water loss in old steam heat residential system

Have anyone took at look at the burners or flooded the boiler to see if the block is leaking . Usually removing the front door and inspecting the burners will give you some insight sometimes you will see evidence of water running down the block but usually it evaporates before making it to the floor . Usually one would flood the boiler block and check for some drips . Checking all radiator supply valves and the nuts is advised .
peace and good luck clammy
you could try reaching out to one of the contractors listed on this site

I wonder how high you could fill the water to?

I suppose if you plugged the main vents you could fill the mains with water as well and check for leaks

If the new copper wet return was wrapped in insulation before the concrete was put back, it should be fine. That is standard practice. And it's fine to use copper for piping that does not carry steam, like wet returns.

There is a very good reason why copper can be problematic in steam carrying piping — but is not, inherently, bad. Expansion. Particularly in near boiler piping the use of copper can cause a good deal of stress on the cast iron or steel of the boiler, as the copper will expand faster — and a lot further — than the iron or steel. Unless provision is made in the piping to absorb that expansion without putting stress on the boiler — or, for that matter, other piping or even sometimes other structure — problems can occur.

These problems show up in two ways: damage to, including cracking, of the boiler if the copper is piped so as to try to pull it apart, and leaks at the soldered joints in the copper (if solder is used) and the movement of the pipes cracks the solder (it will).

Neither of these things are a problem with iron pipe — it expands in the same way as the boiler does, so that's not a problem, and it is very easy to pipe it in such a way as stress on the joints is near zero, as a threaded joint can give a little without leaking.

ProPress joints for copper appear to be able to give a little in rotation, as threaded joints can; they have not been used as long as threaded joints have, so there is some question as to their longevity if constantly stressed.

That said, even with something like ProPress, the piping must be arranged with double swing joints (one advantage of a drop header) so that variations in expansion rate cannot impose any linear stress on the boiler or other pipiing. The alternative is very long expansion loops, so that any potential linear stresses are taken up by bending in the pipe itself between joints. Expansion loops, to be effective, however, take up a lot of space and may not be feasible in steam lines.

@RTW Some contractors use copper on steam piping because it's standard practice on hot water systems, which they are accustomed to dealing with. They have the parts on the truck, they know how to solder, and they might not own pipe threading equipment or the wrenches needed to work with steel pipe. Also, they often don't read the installation manual that came with the steam boiler; so they don't know about the problems Jamie described above.

Since steam heat is generally not installed in new homes, the trade is losing the specialized knowledge that goes with it.

As to my personal experience working on steam, I added three new radiators (used in excellent shape) to my system after purchasing house. I worked with a good plumber/ heating guy who had a pipe threader who knew how to work with steel pipe, but I did all the carpentry and design to place the piping and proper size - with the help of information from this site. I had to learn about venting too and added main vents along with a permanent skim port to boiler. It was a bit of a nail biter after the plumber left as to whether the system would actually work, but surprisingly everything did work and tweaking the venting finished the job - and of course skimming the boiler slow and many times

Regards,

RTW

Any of the folks in New York state in "Find a Contractor"… I might suggest giving Danny Scully a call as a first shot. 516.887.1122 He's right up there with the best. Located in Malverne.