Anti-sieze on union nut and various bolts?

So @mattmia2 are you saying you use ant-sieze lubricant like in the photos for bolts, or do you just use pipe dope for those? I want to use whatever has the best ant-sieze properties regardless of looks. I never want to wrangle another coupling or bolt like I had to wrangle that union.

@EBEBRATT-Ed those are excellent tips, and I’ll get the ant-sieze.
I tried the two hammer trick last night and it didn’t loosen to where I could get it the regular way, but it might have helped in the end.
I was going to try heat (can you even heat up a 1-//2 inch union nut enough to help?) but went to a different way which absolutely saved the day. I’ll put photos below, but let me say first that as you’ll see I could have just cut just above the union where there’s copper to get the union out (which isn’t going to be reused), but then I’d have to deal with 1-1/2 inch copper which seems a bit hard to get except in 10’ lengths for like $120 which is a bit steep for a couple inches, Supplyhouse was out of short lengths. But really it was mainly pigheadedness. I banged on it with hammers and put the 24” pipe wrench on it and banged that and did the scissors thing with that wrench and a regular pipe wrench with a pipe on the handle. It’s quite possible if I REALLY strained it would budge, I have had that experience, but this wasn’t horizontal so I could put my weight on it and it just felt if I pushed too hard I could just pull something, muscle, tendon, earlobe. So I went to Plan C which worked like a charm. A couple pulls on the ratchet and she moved like a (slow) racehorse. This is my new go to technique for stubborn stuff that you can use the scissors method with. I’m sure it’s done out there but I never heard of it and it saved the day.

@mattmia2 yup, cutting was an option. I could have cut the black pipe nipple down below the union and hoped to have better luck unscrewing the whole union off of the copper sweat x male NPT adaptor which is visible just above the union in the first photo (and from which, after I had successfully unscrewed the union nut, the female part of the union did screw off smoothly). Another option was to cut the copper above that copper adaptor and voila!, boiler would be loose from supply riser. But the problem with that is, as you can see from the photo, there's just an itty bitty tiny nipple of 1-1/2 copper between the adaptor and the tee above it that leads to the whole conglomeration of zone valves etc. I would have to make the cut into that nipple, seems to me. Then to connect more riser piping down below to the new boiler I'd have to unsolder what would be left of that nipple (very little) and I shudder at any thought of unsoldering anything, having had a few experiences where something absolutely positively refused to come unstuck. This fear of unsoldering anything was intensified with my next experience in this drama of DIYer vs. Pipe, which I will describe briefly below for those who, after reading what I just wrote, would like a second headache on top of the first.

So, union is off, old boiler is out, new one up on blocks (but only one set of blocks instead of the old four, to make space for a circulator on the supply where there was none before due to some good advice from @EdTheHeaterMan). I line up the supply nipple out of top of boiler with what's hanging down from manifold visually as well as I can … (Footnote. It was not easy lining up this boiler precisely so the new near boiler piping will line up with the risers, and I'm not sure it's even right yet. The thing is 600 lbs., and does not want to be twisted and walked delicately this way and that when its on cinder blocks whose footprint is barely bigger than the boiler's. I do not know how you guys do this stuff, day in, day out. I'm in awe, no exaggeration.)

Anyway, now I'm lined up, within reason, and I start trying to dry fit, in order going up from the boiler: circulator with flanges, nipple, coupler to existing male npt copper adaptor. But the coupler won't screw onto the adaptor, I tried 800 times, I figure the adaptor's cross threaded or has some other thread-related personality disorder. Tried other 1-1/2 fittings in case it was the new coupler's fault, but nothing went on. So I figured I had to unsolder the adaptor and replace it with a new one which, in an unusual fit of brains, I had ordered just in case. But this would mean facing my greatest fear, unsoldering. And sure enough, the damn thing wouldn't budge. I had the torch on it for-absolutely-ever (which I hate, I feel like I'm damaging something nearby, like the ball valves and zone valves located just upward of this fitting I'm trying to remove, or inadvertently unsoldering everything else in the system). YouTube told me that almost always the culprit in these situations is water. This is a vertical riser and although the system is drained and the ball valves closed I still saw drips during the day. So now I waited until it cooled and went up with a paper towel which came out dry, plus I figured that any moisture in that manifold between this fitting and the closed valves would have evaporated. So I tried again but nothing. So anyway in the end I took the female part of the old union which had been on this adaptor and attempted yet again to get it threaded on evenly, and then it did seem to go (with a lot of resistance) so I wrenched it on tight figuring maybe I'm retraining errant threads to be legitimate again. And after that the coupler did seem to go on untilted and evenly, so maybe I'm okay (even though defeated by the unsoldering episode).

My plan of hoping that the distance between the circulator on top of the boiler and the copper manifold (which does have a tiny bit of play up and down) can be matched by a standard black iron pipe nipple may be foiled, but I'm hopeful; I bought a bunch of short iron nipples in various lengths. If not, @mattmia2, I did locate a hardware store that will cut a piece of 1-1/2 for me (I now love that hardware store) and I guess I could do something like this, going downwards: copper female x sweat adapter screwed onto that sweat x male adapter that I can't get off, then short copper nipple cut to length, then sweat x male npt into the top flange of the circulator. That's IF I can get the female copper adaptor onto the weird threads of the old male one.

BTW, someone please reassure me that my applying the torch to the adaptor for more time than four Avengers sequels plus Gone with the Wind did not necessarily harm the five ball valves and then five zone valves that are just above it in the manifold.