In two-pipe steam installations, steam flows from the boiler to the radiators through an inlet pipe. Once the steam condenses it returns to the boiler through a second outlet pipe. You can typically recognise a two-pipe system from the two pipes and lack of steam vent attached to the radiator.
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. As the radiator fills with steam, air escapes the radiator through the open trap. Once radiator is full of steam, the thermostat inside the trap expands and closes the outlet, trapping the steam inside it. Once the steam has condensed, the trap opens again to allow the water to return to the boiler.
Air is vented out of the pipes through one or more master vents 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.
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.