How to Repair a Radiator

How to Repair a Radiator That Is Cold on Top and Hot on Bottom

When a water radiator is cold at the top but hot at the bottom, trapped air is usually preventing hot water from circulating. Without circulating, the water cannot adequately heat the radiator. The hot and cold areas may gradate from bottom to top or they may be spotty or uneven. Bleeding the radiator is the best way to repair the radiator.

1. Turn the System On and Off

   Turn on the radiator system to let it heat up. Once it is hot, turn it completely off. Let the radiator cool down until it is warm but not cool.

2. Hold a Towel Under the Bleed Valve

   Wearing safety glasses and gloves, hold a folded towel under the bleed valve. This small valve will always be found on the top of the radiator, to the side.

3. Open the Radiator Valve

   Insert the radiator key on the valve. Slowly turn the valve counter-clockwise. Radiator valves only need to be turned one-quarter or one-half turn for the air to escape.

   Tip

   A radiator valve key is a T-shaped metal key about 1/2-inch long. Replacement keys cost $5 to $10. Alternatively, newer radiator valves can be turned with a flat-head screwdriver.

4. Let the Air Bleed Out

   If air first escapes from the valve, this means that the radiator had air trapped in its sections. Keep the valve open until the air is followed by warm water. Let some of the warm water leak onto the cloth, then firmly shut the valve.

5. Shut the Radiator Valve

   Shut the radiator valve firmly with the radiator key. Turn on the heating system again and observe the radiator for leaks.

How to Repair a Radiator That Is Hot on Top and Cold on Bottom

If the water radiator is hot at the top but cold at the bottom, that means that there is sludge in the system. Sludge is a collection of impurities like limescale, iron oxide, and other minerals that are heavier than water and will settle at the bottom of the radiator. Hot water cannot reach these blocked areas. The repair is to flush out the radiator.

1. Turn System Off

   Turn off the radiator system and let it fully cool down.

2. Turn the Valves Off and Loosen Them

   Isolate the radiator from the system by turning off the valves, including the thermostatic radiator valve (TRV) and the radiator bleed valve.

3. Drain Some Water and Sludge From the Radiator

   Hold a towel under the radiator bleed valve as you loosen it with a radiator key or flat-head screwdriver. Bleed out all air. Place a bucket under the TRV. With an adjustable wrench, loosen the nut that connects the TRV to the radiator. Let the water and sludge flow into the bucket. Give it sufficient time to fully drain.

   Tip

   This passive draining of the radiator removes much of the water and sludge for transport, but this isn't flushing. Flushing happens later.

4. Remove the Radiator

   Finish removing the nuts that hold the radiator to the pipes. Carefully remove the radiator from the wall. Carry the radiator outside if it is light enough or use a hand truck to wheel it out.

   Tip

   Seal the open valves with plastic wrap when transporting the radiator to prevent water and sludge from splashing out.

5. Flush Out the Radiator

   Turn the radiator upside-down. Remove all coverings on the inlets. Attach the end of a hose to the inlet valve. Turn the water on high volume. Water will enter from one inlet, fill the radiator, then flow out of the second inlet. Keep the water flowing until the water coming out of the second inlet runs clear.

   Tip

   Gently tap the side of the radiator with a piece of wood wrapped in a towel or with a rubber mallet to dislodge impurities.

6. Re-Install the Radiator

   Completely drain out the radiator by turning it upright. Tip it to the side. Move the radiator back into the house, connect all valves, shut the radiator bleed valve, and power up the system to test the radiator.

How to Repair a Noisy Radiator

When a steam radiator whistles or makes other high-pitched noises, the cause is usually a faulty steam radiator vent—the valve located about halfway up the side of the radiator. A faulty steam radiator vent sometimes causes water leaks on that side of the radiator. Replacing the steam radiator vent will repair a noise or leaking steam radiator.

1. Turn Off the Radiator System

   Turn off the radiator system and let the radiator completely cool down.

2. Twist Off the Old Steam Vent

   By hand, turn the steam vent counter-clockwise to remove it from the radiator. Place a towel under the vent to catch drips.

3. Wrap the New Steam Vent With Tape

   Hold the new steam vent so that the threads are facing you. Wrap the plumber's tape two or three times around the threads in a clockwise direction.

4. Install the New Steam Vent

   Turn the new steam vent onto the radiator. The steam vent will need to be perfectly upright, so anticipate this as the vent tightens. If you feel that another rotation will bring the vent upright without breaking the valve, continue to rotate. Otherwise, avoid overtightening the vent.

How to Repair a Leaky Radiator

A steam radiator that leaks water from tiny holes can sometimes be repaired with epoxy putty. Intended for water applications, epoxy putty starts as a malleable putty and dries hard in about an hour.

1. Power Down the System

   Power down the radiator system and let the radiator fully cool down.

2. Clean the Radiator Surface

   If there is rust and corrosion, rub smooth with fine sandpaper. With a soft cloth, clean off the area to be patched with denatured alcohol, acetone, or isopropyl alcohol. Let the area dry.

3. Push the Putty Into the Hole

   Roll the putty into a ball about 1/2-inch in diameter. Warm up the putty until it is soft. Pack small amounts of the putty into the hole, followed by more putty until no more putty will fit into the hole. Smooth off the surface with your finger.

4. Sand and Paint the Radiator (Optional)

   Wait for one hour or until the putty has hardened. Lightly sand the putty until it is flush with the surface of the radiator. If desired, the radiator can be painted.

Important Considerations for Cast Iron Radiators

If you attempt to flush out a cast iron radiator on a hot water heat system, there are several factors to consider. It's important to understand how the system works to be able to shut it down properly. In most cases, valves on the radiator do not hold, they may be old. Even if they do hold, the process is complicated.

The boiler has a pressure regulator which reduces household pressure for the system. Operating pressure can be anywhere from 15-25 pounds. The water must be shut off before you do any work and the pressure must be relieved at the boiler. The boiler must be shut down, the power and switch turned off. Once pressure is relieved, and confirmed, you can disconnect the radiator & remove it to flush it out. This could be a two person project.

If you choose to leave the radiator in place to flush it, you will need one person at the boiler and one person at the radiator. The water can be turned on while while the person at the radiator holds a bucket to catch the sludge. This can only be done if you can access a connection on the radiator large enough. A professional is recommended. In many cases it might be easier to just replace the radiator.

On a hot water system, flush the system thoroughly from the boiler by keeping an elevated pressure and opening a hose bib from the return side of the loop after isolating the system.

When to Call a Professional

The hub of the radiator system, the boiler, heats water in a large sealed tank and sends steam or hot water in a loop throughout the house. When the radiator system's problems stem from the boiler, call a heating and cooling company. First, verify that the company has technicians experienced in working on radiators and boilers, as this may not be common in all areas.

When a radiator accumulates too much sludge at the bottom, flushing the radiator with a hose may not be sufficient. In this case, have a heating and cooling company power flush the radiators. Power flushing injects a pressurized chemical solution throughout the radiator loop and the radiators.