How to Bleed a Radiator

A radiator that is hot along the bottom and cold across the top has air trapped in it, and bleeding that air out through the valve at the top corner is a genuine homeowner job. It takes a radiator key, a towel and about five minutes per radiator. The one thing to get right before you start is what kind of system you have, because the same procedure on a steam radiator accomplishes nothing and can make a mess. If the radiator is cold for some other reason, that is boiler repair territory rather than a bleeding problem.

Is it a hot-water radiator or a steam radiator?

Count the pipes at the floor. A hot-water radiator has two pipes — supply and return — and a small bleed valve at the top of one end, usually with a square stem that takes a radiator key or a slot that takes a screwdriver. A one-pipe steam radiator has a single pipe at one end and a bullet-shaped or cylindrical air vent screwed into the side, roughly a third of the way down. That vent is the steam system's own air-removal device, and it opens and closes on its own as the radiator heats.

This matters in the older housing on the Duluth hillside and in the original neighborhoods of Superior, where both kinds of system were installed and some houses have been converted from one to the other. If you find two pipes and an automatic air vent, or a vent that looks like a steam vent on a two-pipe radiator, stop and have it identified before touching anything.

Telling a steam radiator from a hot-water radiator

What you see	Steam	Hot water
Pipes at the radiator	One pipe, larger diameter	Two pipes, smaller diameter
Air device	Vent on the side of the radiator	Small bleed valve at the top corner
At the boiler	A sight glass showing a water level	A pressure gauge and circulator pumps
Sound when heating	Hissing at the side vent, some knocking	Quiet, sometimes trickling

What you need before you start

Bleeding a hot-water radiator needs almost nothing, but improvising the key is where people get into trouble. Gather these first:

• A radiator key that fits your valve stem — they are not all the same size, and pliers will round off a soft brass stem.
• An old towel and a small cup or dish to catch water.
• A flashlight, because most bleed valves are at the back corner against a wall.
• Knowledge of where the boiler's pressure gauge is, so you can look at it afterward.

The steps to bleed a hot-water radiator

Bleed a hot-water radiator by warming the system, shutting the circulator off, then opening the top valve just far enough to release air. Work through every radiator in the house, starting with the ones on the lowest floor and finishing at the highest — air collects at the top of the system, so the top-floor radiators are usually the worst.

1. Run the heat for fifteen or twenty minutes so the system is hot and the air has moved into the radiators, then set the thermostat to Off so the circulator stops. Bleeding while the pump is running gives inconsistent results.
2. Let everything sit for several minutes. The water calms down, the air rises to the top of each radiator, and the metal cools enough to work near.
3. Put the towel under the bleed valve and hold the cup beneath it.
4. Fit the key on the valve stem and turn it counterclockwise a quarter to a half turn — no further. You should hear a steady hiss of escaping air.
5. Wait until the hiss stops and a steady dribble of water comes out with no spitting, then close the valve snugly. Do not crank it; the seat is soft brass.
6. Repeat at every radiator, lowest floor first, and dry each valve so you can see later whether it is weeping.
7. Turn the heat back on, let the system come up, then look at the boiler's pressure gauge once the house is warm and again when it has cooled.

If nothing comes out at all — no hiss, no water — the radiator is not full of air. Either the valve is plugged with mineral scale or the radiator is not getting water at all, which is a different problem covered further down.

What to check on the boiler afterward

Every bit of air you let out of the radiators is replaced by water, so system pressure falls, and the boiler's gauge is where you see that. Look at it once the system has cooled down. If the needle is sitting noticeably lower than where it normally sits cold, the system needs make-up water — and that is the point where we would rather do it than talk you through it, because an overfilled hydronic system pushes water out of the relief valve and an underfilled one will not lift water to the top floor of a tall house.

Two related things to look at while you are at the boiler. On a condensing boiler that vents out a sidewall, check the outside termination is clear; the same drifting and freezing drizzle that blocks a furnace vent affects a boiler, and what to do about snow or ice blocking a vent applies equally. And because a boiler is a combustion appliance, confirm you have a working CO alarm on every level, which carbon monoxide alarms and your heating system covers.

Why does the same radiator keep needing to be bled?

A hydronic system that needs bleeding more than once or twice a season is taking in air somewhere, and finding that source is the actual repair. Bleeding it again just resets the clock. The usual causes, in the order we find them:

• A waterlogged expansion tank. The tank's air charge is gone or the bladder has failed, so the system cannot absorb expansion and pressure swings wildly between hot and cold.
• A failed automatic air vent on the boiler or at a high point, which should be removing air continuously and is instead leaking it in or plugged shut.
• A slow leak somewhere in the loop. Every gallon lost is replaced by fresh water through the fill valve, and fresh water carries dissolved air and minerals with it.
• A fill valve left open, or set wrong, continuously feeding the system.
• Piping run above the boiler's pressure ability in a tall house — common in three-story houses on the Duluth hillside — where the top floor simply never fills properly.

That last group is why a house in Mahtowa or anywhere with an older gravity-converted system can behave oddly. Many of these were designed for gravity circulation, then had a pump added; the piping is enormous, slow to purge, and holds air in places a modern system would not.

The radiator is still cold after bleeding — now what?

A radiator that stays cold after successful bleeding has a flow problem, not an air problem. Check the obvious first: the valve at the radiator inlet may be closed or only cracked open, and on a two-pipe system the return may have a lockshield valve that has been turned down. Beyond that, the causes need tools — a circulator that is running but not moving water, a zone valve that is not opening, a plugged balancing valve, or sludge in the bottom of an old cast-iron radiator.

In rural service, where a single zone often has to feed a long run out to an addition, we also find loops that were never balanced after a remodel. Houses around Solon Springs and Independence frequently have additions piped onto an original loop, and the added length starves the far end.

Where the homeowner job stops

Bleeding radiators, opening a radiator valve and reading the pressure gauge are the safe end of hydronic work. Stop and call when the bleed valve weeps after you close it, when the relief valve on the boiler discharges, when the gauge climbs well above where it normally sits hot, when you smell gas, or when you have bled everything and the top floor is still cold. Adding water to a boiler, purging a zone with a hose, charging an expansion tank and replacing air vents all involve the pressurised side of the system, and getting them wrong is how a small annoyance turns into a flooded basement. Booking boiler repair and diagnosis for a system that keeps making air is cheaper than replacing a leaking cast-iron section, and if you have no heat at all, the no-heat emergency steps come first.