubing that ran to a steel thermal expansion tank. Is this a good setup?
2. As far as I know, this was a one-zone boiler (only found one t-stat) with one circulator. There was a one-inch feed pipe off the boiler. But the return piping had me confused. There were two 3/4" pipes that joined into a one-inch return near the boiler. Just before the 3/4" pipes joined the common return, was an old Bell & Gosset valve that acted as both a balancing valve and a purge station, with 2 boiler drains. My question: Why would there be two return pipes if there's only one feed pipe? I couldn't find any zone valves, but the basement had a mostly finished ceiling.
3. There was difficulty purging the system. The B&G valve says to turn the arrow on the handle toward either one of the boiler drains when purging (but I'm not sure if the arrow should be pointed at 9:00 and 3:00, or 10:00 and 2:00). Also, the system had an older feed valve that didn't have a fast-fill lever. While 12 pounds of water pressure should be enough to reach the 2nd floor during operation, would it be enough to positively purge the system of air? (The valve to the steel expansion tank was closed prior to purging.) Any ideas why the difficulty purging? My newbie guess would be because the feed valve did not have a fast fill, plus, I have doubts about whether the B&G valve closed all the way. In fact, if I had my way (the homeowner didn't want to spend the money and even insisted on purging the system herself!), I would have removed the B&G valve and put a purge station on the 1" common return. Comments?

JohnNY

It's pretty common for a system like that to go out with a single feed pipe, split off into two loops somewhere down the line and reunite with two returns at the boiler. Personally, I hate that. But there are times when you have to do it.
I think the tapping on that air scoop is 3/4". If so, I'd prefer to have that copper pipe, which is otherwise fine, from it to the expansion tank be run in full size. If the tapping is 1/2", then disregard.

If there is no fast-fill and you are having trouble getting rid of air, put a washing machine hose on the water heater (if there is one) and another on the boiler drain cock and voil