Is it legal?

We're not hearing the whole story here.

I'm guessing here, but one of the major problems with heat distribution in many upscale prewar buildings stems from renovations that dramatically depart from the original floorplans, and in particular if those renovations involve significant "noise insulation".

Would that be the case here?

Have him call 311 and put in a complaint. Boards and managing agents respond well to fines.

It's probably not a pressure problem, though. It takes hardly any pressure -- a few pounds, I think -- for steam to fill the whole system. It may be that the air valves on the radiators or at the top of the risers are clogged or inadequate: if air can't get out, steam can't get in.

Matt: no. No deviation from any floor plans since 1922. No renovations, either.
NWT: the valves everywhere were checked.
So it is the board trying to save a few bucks on the expense of his comfort.
311 sounds good!

Stakan: it's not a question of whether your friend's plans were changed - if any of the floorplans of the apts below were changed, it can impact his heat.

I find that hard to believe.

But again, we're hearing this third-hand on an anonymous board.

Matt, what could be possible gained by making up stories on anonymous board? Ah, whatever...
nyc10023: his problem started 2 years ago, up until then the heat was good.
He said the new board suddenly decided to appear financially responsible.
So the question remains: can a board arbitrarily affect comfort of a shareholder?

How can it be legal? No, it is not legal. And it does not matter if there was a renovation. The renovation is approved by the whole building board and so it applies to everyone.

"And it does not matter if there was a renovation."

Actually, in many cases a renovation DOES matter. It can affect the natural flow of heat THROUGH the floors, which is how most steam buildings were engineered back in the prewar days.

I believe the law requires that minimum temperatures be no less than 55 degrees in an apt

The fastest way to resolve this...

Send a letter to the coop board so it's documented. Give them 48 hours to fix. When/if they don't, he should call an HVAC contractor/heating plumber. I don't know if this is steam or hot water, but in any case they will be able to definitely say whether there is enough pressure/volume for his heat to work. If there is, they can fix whatever problem he does have and he can bill the coop for the work as long as it's reasonable. If not, then the building will need to do some work.

Calling 311 won't get him any action in the early spring, it's just not that cold for much longer, and they focus on HDC buildings anyway, and even if they do get a fine, they all end up paying in a coop, which is silly. First try to be reasonable and work with the board. If that doesn't work, a lawyer is probably a better solution than 311.

"I don't know if this is steam or hot water"

If it's a real prewar building (as opposed to what are called prewar, but are gut renovations with prewar shells) I can just about guarantee it's steam. The bigger issue is whether it is one pipe steam or 2 pipe steam.

stakan: is he getting ANY steam but the apartment is cold or no steam at all?

Also, one solution may be simply to have "top of riser" / "end-of-main" / "Main Vent Valve"" air vents installed. I'll bet there aren't any currently. (see second part of NWT's answer above)

PS Almost all managing agents and even a lot of plumbers who are "experts" in "boliers" know relatively nothing about how steam heating actually works. I'll bet the managing agent doesn't even know what a master top of riser vent is, and even a significant amount of the guys who will show up to diagnose the problem (and unless you mention it to them, won't think of it. But if you do, they will "call the boss" who actually knows something and he will say "off course that's the problem. tell them they need them installed immediately and that it will cost $X per riser. How many risers are there?".