Air trapped in radiators (needing a bleed)

A radiator heats when hot water fills it and rises through the panels, giving up heat to the room as it travels. Air is lighter than water, so it collects at the top of the radiator, and once enough gathers there the hot water can no longer reach the upper panels. The result is a radiator that is warm at the bottom and cold at the top, with the cold area growing as the pocket of air grows.

Trapped air also makes itself heard: gurgling and trickling sounds as water climbs past the pocket. Air can enter a sealed system through the fill side, through tiny leaks that draw air in when the system cools, after any repair that opened the loop, and slowly over the years as dissolved gas comes out of the water. A little air is normal; a radiator that needs bleeding every month is not.

Feel each radiator from bottom to top and note which are cold at the top and which gurgle. If a radiator needs bleeding, do it only with the system cold: the boiler off and the radiator cool enough to hold, because bleeding a hot, pressurised system can spray scalding water.

Turn the bleed key slowly on the small valve at the radiator's top corner, holding a cloth underneath, and let the air hiss out until a steady stream of water appears, then close the valve immediately. Bleed only radiator systems, never in-floor loops, which have their own purging procedures, and check the boiler's cold pressure reading afterwards, because bleeding lowers system pressure and a very low reading means the loop needs topping up by a technician.

Air keeps returning when the system has an entry point: a tiny leak that draws air in as the water cools and contracts, a fill assembly that does not seal properly, or corrosion inside older steel radiators and pipes that generates gas. Systems that were opened for repairs and refilled without a full purge carry air into the loops, and in-floor circuits hold air stubbornly because their long loops hide their high points.

Location matters as much as the source. Radiators at the highest point of the house, or at the end of long runs, are the first to trap air because that is where air naturally collects, and a system without an effective air separator can recirculate the same air instead of removing it. The pattern of which radiators need bleeding most often is itself a clue to where the air is entering.

Call when a radiator needs bleeding more than once or twice a season, when bleeding does not restore the heat, or when the boiler loses pressure along with the air problem, since air and water loss often share one cause. Persistent air can also be the sign of a leak that only draws air in while the system is cold.

If you are not comfortable bleeding a radiator, or a bleed valve will not turn, stop rather than forcing it; the valves are small and break easily. Call ROMA Heating & Cooling at 778-909-5731 during business hours and describe how often the radiators need bleeding and whether the boiler pressure drops as well.

A technician purges the system properly: each zone is filled and vented at its bleed points until only water flows, the high points are checked, and the boiler's pressure is set to the cold figure in its manual. Where air keeps returning, the search turns to the leak or the air-separation problem that is feeding it.

If corrosion is part of the story, the conversation moves to water treatment and to the age of the radiators and piping, because repeated fresh water in an old system feeds the corrosion that makes gas. The technician confirms at the end of the visit that the radiators heat evenly and that the pressure holds, so the cure is verified rather than assumed.