When the first real cold snap arrives, you flip on the heat and some radiators stay stubbornly lukewarm. A common culprit is trapped air, which collects at the top of the radiator and blocks hot water from filling it. The fix, called bleeding, takes about ten minutes per radiator and costs almost nothing. This guide covers hot-water (hydronic) radiators with a bleed valve. If you have steam radiators, electric units or are unsure what you have, check your manual or call a heating professional before you start.

Signs your radiator needs bleeding

  • It is warm at the bottom but cool at the top.
  • It takes much longer to heat up than other radiators.
  • You hear gurgling, hissing or banging sounds.
  • The room stays cold even though the boiler or heating system is working.

What you need

  • A radiator key, a flat-head screwdriver or a small wrench, depending on your valve (hardware stores sell inexpensive keys)
  • An old towel or rag
  • A small container or cup to catch drips
  • A flashlight
  • Gloves, since the water may be hot

Step 1: Turn the heating on, then off

Run the heat for a bit so you can see which radiators have cool spots. Once you know which ones need attention, turn the heating off and let the system cool down. Bleeding a hot radiator is risky because the water and air that escape can be very hot, and the pump will keep pushing water around if it stays on.

Step 2: Find the bleed valve

Look near the top corner of the radiator, on one end. You will see a small valve, often square or slotted in the center. Place the towel underneath and hold the container close.

Step 3: Open the valve slowly

Fit the key or screwdriver to the valve and turn it counterclockwise a quarter turn. Do not open it all the way. You should hear a hiss as air escapes. That is what you want. Keep the container under the valve and be ready to close it.

Step 4: Close it when water appears

Once the hissing stops and a steady stream or spurt of water appears, tighten the valve clockwise right away. Do not overtighten. Wipe any drips with the towel, and put the container to use for any leftover water. Rust-colored or dirty water may be a sign of internal corrosion, which is worth mentioning to a heating professional.

Step 5: Check the system pressure

Many hydronic systems have a pressure gauge on the boiler. Bleeding lets out a bit of water, so the pressure can drop. Check your boiler manual for the recommended range and how to top it up. If you are not comfortable adjusting the pressure, call a professional.

Step 6: Turn the heating back on and test

Turn the system back on and wait. After a while, feel the radiator from top to bottom. It should be evenly warm. If it still has cold spots, repeat the process once. If problems persist, the issue may be a stuck valve, sludge buildup or a balancing problem, all of which are worth a professional diagnosis.

Bleed from the bottom floor up

If you have more than one floor, start with the radiator closest to the boiler on the lowest level, and work your way up. Air tends to rise, and bleeding in this order helps you catch it all.

Safety notes

  • Never open a valve on a hot radiator.
  • Keep towels handy, because water can spray unexpectedly.
  • If the valve is stuck, corroded or leaks when you try to open it, stop and call a plumber or heating technician.
  • Follow the instructions for your specific heating system, as designs differ.

Keep the heat working through winter

Bleeding once a season is usually enough for most households. Combine it with other pre-winter tasks: check the boiler’s service date, make sure furniture or curtains are not blocking radiators, and keep an eye out for leaks around valves.

The bottom line

A radiator that is cold at the top is often a quick, inexpensive fix. With a key, a towel and ten patient minutes, you can bring back even heat and cut down on that clanking, so you can enjoy a warmer home all winter.

Image: Radiator-250558 1280 by ri via Wikimedia Commons, CC0 1.0