If a radiator is warm at the bottom and cool at the top, or the room is colder than the rest of the house, the likeliest cause in a hot-water system is trapped air. Bleeding the radiator releases it and lets hot water fill the full panel again. It's a ten-minute job, but only for the right kind of system.
First: Is This a Hot-Water or Steam System?
This matters, because the advice is different. A hot-water system circulates heated water through pipes and radiators or baseboard. Each radiator typically has a small bleed valve near the top corner on one end. A steam system sends steam to radiators, which usually have a vent valve on the side that hisses as steam fills the radiator and closes when it gets hot.
Do not bleed a steam radiator. Steam systems manage air through those vent valves and the boiler's water level, and a cold steam radiator is usually a venting, pitch, or boiler pressure problem. Our guide on steam versus hot water boilers explains how to tell, and boiler noises decoded covers common steam-system symptoms.
What You'll Need
- A radiator bleed key, available at hardware stores, or a flat-head screwdriver for some valves
- A towel or rag
- A small container to catch water
How to Bleed a Hot-Water Radiator
- Turn the heating system off and let the radiators cool. Hot water can scald.
- Place the towel and container under the bleed valve.
- Using the key, turn the valve counterclockwise a quarter turn at a time. You should hear hissing as air escapes.
- When the hissing stops and a little water appears, close the valve firmly. Don't force it.
- Repeat on other cool radiators, starting with the lowest floor and working up.
- Turn the heat back on and check each radiator after it warms up.
Check the Boiler Pressure Afterward
Releasing air can lower system pressure. Look at the pressure gauge on the boiler. Residential hot-water systems commonly sit around 12 to 15 psi when cold, but check your boiler's label and manual. If pressure has dropped below the normal range, the system may need water added. Many boilers have an automatic fill valve. If you don't know how your system is fed, don't add water on your own, because overfilling can cause problems with the expansion tank and relief valve.
If Bleeding Doesn't Fix It
- A radiator that stays cold even after bleeding could have a blocked valve, a closed shutoff, or sediment in the line.
- Several radiators cold in one zone points to a zone valve, circulator pump, or a problem with the thermostat that controls that zone.
- Gurgling noise throughout the system suggests air that has accumulated more widely, which may be a pressure or feed problem.
- Water at a valve or on the floor means stop and call.
A boiler that has to be bled every few days is telling you something. Air is getting into the system, typically through a leak, a failing automatic air vent, or an expansion tank problem, and that's worth diagnosing instead of repeatedly bleeding the radiators.
When to Call
If the boiler itself is not producing heat, if you smell gas or oil, or if you're unsure what type of system you have, our boiler repair team can diagnose it. For a general no-heat situation across any system type, see heating repair. If the boiler is old and failing often, our boiler replacement cost and repair-versus-replace guide covers how to think about replacement.
