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How it works: One-pipe steam


One-pipe steam systems

In one-pipe steam installations, steam flows from the boiler to the radiators, and water flows back again through the same single pipe. One-pipe radiators are easy to identify because they have just one pipe connected to them, always at the bottom, and an air vent attached to one side, usually about halfway up the height of the radiator.

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.

This is different to two pipe steam installations in which the steam flows from the boiler to the radiators in one pipe, and water returns to the boiler through a separate, second, pipe.

Read an introduction to two-pipe steam systems here.

Windsor XL Chrome - 1.25in Radiator Valve
Windsor XL Chrome – 1.25in Control Valve

One-pipe steam radiator components

Steam enters the radiator through a large-diameter control valve, causing the air in the radiator to exit through an air vent. The air vent closes automatically when the radiator is full of steam. The steam then cools and condenses, turning back to water. The water returns to the boiler through the same control valve.

The control valve must have a large internal bore: Minimum of 1 in. for radiators of 5000 BTUs or fewer, at least 1.25 in. above that.

The inlet, or control, valve for a one-pipe steam radiator must be fully open or fully closed. Throttling the valve (leaving it neither fully open nor fully closed) can lead to very noisy steam hammer.  

The heat from a single-pipe steam radiator is controlled by restricting the air that can exit. Adding a thermostatic valve between the radiator and air vent adds comfort and control. 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? 

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. 

What size valve? 

We recommend a 1″ valve for radiators up to 5000 BTUs or fewer and 1.25″ valves above that.  

Read also: Hydronic heating: The basics

Further reading

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