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Most valves that are different manufactures or series are slightly different dimensions. If you replace the valve you are likely to need to make some adjustments. Regasketing the old valve eliminates this problem.

With (2) 2" risers instead of (1) 2 1/2" you have about 28% more riser capacity than required. With the height of the oversized risers (they must be 35-40") I doubt you will get any water in the header

The guy did a fantastic job. Who did the insulation? Perfecto! The manufacturers specs are minimum standards for reliable performance. I'm a big believer in individuality, artistry & freedom of expression. As Ed said, he used both tappings. You got a very nice job! Mad Dog

OK here is the scoop on Minimum Recommended Pipe Size. You have a diagram that shows that you only need one riser that is 2-1/2" diameter that transitions with a 90° elbow into a 2-1/2" header. The end of the header then transitions into a 1-1/2" equalizer drop where the wet returns connects via a Wye fitting or a short nipple before it connects to the boiler return opening at the bottom. That Wye fitting has a minimum 2" below the water line specification.

Since your contractor did a double 2" riser, that is more that the minimum requirement. The Wye fitting is at 3" below the water line, so that is more than the minimum requirement. the fact that you have a drop header will better facilitate dry steam to the system. the only shortcoming I can see is the short section of Header where it is common to both the double boiler risers and the double system risers may have an increased velocity that may cause some water to find its way into the steam mains. Only time will tell if that actually is a problem. After the system is skimmed and the water conditioned for pH, I doubt that you will have any problem.

If at all possible, I would hold back $500.00 from the final payment (if that is possible) to insure that the system operates properly without major banging noise. (some expansion noise is going to happen on any heating system) If you get major banging that can not otherwise be explained that the header should be increased to 2-1/2" but I don't see that being a problem.

Otherwise you have an excellent system. Give it a month to see if there are any problems that skimming does not solve. Follow @ethicalpaul and his YouTube videos for proper boiler care of your new system.

Then call it a day. JMHO.

You can let @New England SteamWorks answer that himself but i think the manual has some language that allows other industry accepted methods. Because it has the drop header to help the steam and water separate over the long path and turns the 2" is very likely fine for the header.

Your returns are above the water line anyhow until they get to the boiler so the only place that the water could leak out of the boiler is at the piping near the boiler. The lower connection to the hartford loop just means there is a small amount more piping that could leak and allow the water to run out of the boiler. The lwco ("safegard") is there to stop the boiler from firing if the water gets low anyhow so on modern automatically fired boilers the hartford loop is redundancy.