Replacing a Weil Mclain CGA6 and expansion tank

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Hello all new to the forum. I do concrete for a living so bear with me. My expansion tank (plain steel) was leaking at the end of the year from the top, and my boiler also blew a leak. I'm assuming it was a combination of things including my auto fill valve. System was running correctly even with the drip from the expansion tank(12-15lbs). I haven't removed the tank yet too see how high the leak was on the tank(top in joists).

I'm replacing a Weil McLain CGA6 ,with a CGA6(new version), that I actually physically installed myself 22+ years ago? I just had someone wire it for me. My problem is either replacing the plain steel expansion which I cant seem to find, or replumbing for the newer vertical expansion tanks. Now for the replumb and expansion size questions I'm here to ask. When I replaced the prior boiler, the plumbing was straight forward as I was not changing anything but boiler types.

The current plumbing for the CGA6 and the prior boiler worked fine. I'm assuming this system was installed in the 50'-60's as the baseboard radiators were Sears Robucks steel 11" high by various lengths.

Here goes. The boiler is fed off the hot water heater with 3/4" pipe double T. One on/off "quick fill" valve, the other a B&G 12 PSI reducing valve(the auto fill). Off the otherside of those valves the 3/4" pipe goes both up and down(connected completely 4 ways/from either fill and up/down). It goes up to my current steel expansion tank(dead end). It goes down to "what I was told" is an air separator. This separator has the 3/4 feed on the left(feed side), the boiler 1 1/4 OUTPUT on the right(heat pushing up) and a 1 1/4" output to both zones on the top.

I had a bid(outrageous) tell me the air separator needed to be re-plumbed(removed) and my expansion tank needed to be plumbed from the currently capped 3/4 pipe that comes up from particular boiler right next to the output piping(top 1 1/4") . Both my return feeds are B&G series 100 pumps that return to the bottom of the CGA6. I just gotta make sure I plumb it right......a newer style ET and that old "air eliminator"....I dont wanna mess it up.

How to plumb in a new expansion tank?
What to do with the old air eliminator.....Just a simple T?
My existing standard steel tank is 25G approximate, do I need a #60 or a #90(Extrol example)?

Thanks for your time folks.
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Have you already taken delivery on the new CGa-6? That thing puts out 146 BTU/hr, enough heat to theoretically keep my sub-code antique 2400' house + 1600' of sub-grade basement cozy even when it's -175F outside(!).

If it's not already too late replacing like for like repeats the oversizing errors of the past two boilers, which has both comfort and efficiency consequences. Run a fuel-use load calculation using the old boiler as a measuring instrument. The 99% outside design temp for Rockford is -4F, so upsize the boiler output by 1.4x (and not more) from there to be able to cover. Almost all houses in Rockford could be heated just fine with a CGa-25 or CGa-3, and only the big drafty houses could make good use of a CGa-4 or larger.

You probably don't even have enough radiator to emit the full output of the CGa-6- it just cycles on/off during continuous calls for heat. It's pretty typical to find boilers 2x oversized for the radiation and 3x oversized for the actual space heating load. A bigger boiler doesn't heat the place any better or faster- it just cycles on/off more, and could even be short-cycling if the house is broken up into separately controlled zones.
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