Why Is My Radiator Making Noise? Banging, Hissing and Gurgling
The first cold week wakes up every radiator in the house, and some of them complain. The noise tells you what is wrong, once you know whether your system runs on steam or hot water.
Quick answer
A steam radiator that bangs as it heats up has water trapped inside because it has tilted the wrong way; shim it so it slopes slightly back toward its pipe. A steady hiss means a steam air vent is stuck open. In a hot-water system, gurgling means trapped air: bleed each radiator at the start of the heating season, top floor first.
- Difficulty
- Moderate DIY
- Banging (steam)
- Trapped water; re-pitch the radiator
- Constant hissing (steam)
- Air vent stuck open; replace it
- Gurgling (hot water)
- Trapped air; bleed the radiator
- Technician job
- Steam traps, boiler, leaking valves
On this page
Radiators are fed by a boiler, and the boiler sends them one of two things. A steam boiler boils water and lets the steam travel through the pipes on its own. A hot-water boiler heats water and a pump circulates it. The two systems make different noises for different reasons, and a fix for one is wrong for the other.
So the first step is to find out which you have.
Radiator not heating? Where it is cold tells you why
| What you notice | System | Likely cause | Who handles it |
|---|---|---|---|
| One radiator stays cold while the others heat | One-pipe steam | Clogged air vent: air cannot leave, so steam cannot enter | You can replace the vent once the radiator is cold; otherwise a technician |
| A radiator hisses or spits and the room feels damp | One-pipe steam | Air vent stuck open | Replace the vent when cold |
| Some radiators too hot, others too cold | Two-pipe steam | Old steam traps stuck open or closed | Technician: the Department of Energy suggests replacing all the traps |
| A radiator is cool at the top and gurgles | Hot water | Air trapped in the radiator | You: bleed it (below) |
| It fills with air again soon after bleeding | Hot water | A fault the bleed valve cannot fix | Technician |
| Every radiator in the house is cold | Either | Not a single-radiator problem: start with the controls | Check the thermostat first: see thermostat not working. Then a technician |
The steam and hot-water rows come from the Department of Energy’s Energy Saver page on heat distribution (archived copy). Find out which system you have first, from the section below.
Steam or hot water?
| Look for | Steam | Hot water |
|---|---|---|
| Pipes at each radiator | One pipe, or two | Two |
| On the side of the radiator | A small automatic air vent (one-pipe systems) | A small bleed valve |
| Typical radiator | Upright cast iron | Baseboard units, or upright radiators that resemble steam ones |
| How heat arrives | With a noticeable delay after the boiler starts | Gradually, as the water warms |
If a radiator has a single pipe and an air vent, it is one-pipe steam.
Match the noise to the cause
| Noise | System | What is happening | Fix |
|---|---|---|---|
| Banging or hammering as it heats up | Steam | Water has not drained out of the radiator, and incoming steam hits it | Re-pitch the radiator with shims |
| Short hiss as it heats, then silence | One-pipe steam | The air vent is letting air out, then closing | Nothing; this is normal |
| Hissing that does not stop, or steam and water from the vent | One-pipe steam | Air vent stuck open | Replace the air vent |
| Gurgling or trickling | Hot water | Air trapped in the radiator | Bleed the radiator |
| Ticking or creaking as it warms and cools | Either | Metal expanding and contracting | Usually nothing |
| Some radiators too hot, others too cold | Two-pipe steam | Steam traps stuck open or closed | Technician: replace the traps |
Steam: banging when the heat comes on
In a one-pipe steam system, the same pipe that brings steam in also carries the condensed water back out. That only works if the radiator slopes slightly toward the pipe, so the water runs out by gravity.
The Department of Energy explains how that goes wrong. A steam radiator can warp the floor under it, and years of expanding and contracting can dig ruts into the floor. Either one tilts the radiator the wrong way, so water stays inside when it cools. When steam arrives, it throws that water against the metal, and you hear banging.
- Check the slope.Safe check
With the heat off and the radiator cold, put a level on top of it. It should slope slightly down toward the supply pipe on a one-pipe system, or toward the steam trap, at the outlet end, on a two-pipe system.
- Shim the far end.Take care
The Department of Energy's fix is to insert shims under the radiator to pitch it slightly toward the pipe. Cast-iron radiators are very heavy: lift only the feet at the end away from the pipe, only a little, with a pry bar and a helper, and slide shims under those feet. Do not strain the pipe connection.
- Listen at the next heating cycle.Safe check
If the banging continues, or it comes from the pipes in the walls or basement instead of one radiator, the cause is elsewhere in the system and needs a steam heating technician.
Steam: hissing, or a radiator that stays cold
Each radiator on a one-pipe system has an automatic air vent. It lets air out as steam fills the radiator and shuts when steam reaches it. That is the brief hiss you hear at the start of a cycle.
