One-Pipe Steam Radiator | Castrads
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One-Pipe Steam Radiator

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Installing a radiator on a one-pipe steam system


Components of a one-pipe steam radiator

Pipe sizing

A one-pipe steam cast iron radiator requires big pipes. Our radiators must be supplied with a minimum of 1" continuous piping from the riser for radiators up to 5000 BTUs. Above that, 1 ¼" is the minimum. Piping should be black iron or steel.

¾" pipes are too small for proper operation of a one-pipe cast iron radiator. If the existing feed is less than 1", investigate before installing the radiator to see if larger pipes exist downstream. The feed between the radiator and the riser should at no point reduce below the size of the valve.

Site preparation

Ensure the floor surface is well maintained. Don't install a cast iron radiator on unsound flooring.

There is likely to be some residual water inside the radiator from the manufacturing process. This will stain floors, so be sure to protect the area in which you're working.

Cast iron radiators are very heavy. Always protect the floor from scratches.

Know the wall material and measure for wall stays before the radiator is installed - it will make installation much easier. See our wall stay installation guide for more.

Installing valves

Use an adjustable wrench on the valve, or protect surface with a rag. Never use pipe wrench directly on finished surface of the valve - it will damage the decorative finish.

Use a spud wrench to insert spud. Apply sealant such as Teflon tape to the spuds. None is required on the union between the valve and tail nut - the EPDM gasket provides the seal.

Don't connect valve until the wall stays are installed (if using).


The radiator should lean very slightly towards the inlet valve. A rule of thumb is approximately 1/16" for every eight sections of radiator. A quarter coin under each of the legs furthest from the valve is usually all that's required.

Vacuum breaker

Always necessary on thermostatically-controlled radiators. No harm in using them on any steam radiator.

TRV installation

one-pipe steam thermostatic radiator valve in natural brass.
Niva one-pipe steam thermostatic radiator valve in Natural Brass.

We recommend setting TRVs to 3 to start with. Give it time and adjust gradually until you reach a temperature you are comfortable with. Once you have found your preferred setting, your TRV should not require regular adjustment.

Post installation

Ensure inlet valve fully open. We recommend a boiler service following installation of any new radiator.

cast iron radiator in landing of brownstone townhouse. Pharmacy cabinet. Wooden plank floor.
One-pipe steam cast iron radiator in Matt Black with Windsor 1.25" XL valve and Niva one-pipe steam TRV in Natural Brass.


  • Air vent spitting water

Check the pipe sizing is correct
Try lowering the boiler pressure
Check the radiator is pitched correctly
Ensure the valve is fully open

Ensure system is vented adequately

  • Radiator not getting hot

Ensure the valve is open
Is the TRV turned down too low?

Ensure system is vented adequately

  • Noisy / water hammer

Ensure the valve is not throttled
Check if boiler pressure is too high
Ensure the radiator is pitched correctly

Ensure system is vented adequately