How to Tell Which You Have

Look at your radiators. If they have one pipe connecting them to the system, you have a steam boiler. If they have two pipes, you have a hot water (hydronic) boiler. You can also check the boiler itself — a steam boiler has a sight glass showing the water level, and the pressure gauge reads in psi (typically 0.5–2 psi). A hot water boiler operates at higher pressure (12–25 psi) and has an expansion tank.

How Steam Boilers Work

A steam boiler heats water until it turns to steam. The steam rises through pipes to the radiators, releases its heat, condenses back to water, and returns to the boiler by gravity. Steam systems are simple but require careful maintenance — water level, pressure, and venting all need to be right.

How Hot Water Boilers Work

A hot water boiler heats water to 140–180°F and circulates it through pipes to radiators or baseboard heaters using a pump. Hot water systems are more controllable and generally more efficient than steam systems.

Efficiency Comparison

Modern hot water boilers can achieve AFUE ratings of 95%+. Steam boilers are typically less efficient — most operate in the 80–85% AFUE range. High-efficiency steam boilers exist but are less common.

Replacement Considerations

If you have a steam system, you generally need to replace it with another steam boiler — converting to hot water requires replacing all the radiators and piping, which is a major renovation. Hot water systems can be upgraded to high-efficiency condensing boilers more easily.