How often should you bleed radiators?
Updated July 2026
Bleed your radiators once a year, right at the start of the heating season, and again any time a radiator is warm at the bottom but cold across the top. That cold-top pattern is the telltale sign of trapped air, which rises to the highest point of the radiator and blocks hot water from filling it. Bleeding is a two-minute job with a small radiator key and a cloth: you crack the valve at the top corner, let the hissing air escape, and close it the moment water dribbles out.
What changes the interval
| Your situation | Interval | Why |
|---|---|---|
| Start of heating season | Bleed every radiator once | Air collects over the idle summer months |
| Cold at the top, warm at the bottom | Bleed that radiator now | The classic sign of an air pocket at the top |
| Gurgling or banging when heat comes on | Bleed soon | Air moving through the system makes those noises |
| Just topped up boiler pressure | Bleed, then recheck pressure | Adding water can introduce air that needs releasing |
| Upstairs radiators cold, downstairs fine | Bleed the upstairs ones first | Air rises to the highest radiators in the house |
| Radiators bleed brown or dirty water | Bleed, then consider a power flush | Sludge and corrosion may be building inside |
The cost of skipping it
What skipping it costs: trapped air leaves cold spots, so rooms never quite warm up and the boiler runs longer chasing a temperature it cannot reach, quietly padding your heating bill. Worse, the air pocket accelerates internal rust, and the resulting sludge can seize a $300-600 circulator pump or block pipes, eventually forcing a power flush at $400-1,000. Bleeding costs nothing but a $5 radiator key. If radiators keep needing bleeding every few weeks, that points to a leak or a failing part worth investigating; the radiator maintenance checklist covers the rest of the routine.
How to do it (2 minutes per radiator · DIY with a $5 key)
- Turn the heating on, let the radiators get hot, then switch the system off and let them cool so the water settles.
- Hold a cloth under the bleed valve at the top corner and fit the radiator key onto the square nipple.
- Turn the key slowly counterclockwise, a quarter to half turn, until you hear air hissing out.
- When steady water dribbles out instead of air, close the valve snugly, but do not overtighten it.
- Work from the lowest radiator to the highest, then recheck boiler pressure and top it up if it dropped.
Knowing the interval is the easy half. OnOtto reminds you at the right interval, and it won't let you snooze it into next year. Tasks persist until they're done, and completing radiator bleeding schedules the next one automatically.
Put this on autopilot, free for 30 daysFrequently asked questions
How do I know if a radiator needs bleeding?
The clearest sign is a radiator that is warm or hot at the bottom but stays cold across the top, which means an air pocket is sitting where hot water should be. Gurgling, banging, or clicking when the heating fires up points the same way. If a radiator is cold all over instead, the cause is more likely a stuck valve, a blockage, or sludge, not air. The radiator not heating up guide sorts out which is which.
Do I need to turn the heating off before bleeding radiators?
Yes, turn the system off and let the radiators cool first. Bleeding a hot, pressurized radiator can spray scalding water, and with the pump running you may draw in more air rather than release it. Cool radiators let the trapped air rise cleanly to the valve. After you finish, turn the heating back on and check that the once-cold tops now warm up evenly. Then confirm the boiler pressure did not drop too low.
Why does my boiler pressure drop after bleeding radiators?
Because bleeding releases air and a little water from a sealed system, which lowers the pressure. On most combi and system boilers the gauge should read about 1.0-1.5 bar when cold; if bleeding drops it below 1 bar, top it up using the filling loop until it sits back in the normal band. It is normal to bleed, then repressurize. If the pressure keeps falling on its own between bleeds, you likely have a small leak worth investigating.
How often is too often to bleed radiators?
Once a year is normal. If a radiator needs bleeding every few weeks, something is feeding air into the system: a small leak drawing air in, a faulty automatic air vent, a failing pump, or corrosion generating gas inside the radiators. Frequent bleeding of brown, smelly water in particular signals sludge and internal rust. Rather than bleeding endlessly, have the system checked, and weigh whether an old, corroded radiator is worth a repair or replacement.