Heating
Why Do My Radiators Clang and Gurgle When the Heat Comes On for the Season?
That noise almost always means trapped air in the radiator, and bleeding it out is a five-minute job most homeowners can do themselves. Here's how to get your heating system ready before the first cold snap.
That clanging, gurgling, or knocking when your radiators first fire up for the season almost always means air is trapped inside them, and the fix is bleeding the radiator: a five-minute job with a small key and a rag that most homeowners can do themselves. It's one of the most common calls GetFixHero sees every October, and it's rarely the emergency it sounds like.
Why does trapped air cause so much noise?
A hot water radiator or boiler system works by circulating heated water through pipes and radiators. Over the summer, while the system sits idle, small amounts of air work their way into the lines. When the heat kicks on in fall, that air gets pushed around with the water and gets trapped in pockets inside the radiator, usually near the top. Water hammering against that air pocket is what makes the banging and gurgling sound, and it also means less of the radiator is actually filling with hot water, so that room heats slower than it should.
How do I bleed a radiator myself?
Turn the heat off first and let the system cool so you're not working with hot water under pressure. Most radiators have a small bleed valve near the top, usually a square or slotted fitting you turn with a radiator key (sold cheaply at any hardware store) or sometimes a flathead screwdriver. Hold a rag or small cup underneath, turn the valve slowly counter-clockwise, and you'll hear a hiss as air escapes. Keep it open until the hiss stops and a steady trickle of water comes out, then close it snugly. Work through each radiator in the house the same way, starting with the lowest floor and working up. Afterward, check your boiler's pressure gauge: bleeding radiators lets a little water out of the system, and most residential boilers want to sit around 1 to 1.5 bar (check the sticker on your unit for the exact range). If it's reading low, there's usually a filling loop valve to top it back up.
What boiler pressure should I be seeing before winter?
Before you bleed anything, it's worth glancing at the pressure gauge on your boiler. If it's already below the manufacturer's minimum, your system may be losing water somewhere, which bleeding alone won't fix. A pressure that creeps back down within a day or two of topping it up points to a leak in the system rather than just trapped air, and that's worth having a professional look at before the cold really sets in. A pressure that holds steady once you've topped it up and bled the radiators is a good sign you're set for the season.
When is this a job for a plumber instead of something I handle myself?
Bleeding a radiator and topping up pressure through a filling loop are both reasonable DIY tasks. Call a plumber if: the pressure won't hold no matter how many times you top it up, you see water pooling anywhere near the boiler or a radiator valve, a radiator stays cold at the bottom even after bleeding (that can mean sludge buildup inside, which sometimes needs a full system flush), or you're not comfortable with the filling loop and don't want to guess. A boiler service call or diagnostic in the Greater Toronto Area (GTA) generally runs in the same range as other main-system work, and catching a slow leak in October is a lot less disruptive than dealing with a cold house in January.
Why does this matter more in older Toronto homes?
Houses across neighbourhoods like Leaside, the Danforth, and parts of Etobicoke still run original or older-generation hot water radiator systems, and those systems tend to collect more trapped air and sediment over the decades than a newer setup. If your radiators have always been a little noisy every fall, that's a sign the system could use a proper flush rather than just an annual bleed. It's also worth doing this check before the heat is actually needed, since a cold October weekend is a much better time to find a problem than the first real cold snap of November.
Frequently asked questions
How often should I bleed my radiators? Once a year, at the start of heating season, is usually enough for most homes. If one radiator consistently needs bleeding more often than the others, that often points to a slow air leak somewhere in that line worth having looked at.
Is it normal for a little water to come out when I bleed a radiator? Yes, that's expected and it's a small amount. Just have a rag or small container ready, and don't force the valve past where it turns easily.
Can I just leave the noise alone if it's not bothering me? You can, but trapped air means that radiator isn't heating as efficiently as it could, so you're likely paying a bit more to heat that room. It also puts extra strain on the pump over time, so a five-minute fix now is cheap insurance.
A few minutes with a radiator key now beats a cold room in January. Request a plumber or message us on Facebook. Free for homeowners, care of GetFixHero.
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