Hydronic Cast Iron Radiator | Castrads
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Installing a cast iron radiator on a hydronic (hot-water) system


Components of a hydronic radiator

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.

Valve tails

Cast iron radiators have normal thread on one side and reverse (left-hand) thread on the other. When inserting valve tails it is possible to loosen the left-hand thread reducing bush. If the bush is loosened it will leak.

It’s very easy to remedy the problem. Simply tighten the bush up again before installing the radiator.

The Rothenberger Uni Spanner (code 73297) is very useful for inserting valve tails.

Always use the correct sized Allen key or a suitable alternative to insert valve tails and ensure proper leverage.

Never grip the valve tail on the outside as this will damage the finish.

Shrouds must be placed over the feed pipes before the valves are installed. There is no way of installing shrouds once there’s an olive on the pipe.

Ensure the rubber O-ring is properly seated in the recess of the base plate. This allows it to grip the shroud so that it stays in position when pulled to meet the base of the valve and hide the pipe leading into it.

Bleed valves

Matt Black cast iron radiator with polished brass bleed valve.
Mercury 3 Column in Matt Black with Polished Brass 0.5in bleed valve

Check the finish of bleed valve on the radiator label. Orders with multiple radiators may have different finishes of bleed valve on different radiators. The bleed valve finish is displayed on the label stuck to the packaging of each individual radiator.

To install the bleed valve, apply a small amount of thread sealant before tightening with a small spanner. Drying sealants such as LoctiteTM or LSXTM are preferred to prevent movement of the bleed valve once installed. PTFE is also acceptable but the bleed valve may loosen when bled.

Touch-up paint

If the finish of your radiators is damaged during installation, please contact us for a free pot of touch-up paint. To paint over small scratches simply rub the area down with wire wool or very fine abrasive, clean the area with soapy water and leave to dry before applying the paint in thin coats as required.

Balancing radiators

Balancing radiators is an essential part of installation that should never be overlooked. It ensures that your radiators heat evenly without being too hot or too cold.

Post installation

The system should be cleaned and then refilled with appropriate levels of inhibitor. Failure to do so will significantly reduce the efficiency and lifetime of your central heating system. Castrads recommends Sentinel products for all aspects of hydronic system care.

Castrads radiators require very little external maintenance. A damp cloth should remove any dirt and a feather duster is the best way to remove dust from in between the sections.

System maintenance

Inhibitor levels should be checked and maintained on an annual basis. A system cleaner may be used and the system flushed but this shouldn’t be required more than once every few years.

Radiator refurbishment

White 6 column radiator in hallway with chair rug and floor lamp.
Refurbished 6 column radiator finished in Bright White

Cast iron radiators have an almost infinite lifespan. Tired finishes can be removed and any worn parts replaced thanks to their modular construction. If ever you feel your radiator would benefit from a facelift, contact us to discuss refurbishment rather than replacement.


  • Valves knocking, rattling or banging

This occurs when water flows the wrong way over the plunger inside a thermostatic radiator valve because the valve has been installed on the return side of the radiator, rather than the flow. Solution: Swap over the lockshield and thermostatic valve.

  • Leak on an end bush

Leaks can occur between the end bush and the body of the radiator if the bush is loosened during installation. Solution: Tighten the bush by turning it anticlockwise (opposite to usual).

  • Leak on the bleed valve

The bleed valve is either not closed fully or isn’t sealed well into the radiator. If water is seeping from between the body of the radiator and the base of the bleed valve the seal is at fault. Solution: Having emptied the radiator of water first, remove the bleed valve. Add more sealant and insert it again, nipping it tight with a small spanner.

  • Radiator not heating up

There are several factors that may cause one radiator on a system to not heating up as much as the others.

Problem: Lack of water in the radiator Solution: Try bleeding the radiator. Your system may need topping up.

Problem: Radiator valves are not open Solution: Ensure both the control and lockshield valves are fully open. Try removing the thermostatic head completely if applicable. You may need an Allen key or screwdriver to open the lockshield valve depending on the model.

Problem: There’s a blockage Solution: Try forcing all of the water in the system into the one radiator that is affected. Close the valves on all other radiators in the system and run the heating. Failing that, a full system flush may be necessary.

Problem: The system is not properly balanced Solution: This is the most likely issue if the radiators are progressively cooler with distance from the boiler. Call a plumber to balance your system.

  • Not all feet are touching the floor or the radiator leans away from the wall.

This is caused by uneven floors. Try to make the floor surface as level as possible before installation. In older properties this may not be possible. Solution: Small shims or penny coins placed under the feet are useful to even out any small gaps. If uneven floors or walls cause the radiator to lean, wall stays are useful to pull the radiator vertical. Ensure that the wall fixings are strong enough to take the weight.

Polished cast iron radiator on black and white checkerboard tiles.
Rococo II 26in in Etch-Coated Polish with Windsor Satin Nickel TRV. Hydronic installation.