Replacing an old steam boiler with a new steam boiler is not easy and rarely advantageous. Steam boilers were particularly popular seventy or eighty years ago, and many of them are still in service, but their technology is outdated and inefficent.
One and two-Pipe systems
Most residential steam boiler systems are "one-pipe"; when the thermostat calls for heat, the boiler fires and steam fills the system, displacing the air contained in the pipes and radiators; and as the steam releases heat throughout the radiators, it condenses to water and returns through the same pipes to the boiler (hence their name: one-pipe steam systems).
Two-pipe systems are a lot less common in residential buildings, and are different in the sense that they have a return pipe system.
Whatever your system, replacing it with a new steam boiler system, keeping the existing distribution system, is a difficult task. Only steam experts will be able to solve potential compatibility and design problems. And, as already mentioned, it will not be a wise choice and you will not get energy savings. Steam boiler technology is mostly outdated and energy-inefficient.
Anyway, it may be worthwhile to improve the efficiency of your system, or to solve some possible problems. Below we list common problems or sources of energy-inefficiency, and how to solve them.
The most common energy problem with one-pipe steam systems comes from clogged and undersized air vents. Uneven heating is usually caused by parts of the system being air-locked. And the solution is to replace the existing vents, or add new large-volume air vents to the main supply pipes. Replacing malfunctioning or undersized air vents can make a significant difference.
New thermostatic air vents are a good choice for oversized radiators (a common situation) since they control the flow of steam into the radiators, thus avoiding overheating problems and improving energy efficiency.
Valve problems and Thermostatic valves
Two-pipe steam boiler systems are prone to problems in their valves (steam traps). They do not last longer (often 3-6 years) and when they fail, they cause water hammer - steam pressure hurling the water against the pipe joints -, which will damage the other valves.
So, replace malfunctioning valves, or consider replacing them with steam radiator orifices (metal disks with a hole in their center, able to regulate the steam flow).
Also consider thermostatic radiation valves (TRVs). They can fix many overheating problems and improve the system efficiency.
grossly oversized steam radiator systems, Low energy efficiency
Many steam radiator systems - in old buildings that have been renovated and benefited from insulation and air sealing improvements - are grossly oversized.
If that is your case, you should try to reduce the capacity of your steam system, by installing thermostatic valves or radiator plugs. That's important for energy savings.
Heat Reflectors for energy savings and energy improvements
Steam radiators are prone to heat loss, especially when they are installed in walls with low levels of insulation; to prevent it consider the installation of heat reflectors at the back of the radiators.
Reflectors - foil-covered cardboard, foil and foam board… - can be purchased in home improvement stores and should at least match the size of the radiators. That's a DIY job and a way of improving the steam radiator efficiency and getting some energy savings.
Placing Shims under the feet of steam radiators
If your steam radiators are making banging sounds and tilted, see if it is possible to fix the problem by placing shims under the feet of the radiators: pitch them slightly toward the pipe (one-pipe systems) or toward the steam traps (two-pipe systems). That may interfere with its efficiency.