Bleeding Radiators & In-Floor Heat: The Basics

Why air gets into the system

Hot-water heating is a sealed loop, but it is never perfectly sealed. Fresh water added to the system carries dissolved air, tiny amounts sneak in at fittings, and heating the water drives the air out of solution. The bubbles rise and collect at the high points, which are usually the tops of radiators or the upper runs of piping. An air pocket blocks water flow, and where water cannot flow, heat cannot arrive.

The telltale symptoms

A radiator that is hot at the bottom and cold at the top has air sitting in its upper chamber. Gurgling or trickling sounds in the pipes are air moving with the water. A whole zone that stays cold while others heat may be air-locked at a high point in that loop. These symptoms are worth recognizing because they look like a broken system but often need only a simple bleed.

How bleeding a radiator works

Each radiator has a small bleed valve at the top, opened with a bleed key or flathead screwdriver. With the system warm and circulating, the valve is cracked open until air hisses out, then closed the moment water appears. Keep a rag handy and work top floor first on multi-story homes. Afterward, the system pressure should be checked at the boiler gauge, because the water that replaced the air had to come from somewhere and pressure may need topping up.

In-floor radiant is a different animal

Radiant floor tubing has no bleed valves along its loops; air is removed at the manifold by purging each loop with fresh water under pressure until it runs bubble-free. That usually takes a purge cart or a hose-and-valve procedure at the manifold, and doing it wrong can push more air in than it takes out. Radiant is also slow by design, so a floor can take hours to respond after a fix, which fools people into thinking the repair failed. Patience and a manifold thermometer tell the real story.

When air keeps coming back

Bleeding once a season is normal on older systems, but needing it every couple of weeks means air is being made or admitted somewhere. Suspects include a waterlogged expansion tank, a failing automatic air vent, a pinhole leak drawing in air, or a fill valve constantly adding fresh oxygenated water. Chronic air also accelerates internal rust, so it is worth solving rather than living with. That diagnosis is a job for someone with hydronic experience, not a bigger bleed key.