Best Of

Those mentioned above are good. I'm a bit partial to Rinnai. Give them a look.

Look at what you want to do with it too. Some Lochinvar and Viessman models especially have built in controls for multiple zones with multiple setpoints and things like that. If you just need a similar boiler HTP's now discontinued UFT line was inexpensive and lacked some of the controls fancier models have but was a reliable boiler for a simple installation. Also look at turndown ratio and what your needs for turndown are. Many boilers are 10:1 now which may or may not be important depending on what zoning you are doing and what sort of load you have on a mild day vs design day.

Maybe IBC? With any of the brands I would suggest avoiding the budget models with a "tankless water heater" style heat exchanger IBC calls that one the SFB.

Most of the firetube style residential boilers have heat exchangers made by AIC, and have very similar internal parts. The brands mentioned above have a good track record of honoring any warranty issues. My usual suggestion for brands would include to buy one from a source you are comfortable with. Ideally someone has parts and knowledge nearby.

I put a Viessmann B1HE-85 in my house. I know the boilers and stock parts myself so its a no brainer for me.

Lochinvar, Viessman, HTP. There is another one which I keep confusing with the one with the chronic HX failures.

Ahh, the insulation…. It was like that when I moved in this past summer. I am speculating it used to have asbestos wrap and after remediation they replaced with fiberglass on the straight sections only.
No hammering the past several days so I’ll see if I can localize it more next time it happens.
FWIW this is the only photo I have of the leaky pipe layout prior to their replacement.

Following up, I have a workable solution for now. If I really choke down the garage baseboard, I can keep the 1st floor living and 2nd Floor west loops from reversing. It's not ideal because the garage baseboard isn't heating up nearly as much as before but for now it won't freeze up and everything else works properly.

The baseboard in the garage is cast iron and made by slant fin. I think it's their "Rhino Cast". I found some install instructions and it looks like the supply and returns should be on the bottom fitting on opposite sides from each other, although I am not sure if it would work just a well going into one top fitting and out the bottom fitting on the opposite side. The bleeder valve goes on the return end upper fitting of course.

Anyways, I appreciate all the help so far. I'm going to eventually make the changes to the mixing valve and rework the garage piping. I'll probably lower the radiator to the ground at the same time. I do think that my idea of cutting the garage return pipe off before the tee with the radiant floor return and connecting it in with 1/2 piping to the return stack at the bottom will improve / solve the issue. If not then it's time for check valves. Replumbing the supply and return stacks to a larger diameter 1 1/4 isn't a practical solution.

PS, Naviclean is clean and unblocked; I checked it.