If you have a home with hot water radiators and some of them feel cool at the top while warm at the bottom, or make gurgling and banging sounds when the heat runs, the problem is almost certainly trapped air. Air gets into the system over time, rises to the top of the radiators, and prevents hot water from filling those sections. The fix is called bleeding the radiator, and it is one of the simplest HVAC maintenance tasks a homeowner can do.

How Hot Water Radiator Systems Work

A boiler heats water and circulates it through pipes to radiators throughout the house. Each radiator has a bleed valve, also called an air vent, near the top of one end. When air accumulates inside a radiator, it occupies space that should be filled with hot water, reducing the heating surface area and creating cold spots. The process of bleeding releases that trapped air so hot water can fill the entire radiator.

When to Bleed Your Radiators

Bleed radiators at the start of heating season each year as part of routine maintenance. Also bleed them any time you notice:

  • Radiators that are warm at the bottom but cool at the top
  • Gurgling, knocking, or banging sounds during operation
  • Reduced heat output from a radiator that was previously working well
  • After any work that required draining or opening the system

What You Need

You need a radiator bleed key, which is a small square or rectangular key that fits the bleed valve. These cost under two dollars at hardware stores and fit most residential radiators. You also need a small container to catch any water that drips out, and a cloth or paper towels.

Step-by-Step Instructions

Turn your heating system on and let it run until the radiators are fully warm. This pressurizes the system, which makes the bleeding process more effective.

Working from the top floor of your home downward, and from the radiator farthest from the boiler to the closest, locate the bleed valve on each radiator. It is typically a small brass fitting with a square or slotted head, located at the top corner of the radiator.

Hold your cloth or container under the valve to catch any water. Insert the bleed key and turn it counterclockwise, slowly, about a quarter to half a turn. You will hear a hissing sound as air escapes. This is exactly what you want.

Keep the valve open until the hissing stops and water begins to drip out steadily. Once you have a steady flow of water with no air, close the valve by turning it clockwise. Do not overtighten; snug is sufficient.

Move to the next radiator and repeat. Work your way through every radiator in the system.

When you are finished bleeding all the radiators, check the boiler pressure gauge. Bleeding releases air and some water, which can drop the system pressure. Most residential hot water systems should be maintained between 12 and 18 PSI. If the pressure has dropped significantly, you may need to add water to the system using the boiler’s fill valve. Consult your boiler manual for the correct procedure, as it varies by system.

After Bleeding

Run the heating system for an hour and check that the previously cold radiators are now heating uniformly from top to bottom. If cold spots remain, repeat the bleeding process, as some air pockets can be stubborn.

Bleeding radiators takes about 20 minutes to work through an entire house and requires no special skills. Done once a year, it keeps your system running quietly and efficiently all winter.