The vent fails in two ways, according to the Department of Energy:
- Clogged. Air cannot leave, so steam cannot enter, and the radiator stays cold.
- Stuck open. Steam keeps escaping into the room, which raises indoor humidity and wastes fuel. If your windows are suddenly streaming with water, this is one possible reason; see condensation on windows.
Vents can sometimes be cleaned by boiling them in water and vinegar, but the Department of Energy notes that they usually need to be replaced. Do that only with the heat off and the radiator completely cold, or have a technician do it.
Steam burns. Never loosen a vent, valve or fitting on a steam radiator while the system is running or still hot. Keep children away from a vent that is spitting steam or water, turn the thermostat down, and deal with it once the radiator is cold.
Hot water: gurgling and radiators that do not heat fully
The Department of Energy calls unwanted air the most common problem in hot-water systems. Air collects in radiators, the water cannot fill them, and you hear it gurgle past the bubble.
The fix is bleeding, and the Department of Energy’s method is short:
- Do it at the start of the heating season, with the system running.Safe check
That is when a summer's worth of air has collected.
- Start on the top floor and work down.Safe check
Air rises, so the highest radiators hold the most.
- Open each bleed valve slightly.Take care
Hold a cup and a cloth under the valve and open it a little with a radiator key or screwdriver. Air hisses out first. The water that follows is hot.
- Close it when water starts to escape.Safe check
A steady dribble of water means the air is out. Close the valve and move to the next radiator.
If a radiator keeps filling with air after you bleed it, have a heating technician check the system.
Two-pipe steam: uneven heat
Two-pipe steam systems have a steam trap at the outlet of each radiator. The Department of Energy notes that older traps often stick open or closed, which unbalances the system: some radiators give too much heat and others too little. The usual remedy is to replace all the traps, which is technician work.
When to call a technician
- Banging that comes from the pipes, shakes the floor or started suddenly.
- Any leak at a radiator valve, a pipe joint or the boiler.
- A hot-water radiator that fills with air again soon after bleeding.
- Uneven heat on a two-pipe steam system.
- A boiler that has not been inspected this year. The Consumer Product Safety Commission recommends a professional inspection of all fuel-burning heating systems, boilers included, every year.
One more thing for unused rooms: the Department of Energy advises against shutting the heat off entirely in part of a house. Keep cooler areas at around 50°F so water pipes do not freeze. If a pipe does freeze, see how to thaw frozen pipes.
Frequently asked questions
Why is one radiator not heating when the others are?
On a one-pipe steam system, the Department of Energy says a clogged air vent keeps a radiator from heating up, because air cannot leave so steam cannot enter. On a hot-water system, the most common problem is air trapped in the radiator: bleed it with the system running, top floor first. If radiators on a two-pipe steam system run too hot and too cold, stuck steam traps are a likely cause.
Should I turn off the heat to rooms I do not use?
Not completely. The Department of Energy says cooler parts of the home should be kept around 50°F to prevent water pipes from freezing, and never to shut off heat entirely in an unused part of the home.
Is a banging radiator dangerous?
Banging means water is trapped where steam needs to pass. The radiator is not draining as designed, and that is worth fixing. Have a heating technician look at the system if banging is violent, comes from the pipes rather than one radiator, or started suddenly.
Should a steam radiator hiss?
A short hiss as the radiator heats is normal on a one-pipe steam system: the air vent lets air out as steam fills the radiator, then closes when steam reaches it. A vent that keeps hissing or spits steam and water is stuck open and should be replaced.
How often should I bleed hot-water radiators?
The Department of Energy recommends doing it at the start of each heating season, with the system running. In a multi-level home, start on the top floor and work down.
Can I bleed a steam radiator?
No. Bleeding is for hot-water radiators, which have a small bleed valve. A one-pipe steam radiator vents its air automatically through the air vent on its side. Never open or remove parts of a steam radiator while the system is hot.
Why is one radiator cold when the others are hot?
On one-pipe steam, a clogged air vent keeps steam from entering the radiator. On two-pipe steam, a steam trap stuck open or closed unbalances the system. On hot water, the usual cause is air trapped in that radiator.
Sources
- Heat Distribution Systems U.S. Department of Energy, Energy Saver (archived copy, 2023)
- Heat Distribution Systems U.S. Department of Energy, Energy Saver (archived copy, 2026)
- Furnaces and Boilers U.S. Department of Energy, Energy Saver (archived copy, 2025)
- Keep Warm and Safe This Winter: Tips for Using Generators, Furnaces and Space Heaters (January 23, 2026) U.S. Consumer Product Safety Commission
First published October 8, 2026. Last reviewed October 28, 2026. Spot an error? Tell us and we will check it against the source.