When radiators need bleeding
- Radiator is cold at TOP, warm at bottom
- Gurgling, knocking, or banging when boiler runs
- One zone or room runs cooler than others
- Recently had system maintenance, water refill, or major repair
Air gets into closed-loop hydronic systems gradually — fresh fills, small leaks, microscopic PEX permeation. Annual bleeding is normal maintenance.
What you need
- Radiator key ($3-$5 at any hardware store). Some radiators use a flathead screwdriver slot.
- Towel to catch the water that comes out.
- Small cup or bowl for tight spaces.
Step-by-step
- Turn the boiler off via the thermostat. Let the system cool 15-20 minutes. Bleeding a hot system is dangerous — water can spray under pressure.
- Identify the bleed valve. Small square or round nut at the TOP of each radiator, usually on one end. Plastic-topped auto-vents bleed themselves; no intervention needed.
- Position towel/cup directly below the valve.
- Insert the key, turn SLOWLY counter-clockwise (about 1/4 turn). You'll hear hissing as air escapes.
- Let air out until water (not air) flows steadily.
- Close immediately when water flows. Turn clockwise. Snug — don't overtighten.
- Repeat for each radiator, starting FURTHEST from the boiler and working toward it.
- Check system pressure. If it dropped below 12 PSI, top up via fill valve to 12-15 PSI.
- Restart the boiler. Verify radiators heat evenly across full surface.
If something goes wrong
Valve won't open: seized from corrosion. Don't force it. Call us.
Air keeps coming out, never water: system pressure too low. Stop, top up to 12 PSI, resume.
Pressure drops below 8 PSI during bleeding: stop, refill, continue. Don't run below ~8 PSI.
Air keeps coming back days later: leak introducing air somewhere. Check for damp spots, listen for trickle sounds. Time to call.
Valve won't seal after bleeding: washer is shot. Won't get worse immediately, but call within the week.
Quick answers
Once a year, typically early fall before heavy use season. More often if you've had recent system work, refills, or noticed cold-top symptoms.
Slightly. Trapped air reduces heat transfer efficiency. Bleeding restores full heating capacity, reducing run time. Typical savings: 3-8% during heating season.
Minimal if the system is cool. Hot systems can spray scalding water under pressure — that's the main risk. Otherwise routine maintenance.
Those bleed automatically. You don't need to do anything. They occasionally fail (start leaking continuously) — quick service call to replace.