How to Bleed Air From a Hot Water Heating System (5 Steps)

TL;DR: Trapped air pockets in a hot water (hydronic) heating system block hot water from reaching radiators or baseboard units, causing cold spots and gurgling noises. Bleed each radiator with a radiator key by opening the small valve until water flows steadily with no hiss, usually 10-30 seconds per unit, then check the boiler's pressure gauge afterward and top it up if it dropped below 12 psi.

_Last reviewed: July 2026 · 6 min read_

One radiator in the back bedroom never gets warm no matter how high the thermostat goes, and you can hear a faint gurgle in the pipe wall every time the system kicks on. That's trapped air, not a broken radiator, and it's one of the few heating fixes a homeowner can handle without tools beyond a radiator key and an old towel.

Okoniq Property Hub lets you log the date and PSI reading every time you bleed a zone, so you can spot a pattern before a small air pocket turns into a pump problem.

Why does air get trapped in a hot water heating system?

Air enters a closed hydronic loop through fresh makeup water, small leaks at fittings, or after any repair that opens the system to atmosphere. Every gallon of tap water added to the loop carries dissolved oxygen, and over months that gas comes out of solution and collects at the highest points, usually the radiators or baseboard units on the top floor.

Once an air pocket sits at the top of a radiator, water can't fill that space, so heat transfer drops in that section even though the boiler is working fine. This is different from a baseboard unit that's simply dirty or blocked; if bleeding doesn't fix a cold unit, the next step is checking the fins and covers as covered in baseboard heater maintenance.

How do you find which radiators have trapped air?

Feel each radiator top-to-bottom while the system is running; a unit that's warm at the bottom and cold at the top has air in it. Gurgling, clicking, or a hissing sound near the pipe as the system heats up is another tell, and so is a radiator that takes noticeably longer than the others to warm.

Start with the units farthest from the boiler and on the highest floor, since air migrates upward and outward through the loop. If every radiator in the house is lukewarm at once, the issue is more likely low system pressure or a circulator pump problem rather than air alone, and it's worth comparing the boiler's normal noises against the patterns in 7 water heater sounds and what each one means before assuming it's just air.

What's the step-by-step process to bleed a radiator?

Turn off the heating system first, then let the radiators cool for about 20 minutes so you're not working with pressurized hot water. You'll need a radiator bleed key (a small square-socket key sold for about $3 at any hardware store), a cloth, and a small cup or bowl.

1. Locate the bleed valve, usually a small brass fitting at the top corner of the radiator opposite the inlet pipe.
2. Fit the key onto the valve and hold a cloth underneath to catch drips.
3. Turn the key counterclockwise a quarter turn. You'll hear a hiss as trapped air escapes.
4. Wait until the hiss stops and a steady stream of water comes out, then close the valve immediately by turning clockwise.
5. Move to the next radiator, working from the lowest floor to the highest, and closest to the boiler to farthest.

Each radiator usually takes 10 to 30 seconds of bleeding. If water sputters instead of flowing steadily, there's still air in that section; wait a moment and try again.

| Step | What to check | Time | |---|---|---| | Before bleeding | System off, radiators cooled | 20 min wait | | During bleeding | Steady hiss, then water flow | 10-30 sec per unit | | After bleeding | Boiler pressure gauge reads 12-15 psi | 5 min |

What happens to the boiler's pressure after bleeding, and how do you fix it?

Bleeding radiators releases both air and some water, so the boiler's pressure gauge will usually drop below its normal 12 to 15 psi operating range. Most residential boilers have a fill valve or feed valve near the bottom; open it slowly while watching the gauge until it reads back in that range, then close the valve fully.

Never leave the fill valve open unattended, since overfilling can trip the pressure relief valve and cause a mess. If the pressure keeps dropping again within a few weeks after bleeding, that usually points to a slow leak somewhere in the loop rather than normal air buildup, and it's worth checking connections the same way you'd check water pooling under a water heater for the source.

How often should you bleed a hot water heating system?

Most systems need bleeding once a year, typically in early fall before the heating season starts, since that's when air pockets from a summer of sitting idle tend to show up as cold spots. A system that needs bleeding every month has a bigger problem, most often a slow leak, a failing expansion tank, or an automatic air vent that's stuck and letting air back into the loop instead of venting it out.

If you're managing more than one property, logging each bleed date and the pressure reading afterward makes it obvious when a particular unit or zone is drifting from normal, which is exactly the kind of pattern worth tracking rather than trying to remember.

FAQ

Why is my radiator cold at the top but warm at the bottom?

That's the classic sign of trapped air. Air rises to the top of the radiator and blocks hot water from reaching that section, so bleeding it with a radiator key for 10 to 30 seconds usually restores full, even heat.

Can too much air in the system damage the boiler?

Yes, over time trapped air can cause the circulator pump to run dry or cavitate, which wears out the pump bearings faster and can lead to a $300 to $600 pump replacement. Regular bleeding once a year prevents this.

Is it normal to hear gurgling after bleeding a radiator?

A little gurgling right after bleeding is normal as remaining air pockets settle and move toward the vent point. If gurgling continues for more than a day or two after bleeding all radiators, there's likely still air trapped somewhere in the loop or a pressure issue.

Do I need to bleed baseboard heaters the same way as radiators?

Yes, hot water baseboard units have the same type of bleed valve, usually at one end of the unit, and the process is identical: open with a key, wait for steady water flow, then close.

What if water won't come out when I open the bleed valve?

That usually means the system pressure is too low to push water to that point. Check the boiler's pressure gauge first, top it up to 12-15 psi through the fill valve, then try bleeding again.

This is educational information, not HVAC or plumbing advice. Call a licensed heating technician if pressure keeps dropping after bleeding, if you're unsure how to operate the fill valve, or if the boiler shows signs of a leak.