The heat kicks on for the first cold night, and the house starts making noises. Something knocks inside the wall like someone's at the door. The radiator under the window hisses at you. If you live in one of the older homes in Montclair, Bloomfield or West Orange, you've probably lived with radiator banging. It's Halloween week, but we promise it isn't a ghost.
Usually, it comes down to something ordinary: water sitting where steam should be, air trapped where water should be, pressure that's set wrong, or pipes expanding as they warm up. Most of it has a simple fix, and a few things are worth a phone call.
First, figure out which kind of heat you have
The same noise can mean different things depending on your system, so start in the basement.
Steam heat boils water and sends the steam up to the radiators. Hot water heat uses a pump to push heated water through radiators or baseboard and back to the boiler.
The surest way to tell is at the boiler. A small glass tube on the front with water in it is called a sight glass, and it means you have steam. No sight glass, but a round gauge that shows pressure and temperature? That's hot water. On the radiators themselves, a little bullet-shaped vent on the side means one-pipe steam. Some steam radiators have two pipes and no side vent, though, so go by the boiler.
Why steam radiators bang
In a steam system, steam rises to the radiators, gives off its heat and turns back into water. That water has to drain back down to the boiler. When it gets stuck in a pipe or a radiator, the next rush of steam slams into it and throws it against the next fitting. That's the bang. In the trade it's called water hammer, and it usually traces back to one of these:
- A radiator valve that's partly open. On a one-pipe system, the valve at the bottom of the radiator has to be fully open or fully closed. Anywhere in between leaves steam coming in and water trying to get out through the same small opening, and it bangs.
- A radiator that tilts the wrong way. A one-pipe radiator should lean slightly back toward its valve so the water can run out. If it leans toward the vent end, water pools inside. Set a level on top and you'll know. Two-pipe steam radiators are the reverse. They should lean toward the outlet end, away from the supply valve.
- Basement pipes that have sagged. The main steam pipes need a steady slope so the water can drain. Where the water drains the same way the steam flows, that slope should be one inch for every 20 feet. When a pipe sags on its hangers, it holds a puddle, and the steam finds it.
- Too much water in the boiler. If the boiler was overfilled, by hand or because a feed valve was left open, water can get pushed up into the pipes. A sight glass that's full to the top is a strong hint.
- Pressure set too high. Home steam heat is meant to run on very little pressure. Most one-pipe systems need no more than about a pound and a half at the boiler, and many smaller ones get by on a few ounces. Run it higher and you waste fuel, and the extra pressure can back water up into the pipes, where it bangs. The gauge on most steam boilers barely moves at pressures that low, so leave this one to us.
- A boiler that surges. If the water in the sight glass bounces around wildly while the boiler runs, the boiler may need cleaning, or it may be firing too hard or piped wrong.
- A failed steam trap. On a two-pipe system, each radiator has a trap at its outlet. When one sticks open, steam gets into the return pipes and bangs there.
You can check the first two yourself, once the radiator is cool. Turn a partly open one-pipe valve all the way open or all the way closed, but don't force one that won't turn. If the radiator leans toward its vent, lift the vent end a little with a flat pry bar and slide thin shims under the feet. Stop if the pipe at the valve starts to move. Everything else on that list needs someone who knows steam, so call us.
Hissing and spitting from a steam radiator
A short hiss when the heat comes on is normal. That's the vent letting air out as steam fills the radiator. Once the steam reaches the vent, the vent should close and go quiet.
If it hisses the whole time the radiator is hot, the vent isn't closing, and you're losing steam into the room. Spitting water can mean a worn vent, but often it means water is building up inside the radiator, which points back to the tilt or the valve. Swapping a worn vent is a small job, done with the heat off and the radiator cool. If the new one spits too, the problem is the radiator or the system, not the vent.
Hot water heat: gurgling and cold spots
On hot water heat, gurgling or knocking usually means air is trapped in the pipes or radiators, or the system pressure is too low to push water up to the top floor. Trapped air also shows up as a radiator that's warm at the bottom and cold at the top.
