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How it works: Two-pipe steam


Two-pipe steam radiators

In two-pipe steam installations, steam flows from the boiler to the radiators through one pipe. It then condenses and returns to the boiler through a different, second, pipe. Two-pipe systems are often easy to distinguish from a one-pipe system because there are two pipes connected to the radiator and no steam vent on the side.

Components of a two-pipe steam radiator

One-pipe systems are different than two-pipe systems in that they usually have a single, large-bore pipe that both delivers steam to the radiators and condensate to the boiler. One-pipe steam radiators always have a steam vent on one side, usually about halfway up their height.

Read an introduction to one-pipe steam radiators here.

Hoffman Steam Trap for Two-Pipe Steam Radiator
Hoffman Steam Trap

Two-pipe steam radiator components

Steam enters the radiator through a control valve. A steam trap allows air and water to exit, returning toward the boiler, but ensures that steam stays trapped inside the radiator. Air is vented out of the pipes through a master vent, usually near the boiler, and the condensate flows back into the boiler to repeat the process.

The control valve on the radiator may be manual or thermostatic. A thermostatic radiator valve adds comfort and control. The modern energy efficiency of TRVs can give a dramatic saving on fuel bills.

Thermostatically-controlled steam radiators require a vacuum breaker to ensure the condensate can always return to the boiler. Ours are supplied with one as standard.

Which radiators to use with two-pipe steam? 

We’re definitely biased, but cast iron really is the time-proven material of choice for steam heating. Steam puts the system through a heck of a time: large temperature swings that cause the metal to expand and contract with every heating cycle; acid or alkaline conditions dependent on the chemical makeup of the water; and, if the system isn’t well designed or maintained,  intense shocks from steam hammer.

All of this goes against using thin-walled steel radiators with welded joins. We don’t sell our steel radiators for use on steam, only cast iron. It’s the time-tested material. Browse our full selection of cast iron radiators

When it comes to valve connections on steam, we only recommend threaded, mechanical joins with steel or brass pipe. While compression fittings are perfect for hydronic systems, we prefer the time-tested, heavy-duty security of a threaded join. 

Read about one-pipe steam and hydronic radiators. 

Further reading

Dan Holohan: The Lost Art of Steam Heating Revisited
Dan Holohan: Greening Steam