Best Of

After making sure main venting is sufficient I would start out thinking generally 4's for first floor, 5's for second and 6's for 3rd. Modify this by radiator size (and distance from main supply riser) such that small radiators on third floor drop to a 5, large or small radiators on second floor raise or lower to 6 and 4 and larger radiators on 1st floor bump up to 5's. This would be the starting point. Tune further with patience based upon room comfort level. In your own livable space on the third floor you could use adjustable valves like a Hoffman 1a to help your balancing. If using MoM vents you can also get a pack of orifices which essentially makes them adjustable.

That plumber has absolutely no idea what he is doing. I'm sorry — really — that you are a few grand into the job, but don't even think abut trying to go on with him. There's an old saying: when you find yourself in a hole, stop digging.

It's a little hard to say from the description just what got messed up — but that wet return was essential to the operation of the system. I wonder where the water is going now…

And no, there is nothing you can add to the boiler to fix the problem.

All four of the guys mentioned above are known to me, and are dead honest and do quality work.

Totally agree. And it is surprising how "little" overfill will do this. In many installations the pressure switch and the LWCO (and maybe other safeties) will be wired all in series (there are other way to do this, but this is very common indeed). So — I'd say everything worked the way it was supposed to work.

just… sad