How to Bleed Hot-Water Radiators and Baseboard, Step by Step

A hot-water heating system is a closed loop: the boiler heats water, a pump pushes it through radiators or baseboard, and it comes back to be heated again. Air gets into that loop over time, mostly from fresh water added to the system, and it rises to the top of the radiators, where it keeps hot water out. The symptoms are a radiator that is warm at the bottom and cold at the top, gurgling in the pipes, and a room that never quite heats. Letting that air out, called bleeding, takes a small key and a few minutes per radiator.

This guide covers the check you must make first, because steam radiators look similar and must never be bled, then the procedure, what the boiler pressure gauge should read afterward, and the signs that the air is a symptom of something a heating technician needs to fix. The U.S. Department of Energy recommends bleeding hot-water radiators once or twice a season and calling a professional if you are unsure how. If your house has a furnace and air vents instead, see furnace blowing cold air.

Before You Start

Confirm it is hot water, not steam

Only hot-water (hydronic) radiators, convectors, and baseboard get bled by hand. Steam radiators have automatic air vents that let air out on every cycle and close when steam reaches them; opening one can release steam and scalding water. To tell them apart:

• Look at the radiator. A one-pipe steam radiator has a single pipe near the floor at one end and, on the other end partway up the side, a vent valve: a metal canister a couple of inches tall. A hot-water radiator usually has two pipes and a small bleed valve at a top corner, often no bigger than a thumbnail.
• Look at the boiler. A steam boiler has a glass sight tube on the side showing a water level, and its gauge usually sits near zero. A hot-water boiler runs completely full, has no sight glass, and its gauge reads around 12 psi or more even when cold.
• Watch for two-pipe steam. Some older steam systems have two pipes per radiator and no vent. Two pipes plus a sight glass on the boiler means steam. When in doubt, ask the technician at the next service.

What you need

• A radiator bleed key for valves with a small square stem, or a flat screwdriver for slotted valves.
• A rag, and a small cup or jar lid to catch the water.
• A flashlight and a pencil, to note the boiler pressure before and after.

Bleed valves vary, so try the key on one valve before the first cold night and keep a spare by the boiler. A cheap key can crack if forced; if a valve will not turn with light pressure, stop.

Step-by-Step

1. Run the heat and find the cold spots. Turn the thermostat up until the radiators warm. Quickly touch the top and bottom of each one, or each baseboard cover, with the back of your hand; they can burn. Note which are cold at the top or gurgle, and write down the boiler's pressure reading (use the psi scale if the gauge also shows temperature or feet of water).
2. Turn the heat off and let it cool. Turn the thermostat down so the boiler and pump stop, and wait until the radiators are lukewarm, usually an hour or more. The water in a running system is hot enough to scald, and it spits when the valve first opens. With the pump off, the air also settles at the top, by the bleed valve.
3. Start low and far. Begin on the lowest floor at the radiator farthest from the boiler, then work toward the boiler and up a floor at a time. Weil-McLain's boiler manuals also start venting on the lowest floor. If your boiler's manual gives a different order, follow it.
4. Find the bleed valve. On a radiator it is at a top corner, usually at the end opposite the supply valve. On baseboard it is usually under the end cover, on the pipe at one end of the run. Some baseboard has none; that air is removed at the boiler, by the technician.
5. Get the rag and cup in place. The water can be dark or rusty and stains floors.
6. Open the valve a quarter to half turn. Turn counterclockwise slowly until it hisses. That is the air. Open it no further; on many valves the small stem unscrews completely and is hard to get back in against the water pressure.
7. Close it as soon as water appears. The hiss becomes a sputter, then a steady trickle of water with no air. Close the valve snug, not forced, and wipe it dry.
8. Repeat at every bleed valve, in order, including radiators that seemed fine.
9. Check the boiler pressure. Bleeding lets out a little water, so the reading may drop. A typical cold pressure for a two-story house is about 12 to 15 psi, but the right number for your system is in the boiler's installation or owner's manual, or ask your technician to mark it on the gauge. The pressure on the boiler's rating plate is its maximum, the relief valve's limit, not a fill target. Many boilers have an automatic fill valve that tops the system up on its own. If yours does not, or the pressure stays low, follow the owner's manual for adding water or call a technician. Do not add water by guesswork: an overfilled system pushes the pressure toward the relief valve's setting.
10. Turn the heat back on and check. Run a full cycle. The radiators should warm evenly top to bottom, the gurgling should stop, and no bleed valve should drip. Heating drives dissolved air out of the water, which is why boiler manuals call for venting again after the system has heated; bleeding the worst radiators again a day later is normal.

