How to Bleed Air from Radiators and Baseboard Heaters
Short answer: With the boiler running, open each radiator's bleed valve (or baseboard's bleeder) slightly with a radiator key until water — not hissing air — comes out, then close it. Work from the lowest floor upward, and check the boiler's pressure gauge afterward, topping up to 12–15 psi cold if needed.
Short answer: With the boiler running, open each radiator's bleed valve (or baseboard's bleeder) slightly with a radiator key until water — not hissing air — comes out, then close it. Work from the lowest floor upward, and check the boiler's pressure gauge afterward, topping up to 12–15 psi cold if needed.
Trapped air is the most common cause of cold radiators in a working boiler system. Air gets in during servicing, through tiny leaks, or dissolved out of fresh fill water — and once it's in a radiator or baseboard loop, it blocks hot water from circulating. The result: radiators cold at the top, baseboard sections that never warm up, and gurgling noises. Bleeding releases the air and restores flow. It's the one boiler maintenance task most homeowners can safely do themselves.
Bleeding didn't fix it? Call Ember Heat: (860) 650-9760
What You'll Need
- A radiator bleed key (a small brass key, available at any hardware store — buy the standard square-key type)
- A small cup or rag to catch water
- For baseboard: the same key, or a screwdriver if your bleeders are screw-type
That's it. No tools beyond the key, no boiler disassembly, no risk if you follow the steps.
Bleeding Radiators, Step by Step
1. Turn the heating on. The system should be running and up to temperature. Heat expands the air and pushes it to the high points — the tops of radiators — where the bleed valves are. Bleeding a cold system works, but a hot one works better.
2. Start at the lowest floor. Air rises, so begin with ground-floor radiators and work upward. Within each floor, start with the radiator furthest from the boiler.
3. Open the bleed valve slowly. The bleed valve is the small fitting at the top corner of the radiator (on steam radiators, it's the air vent — different device, different process; see below). Turn the key counterclockwise about a quarter turn. You'll hear hissing — that's the trapped air escaping. Hold the cup underneath.
4. Wait for water. The hiss will sputter, then turn to a steady stream of water. The moment you get solid water with no air sputter, close the valve — snug, not gorilla-tight. These are small brass fittings; overtightening cracks them.
5. Repeat for every radiator. Don't skip rooms. Air distributes unevenly, and the radiator that seems fine may still hold a pocket.
6. Check the boiler pressure. Bleeding releases system water along with air, so pressure drops. Look at the boiler's pressure gauge: on a cold system it should read about 12–15 psi. If it's low after bleeding, add water via the fill valve (the pressure-reducing valve on the water feed line) until you're back in range. If pressure was already low before you started, the system may have a leak worth investigating.
Bleeding Baseboard Heaters
Baseboard loops bleed the same way, but the bleeders are at the end of each loop — usually a small valve on the last baseboard section's piping, sometimes hidden behind the end cap. Same process: heating on, open slightly until water runs solid, close, check boiler pressure after.
Some baseboard systems have automatic air vents at high points — small brass or bronze devices that bleed air on their own. If yours do and you're still getting air problems, the auto vent may have failed (they do, eventually). A failed auto vent is a quick technician replacement.
Steam Radiators Are Different
Important distinction: steam radiators don't get "bled" — they have air vents, not bleed valves. The air vent (a small bullet-shaped device on the side of the radiator, about halfway up) lets air out automatically as steam enters. If a steam radiator stays cold, the vent has usually failed — and the fix is replacing the vent, not bleeding anything. Don't open a steam air vent manually while the system runs; you'll get a faceful of steam. If your steam radiators aren't heating, that's a service call, not a DIY bleed.
When Bleeding Doesn't Fix It
Bleeding solves trapped air. It doesn't solve everything that mimics trapped air:
- Radiator cold at the bottom, hot at top: likely sludge buildup, not air. Sludge needs professional flushing.
- Whole zone cold: a stuck zone valve, failed circulator, or thermostat/control issue — not air.
- Air keeps coming back: you have a leak admitting fresh air (and fresh oxygenated water, which corrodes). Systems shouldn't need frequent bleeding. If you're bleeding monthly, something's letting air in — find it.
- No bleed valve, or valve broken: older radiators sometimes have painted-over or seized bleeders. Don't force a seized valve — snapped bleeders mean a service call. We deal with these regularly.
A Note on Boiler Pressure
Your boiler's pressure gauge is worth understanding. Cold system: roughly 12–15 psi is normal for a typical two-story home. Hot system: pressure rises as water expands — up to around 20–25 psi is normal. If the gauge drops to zero repeatedly, you have a leak or a failed expansion tank. If it climbs past 30 psi and the relief valve discharges, the expansion tank has likely failed. Both are technician fixes, not bleeding issues.
After any bleeding session, glance at the gauge. Topping up to the correct cold pressure is part of the job — a system bled of air but left at 5 psi won't circulate properly upstairs.
Make It Part of Fall Startup
The best time to bleed radiators is early in heating season — October, when the system first fires up after months idle. Air accumulates over the summer, and bleeding at startup means even heat from the first cold night. Pair it with your annual boiler tune-up and your system starts winter clean, quiet, and balanced.
Cold radiators that bleeding won't fix? Call Ember Heat: (860) 650-9760 215 E Main St, Torrington, CT 06790 — boiler repair, maintenance, and installation across Torrington, CT.
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