It's about time to bleed radiators

Inspector's Eye

Trapped air reduces efficiency of system

October 07, 2001|By Dean Uhler

We've had our first cool days and cold nights of the season, and soon it will be time to turn on the heat.

If you have a circulating hot-water heating system - typically radiator or baseboard heat - it's a good time to bleed the air out of the system.

Air trapped in the radiators and pipes keeps the hot water from reaching parts of the system, reducing heat output. If you've noticed that a radiator doesn't get hot, or its top half doesn't warm properly, it probably has air trapped inside that is keeping it from filling with water.

Air in the pipes can also make the heating system noisy as it gurgles through.

Bleeding a radiator is usually a simple procedure.

You will first need to look at the bleed valve on a radiator to see what tool it takes to turn it. The bleed valve is threaded into the side of the radiator, near the very top - if it's not there, you may have steam heat, and none of this applies to you.

The bleed valve will most often have a square stem that fits a radiator key, available at hardware stores. Less often, the bleed valve will have a slotted stem for a screwdriver, or will have a knob attached. The other tools to have handy are a cup and old rag.

The procedure is to open the bleed valve enough to let the air escape from the radiator as it is displaced by water filling the radiator. Slowly turn the valve counterclockwise until air or water comes out of it. If it's water, close the valve, without over-tightening it. If air is coming out, wait until all the air bleeds out and you start to get water. Don't walk away with the valve open! Bleed the radiators at the lowest level of the house first, working up a floor at a time.

Sometimes when you open the bleed valve, nothing comes out - neither air nor water. This will occur if the system lacks enough pressure to displace the air. It gets more complicated in this event, and you may want to call in a professional. But for the knowledgeable or bold, the next step is to add water to the system.

Find the feed valve on the pipe that connects the boiler to the water supply pipes, usually in the basement. Open it and add water until the pressure gauge on the boiler reads about 14 pounds per square inch. Then bleed the radiator. You may need to add water a number of times as you bleed all the radiators.

Inspector's Eye

Dean Uhler has been a home inspector for more than 12 years and is president of Baltimore-based Boswell Building Surveys Inc. Uhler is a member of the American Society of Home Inspectors (ASHI) and is the treasurer of the Greater Baltimore Chapter of ASHI.

Questions, with name, address and daytime telephone number, about homes and home inspections can be faxed to 410-783-2517, e-mailed to or mailed to Inspector's Eye, Second Floor, 501 N. Calvert St., Baltimore, Md. 21278-0001.

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