What Normal Looks Like

Normal is a hiss of a few seconds, longer on a radiator that was cold at the top, then a sputter, then water, and a gauge that dips slightly and returns to its usual cold reading. Small amounts of dark water from an older system are normal. Afterward the radiators heat all the way across the top, and the pipes are quiet apart from the ticks of metal expanding.

Not normal: a valve that drips after it is closed; a top-floor valve that gives no hiss and no water, which can mean the pressure is too low to push water that high; a radiator cold at the bottom but warm at the top, which points to sludge rather than air; and a radiator that stays cold all over. For that last one, check that its supply valve handle is open, but if the valve is stiff or weeps at the stem, leave it for the technician.

When to Call a Pro

• Pressure that keeps dropping. A gauge that falls again over days, or a system that needs regular topping up, is losing water somewhere: a joint, a valve, the boiler, an air vent, or a weeping relief valve.
• A leaking valve. A bleed or radiator valve that drips when closed needs repair, which may mean draining part of the system.
• Air that keeps coming back. Bleeding the same radiators every few weeks usually means a leak bringing in fresh water, a failed automatic air vent at the boiler, or fill pressure too low for the top floor.
• A relief valve that discharges. Water from the relief valve's pipe means the pressure reached its setting, often from overfilling, a failed expansion tank, or a faulty fill valve. Call the same day, and never plug the valve.

Our DIY vs. hire-a-pro guide covers hiring well: written quotes, a license, and insurance. If a room stays cold while you wait, an electric space heater run by the safety rules in that guide can bridge the gap.

How Often

Once or twice a heating season, as the Department of Energy recommends: early in the season, after the system has run a few days, and again whenever a radiator goes cold at the top or the pipes gurgle. Bleed again after any repair that drained the system, since refilling brings in air. The fall checklist schedules the annual boiler tune-up, and the first bleed fits right after it; the maintenance calendar lists it in October. For keeping the rest of the house safe in a cold snap, see the winter guide.

Frequently Asked Questions

Should the heat be on or off when I bleed radiators?

Off, with the system cooled down. Turn it on first only to find the cold radiators. The water in a running system is hot enough to scald, and with the pump stopped the air settles at the top where the valve can release it.

What should my boiler pressure be after bleeding?

What your boiler's manual or your technician calls for. As a rule of thumb, a two-story house reads about 12 to 15 psi cold, and the reading rises as the water heats. An automatic fill valve brings it back up on its own; without one, top up per the owner's manual or have a technician do it.

Can I bleed a steam radiator?

No. A steam radiator's vent is automatic: it lets air out as steam fills the radiator and closes when steam reaches it. A cold steam radiator usually has a clogged vent, which gets replaced, or a pipe holding water. Opening anything can release steam and scalding water.

Why do my radiators keep filling with air?

A little air early in the season is normal. Air that returns every few weeks means water is being lost and replaced, an air vent at the boiler has failed, or the pressure is too low to keep the top of the system full. That is a technician's diagnosis; more bleeding will not fix it.

The Bottom Line

Confirm it is hot water, not steam. Run the heat to find the cold radiators, then turn it off and let it cool. Start on the lowest floor, farthest from the boiler, open each bleed valve a quarter to half turn, and close it when water appears. Check the boiler gauge against the cold fill pressure in its manual, typically about 12 to 15 psi in a two-story house, and let the automatic fill or the owner's manual handle any top-up. Pressure that keeps falling, a leaking valve, air that keeps coming back, or a relief valve that discharges is a call to the heating technician.

Sources