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Precharge Expansion tank: old school non-bladder
I hope I'm not being redundant, I couldn't find the answer in a quick search...
I've got a job with a largish residential radiator system retrofit from oil to gas a few years ago. I reused the expansion tank, which is about 30g and is old-school steel tank in the ceiling joists, filled merely with air. It has worked for several years since the retrofit (and decades before), but now the system is overpressured (it's a combi-boiler, so it gets some summer use too), and I have to assess whether the expansion tank merely filled up, or if the auto-fill regulator needs replacement (the only other salvaged part from the oil boiler).
Anyway, I've drained the expansion tank (didn't try to assess if it was full, that was probably a lost diagnostic opportunity, but at near-40-psi there was a lot of water in there), and I could just open it back up to the system now, which I'm sure is what I did the first time (I'm not sure I even drained it before). But it occurs to me that it could make sense to pre-charge it to, say, a few psi below idle pressure (let's say 12 psi), to leave more expansion space... Since these tanks have generally fallen out of favor, I don't see much info on this procedure.
Please don't spend a lot of time trying to convince me to switch to a bladder tank, though pithy insightful suggestions are always welcome. A 30g bladder tank would be expensive and hard to fit into the room. Thanks for any help!