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As the folks have noted, the piping to your son's radiator is marginal in size. It's also long. This makes it all the more important that it be absolutely straight from one end to the other, and that it have adequate pitch — it must slopw back to the main, and I'd want an inch or two in that length if I could get it. That may be where your hammering is coming from, especially at the beginning of a run as the pipe is heating up.

I did my own boiler in 2010 because my house had a cracked one but no one around to replace it in the SE. Used US fittings and pipe. Easily cost of boiler X4. And I didnt pay myself!

3x the boiler cost is a fair starting point, but have some contingency money as well for like asbestos removal, chimney liners, radiator valves and condensate return lines, etc.

………………ducting to bring fresh air to the boiler room from outdoors……………..with fire damper

……………… 2.5" risers, 4" drop header……………………………………………………………………………………..

…………………removing cement encased 500g oil tank in an oil to gas connversion……………………..

………………..repiping the HWH..no 1/2" permitted anymore………

………………..Installing CO detector with boiler shutoff…………

………………..Tekmar 279 in lieu of thermostat……………………………

…………………40' SS chimney liner………………8"…………………………………

……………….1" insulation on all NBP and 100' of 2.5"…………………………………………

………………..2.5" King Valves (2)……………………………………………………………………..

………………..timer to delay feed water for 12 minutes after shutdown……………

……………….. (2) Gorton 2, (4) Gorton 1, (8) Gorton 4, (10) Gorton 5, (4) Gorton 6..

……………..in NYC………………………………………………………………………………………………………………………………………

6X cost of boiler.

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Did anyone mention the chimney flue?

That can get into some big numbers.

I had to do a 45' 10" SS and that cost was about like buying another boiler.

In the Hot Water heating side, the chimney option might turn the job into a Mod Con rather than simple CI.

I'lll bet what was happening was the vents were stuck closed, or mostly closed…

Can you provide a photo of the boiler header and the piping connection for that runout to Radiator 3-1?

It seems your principal problem now is the poor operation of that Radiator 3-1 .

There is no plugging of your steam pipes.

The 9-foot long 1-1/4" runout to your 16-section problem radiator is marginal, and will require good piping elsewhere. A large vent on this radiator will encourage steam flow but quickly produce lots of condensate. A slow vent will reduce condensate flow but retard heating that radiator. Experiment with that. A larger runout will be more forgiving. Steam flow is fighting condensate. There may be ways to improve this without changing that piping.

A slower vent on Radiator 3-2 may encourage a more balanced flow toward Radiator 3-1.

Let us know what steam return you have in the vicinity of that radiator or steam main.

Reducing your steam pressure to no more than 1-1/2 pounds should help performance. That should be a goal.

You mention that your boiler is cycling on pressure. It should not be doing that unless it's very cold outside, returning from a setback or very oversized. Have you calculated the proper size of the boiler vs. what you have?

Generally you can improve system performance by using a single brand of radiator vents that share the same characteristics. Mixing vents from various suppliers often retards performance.

Nice diagrams and good work!