The fix for trapped air is called bleeding the radiator. With the heat off and the radiator cool, hold a rag under the small valve near the top, open it a little with a radiator key, and let the air hiss out. Close it as soon as water starts to come out. Do the same on any radiator that runs cold at the top.
Bleeding lets out a little water, so check the boiler gauge before you start and again after. If the pressure keeps dropping, or you're bleeding the same radiators over and over, there may be a leak or a problem with how the system refills. At that point, give us a call.
A rumble from a hot water boiler, like a kettle about to boil, often means scale or sludge has built up inside it, or water isn't moving through it fast enough. That's a job for a technician.
When to call right away
Most radiator noise is annoying, not dangerous. These are the exceptions:
- You smell gas (rotten eggs or sulfur). Don't touch any switches, flip a breaker or use your phone inside the house. Get everyone out, then call your gas utility and 911 from outside.
- Your carbon monoxide alarm goes off. Get everyone out and call 911 from outside. Don't go back in until the fire department says it's safe.
- Water or steam is pouring out of the boiler's relief valve. That valve opens when the pressure gets too high. Stay clear of the pipe coming off the valve, because the water is scalding, and never cap or plug it. Turn the boiler off by closing the gas or oil shutoff valve on the pipe feeding it, and leave the power on so the pump on a hot water system keeps running. A slow drip is less urgent, but get it checked soon.
- The sight glass on a hot steam boiler is empty. Don't add water. Cold water in a hot, dry boiler can crack it. On a gas boiler, close the gas shutoff valve on the pipe feeding the boiler, then use the emergency switch. On an oil boiler, use the emergency switch. Leave it off and let it cool until a technician has looked at it.
- The banging gets louder every week, or water is leaking from the pipes or the radiator valves.
Once everyone is safe, we'll come out and find the cause. Our service call or diagnostic fee runs roughly $75 to $150, and it's usually credited toward the repair if you go ahead with the work. We offer same-day or next-day service Monday through Friday, 8:30 to 5.
Frequently Asked Questions
Is it normal for radiators to bang when the heat first comes on?
A little ticking is normal as cold pipes warm up and expand. Loud banging on every cycle isn't. On steam heat it often means water is stuck somewhere it shouldn't be, and one of the steam checks above may fix it. On hot water heat, trapped air is the usual cause.
Should I turn the valve down on a steam radiator to make a room cooler?
Not on a one-pipe steam system. That valve should be fully open or fully closed, and leaving it partway open causes banging. If one room runs too hot, a thermostatic radiator valve, which fits between the radiator and its air vent, is the better way to cool it down.
How do I bleed a hot water radiator?
With the heat off and the radiator cool, hold a rag under the bleed valve, open it with a radiator key, and close it once water comes out. Then check the boiler gauge. Only hot water radiators get bled. Steam radiators vent on their own, so never open or remove a steam vent while the heat is on.
Do banging radiators mean I need a new boiler?
Usually not. Most banging comes from the radiators and pipes, and those fixes cost far less than a boiler. If the banging keeps coming back after service, or your boiler is over 20 years old and needs frequent repairs, we'll tell you honestly whether repairing or replacing it makes more sense.
Getting Your Radiators Quiet This Season
Radiator banging in an older North Jersey house almost always has a cause you can find. On one-pipe steam, make sure no radiator valve is left partway open and put a level on the ones that bang. On hot water, bleed the radiators that run cold at the top. If the noise is still there after that, it's coming from the piping or the boiler, and that's a job for us.
Air Temp Service Co has served North Jersey homeowners since 1999, family-owned by Mario and Lori Riccobono. Both NJ licenses are on file (HIC 13VH00127600 and Master HVACR 19HC00254600), and we hold Mitsubishi Diamond Elite and Trane Premier Contractor credentials. We repair and replace boilers across Passaic, Bergen, Essex, and Morris counties, with upfront pricing and rebate paperwork handled for you.
Call (973) 754-1232 to book a visit. We'll find what's making the noise and quote the fix before any work starts.