Short definition
An air lock is a trapped pocket of air at a high point in your plumbing that physically blocks water from flowing past. It usually shows up as a spluttering tap or no flow at one fixture after a shutoff or repair. Bleeding the system by opening the highest faucets first usually clears it in a few minutes.
What it is
Water is incompressible; air isn’t. When air gets trapped at the top of a pipe loop or fixture branch — usually after the main is shut off, a water heater is drained, or a boiler is refilled — that air pocket holds station and stops water from pushing through. The classic symptom is a faucet that splutters, pulses, and barely flows; sometimes nothing comes out at all.
You’ll almost always see this right after a service event:
- After a whole-house shutoff for a repair.
- After a water-heater replacement (first hot draw splutters).
- After a hydronic boiler refill (radiator stays cold at the top).
- After a well-system service or pressure-tank replacement.
The fix is to give the air a route out. Open the highest faucet in the house first, then work down — air rises, so the highest opening is where it’ll discharge. Let each tap run until it stops spluttering and runs steady.
Why it matters to a homeowner
Air locks look alarming — “the water just stopped working” — but they’re benign and usually self-clearing. Knowing the term saves you a service call after any shutoff or water-heater swap. If a contractor leaves and you’ve got spluttering taps, the cure is almost always to bleed the system, not to call them back.
For hydronic (radiant-baseboard or radiator) heating, air locks show up as a cold spot at the top of one radiator while the rest of the system runs hot. Each radiator has a small bleed valve — opening it for a few seconds with a key or screwdriver releases the trapped air and restores even heat.
If air locks recur every time you shut the water off, something else is wrong — a service-valve weep letting air in, a failing well-pump check valve, or a water-heater issue. That one calls for a plumber.
Common failure modes
- Air lock after main shutoff. Bleed by opening highest faucet first; work down.
- Air lock in water-heater hot side. First hot draw splutters; expected after refill.
- Air lock in radiator. Bleed via radiator key; cold spot clears.
- Recurring air lock. Service-valve weep, bad check valve, or vent issue — diagnose, don’t keep bleeding.
- Solar thermal or boiler loop air lock. System-specific bleed point; consult installer.
Common variants
- Air lock vs. waterlogged pressure tank. Air lock is unwanted air in supply pipes; a waterlogged tank is the opposite — a pressure tank that’s lost the air pre-charge it’s supposed to have.
- Air lock vs. element burnout. Both can produce a spluttering hot tap; air lock clears with bleeding, a failed element doesn’t.
- Air in supply (homeowner term) vs. trapped air in hydronic radiator (UK trade term, “air-locked”). Same physics, different fix locations.