A gurgling sound in your boiler system is a sign of a problem. Your boiler system may not start, or may struggle to heat areas of your home. Both major and minor issues can cause a boiler to gurgle. If you check the boiler system for the most common causes, you should be able to identify the reason for the noise.
A steam boiler creates steam and circulates it via pipes to a radiator, while a water boiler sends hot water to baseboard radiators or specially designed systems under floors. A boiler with a coil system heats water passing through the coil on demand and warms the home by releasing heat into the air though vents; forced air is also the method by which a traditional furnace heats a home.
A boiler makes a gurgling noise if air is trapped in the lines. The sound you're hearing comes from the mix of water and air in the system; only water should pass through the lines.
Outside condensation traps or discharge pipes may freeze in cold weather. You'll hear a gurgling sound if the water can't properly circulate throughout the condensation boiler system because of a frozen trap or pipe.
If you suspect air is the problem, test the theory by carefully touching the top of the highest radiator in your system. The top will feel cooler than the rest of the radiator if air is trapped inside. Radiators have bleed keys you can turn to release trapped air. Hot water will pour from the bleed valve, so don't let the drainage touch your skin.
Resetting a condensation boiler — if the model has a reset option — might defrost a partially frozen pipe or trap. If the unit won't start after a reset, you'll have to locate the pipe or trap and defrost it by wrapping a hot water bottle or heat wrap around a frozen part you can easily reach.
Never pour hot water directly on the pipe or trap or use fire, and don't try to thaw a pipe that's above ground level without professional help. You may injure yourself or damage the pipe. If your pipe or trap is frozen near an outside connection, you might not be able to get to it.
Boilers shouldn't need frequent line bleeding. If you're releasing air from the system often, have a professional check your boiler.