One-Pipe Steam Radiators

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A primer on one-pipe steam radiators. Includes component identification, pipe sizing and thermostatic control.

Fundamentals Of One-Pipe Steam Radiators

In one-pipe steam installations, steam flows from the boiler to the radiators, where it displaces the cold air by pushing it out through a vent on the radiator. The air vent closes automatically when the radiator is full of steam. The heat energy in the steam is then transferred to the room, as this occurs the steam cools and condenses into water which pools at the bottom of the radiator. This condensate then flows back again through the same single pipe.

Due to the steam and water flowing in opposite directions through the same pipe, the diameter of this pipe is typically more than 1". So one-pipe radiators are easy to identify from the single, fairly large pipe connected to them, always at the bottom, and an air vent attached to the opposite side, typically halfway up the height of the radiator (see below).

Components of a one-pipe steam radiator. Vacuum Breaker: Ensures condensate can return to the boiler. Included as standard on all Castrads steam radiators. Air Vent: Installed about halfway up the radiator, the air vent allows air to escape from the radiator as it fills with steam. Regulating it with a thermostatic valve (see Niva) adds comfort and control, and can dramatically reduce energy use. Control Valve: Control Valve Always installed at the bottom, this allows the steam to enter and the condensate to exit. The control valve should always be left fully open. Counter-intuitively, the way to control the heat from a one-pipe steam radiator is by regulating the air as it exits the radiator rather than the steam as it enters.

Read an introduction to two-pipe steam systems here.

One-pipe steam radiator components

Windsor XL 1.25in Radiator Valve - Chrome

 The inlet, or control, valve must have a large internal bore: minimum of 1" for radiators of 5000 BTUs or fewer; at least 1-¼" above that. On a one-pipe steam radiator it must be fully open or fully closed. Throttling the valve (leaving it halfway open) can lead to very noisy steam hammer. The heat from a single-pipe steam radiator is controlled by restricting the air that can exit.

Steam vents control the flow of heat into the radiator by throttling the air that exits. This occurs by the means of a thermostat in the valve which will close when it reaches temperature at 212 °F. 

Adding a thermostatic valve between the radiator and air vent allows for a greater degree of control, by stopping steam flowing into the radiator at temperatures lower than the steam vent. 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 one-pipe steam?

Cast iron really is the time-proven material of choice for steam heating. Steam puts the system a lot of stress: 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. Cast iron also forms a passive rust coating protecting the bulk of the material from further oxidation. All of this goes against using thin-walled steel radiators with welded joins, they just don't last.

We only offer cast-iron radiators for steam systems, never steel. Browse our full selection here. 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.

What size valve?

We recommend a 1" valve for radiators up to 5000 BTUs or fewer and 1-¼" valves above that. Read also: How it works: Hydronic heating.

Further reading

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