If a small radiator is not getting hot, then a bigger radiator will also not get hot. I don't think replacing the radiators is going to solve the problem.

Do you have any vents on the supply pipes in the basement? Perhaps they need to be replaced.

The pressure setting may be too low to reach the third floor. What are the cut in and cut out pressures? A small increase in pressure (0.5 pounds) could have a significant affect on the system. Normally the cut in and cut out pressures are 0.5 and 2.0 pounds respectively. Don't go any higher.

Here is a forum devoted to steam heating help:

http://www.heatinghelp.com/forum-category/93/Strictly-Steam

Let us know if you find the solution.

Below is a link to a good book on steam heating. I have never read it, but it gets good reviews.

Here is a link that might be useful: The Lost Art of Steam Heating

Wow, Mike, that's helpful. Thanks. I just ordered the book, and I've been reading up online about steam systems and pressure settings. My knowledge base is pretty poor to start with... Our cut in is set at 2 psi, and out (I assume that's the one that says "Main" and can be adjusted from zero to 15...?) at 7. Obviously this is much higher than what you said is normal, and you say not to go any higher. I am reading a lot online about how having the settings adjusted is important, but mostly warnings to have a professional do it. I have spent an incredible amount of money (to one of the most reputable and expensive plumbing and heating companies in my area--someone considered on multiple forums as the BEST) to address the problem of the last apartment not getting heated thoroughly, and each "fix" is a very small one and never does the trick. Those guys are the ones who adjusted the pressure settings, so I want to assume they are appropriate, but heck, something's gotta give! Any thoughts on the numbers? What's the adjustment I should try?

Also, I keep reading that it can be a good idea to have multiple air vents on the main. I see all these pictures of five or six of them right next to one another--not a set-up we have and not something our heating guy suggested. I wonder if that's something worth trying...?

Thanks again for the help and direction.

Wow, Mike, that's helpful. Thanks. I just ordered the book, and I've been reading up online about steam systems and pressure settings. My knowledge base is pretty poor to start with... Our cut in is set at 2 psi, and out (I assume that's the one that says "Main" and can be adjusted from zero to 15...?) at 7. Obviously this is much higher than what you said is normal, and you say not to go any higher. I am reading a lot online about how having the settings adjusted is important, but mostly warnings to have a professional do it. I have spent an incredible amount of money (to one of the most reputable and expensive plumbing and heating companies in my area--someone considered on multiple forums as the BEST) to address the problem of the last apartment not getting heated thoroughly, and each "fix" is a very small one and never does the trick. Those guys are the ones who adjusted the pressure settings, so I want to assume they are appropriate, but heck, something's gotta give! Any thoughts on the numbers? What's the adjustment I should try?

Also, I keep reading that it can be a good idea to have multiple air vents on the main. I see all these pictures of five or six of them right next to one another--not a set-up we have and not something our heating guy suggested. I wonder if that's something worth trying...?

Thanks again for the help and direction.

I grew up in a house which had steam radiators. I am not an expert so my advice is limited. One pipe steam systems are really simple, but they can drive you crazy.

I think your pressures are way too high. At those pressures I would expect to see water leaks. You don't have a pressure problem.

Are you able to check the pitch on the pipes feeding the radiators on the third floor? It is a likely a 150 year old house has settled and pipes are no longer pitched properly.

What happens if you shut off all the radiators on the second floor? Does that make the problem better or worse?

Interesting questions, Mike... I can't really check the pitch of the pipes without ripping up the floor, I don't think. I can check the pitch of the radiators themselves, and they are fine (pitched very slightly toward the pipe). As for shutting off all the second floor radiators. I don't know! I can check for sure. I would imagine that would help because the boiler would be firing constantly to keep the second floor heated enough to hit the right temp on the thermostat, but I wonder if the upper radiators would stay on at 2/3 capacity or would have the time to actually heat up all the way. Good question. (But our heating guy said I should be sure no one shuts off any radiators because that can lead to pipe banging. Do you think that's not so?)

I suggested shutting off the second floor radiators as an experiment to see how it affects the third floor. It may give a clue as to the cause of the problem. I am not suggesting you this as a long term solution.

Do you have have floor to ceiling pipes on the second floor feeding the third floor? Are these getting hot from bottom to top? Do any of these pipes have vents?

"Do you have have floor to ceiling pipes on the second floor feeding the third floor? Are these getting hot from bottom to top? Do any of these pipes have vents?"

There is one that is visible and yes, it gets very hot. No vent!!