How to Bleed Air Out of a Hot Water Heating System
Air in the loop is one of the most common heating complaints and one of the few you can genuinely fix yourself. The order matters.
By Derek Mills, Co-owner, service lead ·
Gurgling in the pipes, a zone that will not warm up, rooms that heat unevenly. Nine times out of ten in an older hot water system, that is air.
This is one of the few heating repairs I am happy to talk a homeowner through, because the tools are a rag and a small key, and the worst case is you make a mess.
Why air gets in
Water carries dissolved air. Heat it and the air comes out of solution, then collects at the high points of the system where it forms a pocket. That pocket blocks circulation the same way a kink in a hose would.
Fresh water makes it worse, because fresh water carries more dissolved air. Which is one more reason a system that needs topping off every month is a system with a problem worth finding.
What you need
- A radiator bleed key, or a flathead screwdriver depending on your valves. Hardware store, a couple dollars.
- A cup and a rag.
- That is the entire list.
The order
This is the part people get wrong. Work from the lowest and closest to the boiler outward and upward. Basement first, then first floor, then second floor, and within a floor, nearest the boiler first.
Bleed the top floor first and you push air from the lower runs up into lines you already did.
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Run the system. Turn the thermostats up so the boiler fires and the circulators run. You want the water moving and warm. Give it fifteen or twenty minutes.
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Find the bleeders. On baseboard, small valves at the ends of the runs, often under the end caps. On cast iron radiators, a small valve near the top on one side.
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Open one slowly. A quarter turn is plenty. Cup underneath. You will hear hissing.
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Wait for water. Air hisses, then you get spitting and sputtering, then a steady stream. Steady water means that point is clear.
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Close it. Snug, not gorilla tight. These are soft brass and they strip.
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Move to the next one and repeat, working outward and upward.
Afterward, check your pressure
This is the step people skip and it matters. Every bit of water you let out drops the system pressure. Go look at the gauge on the boiler.
If it is below about 12 psi cold, the system needs water added at the fill valve. Go slowly, watch the gauge, stop at 12 to 15.
If you bled a lot of zones, you may need to add water partway through, then bleed again, because the new water brings its own air. Two passes is normal on a system that has never been done.
If it comes right back
Bleeding is treating a symptom. If you are doing this every few weeks, something is introducing air continuously:
- A leak somewhere. As water leaves, the fill valve adds fresh, oxygenated water. Often the leak is small enough to evaporate before it puddles.
- A failed or waterlogged expansion tank. Pressure swings pull air in past fittings.
- An air separator or automatic vent that has failed. Most systems have one near the boiler whose whole job is removing air continuously. They wear out.
- A fill valve stuck open, constantly adding fresh water.
Continuous fresh water in a boiler is genuinely destructive. Fresh water means fresh oxygen, and oxygen plus steel equals rust from the inside. It also brings minerals that scale up the heat exchanger. Systems fed this way die noticeably early.
So if you bleed it and it stays quiet, great, you are done. If you are back at it in a month, call somebody. The bleeding is not the fix at that point, it is the symptom telling you where to look.
What not to do
Do not bleed a steam system this way. Steam radiators have air vents that work differently and opening things on a steam system is a good way to get burned. If your radiators are fed by a single pipe and the boiler has a sight glass, that is steam, and that is a different article.
Do not force a stuck bleeder. Old ones seize and snapping one off means draining the zone to replace it.