| Getting your Trinity Audio player ready... |
Last Revised: 2/19/26
If your radiator is hot at the bottom but cold at the top, you most likely have trapped air in the system. The fix is a process called “bleeding” the radiator, and it’s something most homeowners can do themselves in about 15 minutes with a couple of basic tools.
At Mattioni, we’ve been helping homeowners maintain their heating systems since 1948. And we’re here to help you, too. In this article, we’ll cover what causes the problem, the symptoms to look for, and a clear step-by-step guide to bleeding your radiators, so they heat evenly again.
Let’s start by looking at the issue first.
Why Is Your Radiator Hot at the Bottom but Cold at the Top?
Hydronic (water-based) heating systems work by circulating hot water through radiators or baseboard radiant heaters. When everything is running properly, that water fills the entire radiator and heats it evenly. But over time, air works its way into the system. This happens naturally when gases escape from the water as it heats and cools. Small leaks can also introduce air, and routine maintenance can let some in, too.
Air is lighter than water, so it rises to the top of the radiator and gets trapped there. That air pocket pushes the hot water down, creating the pattern you’re feeling: warm at the bottom, cold at the top. The air acts as a barrier that prevents hot water from reaching the upper portion of the radiator.
It’s not dangerous, and it doesn’t mean your boiler is failing. But it does mean your radiator isn’t heating your room efficiently, and if left alone it’ll get worse over time.
Signs Your Radiators Need Bleeding
The cold-at-the-top pattern is the most obvious sign, but it’s not the only one. Here are a few other indicators that trapped air is the culprit:
- Gurgling or bubbling noises
- Radiators are taking longer to heat up
- One radiator is not working while the rest are fine
- Uneven cold spots across the radiator surface
What is “Bleeding” a Radiator?
Bleeding a radiator means releasing the trapped air so hot water can fill the entire unit again. Every radiator has a small bleed valve (usually at a top corner) that you open slightly to let the air escape.
Once water takes that air’s place, the radiator heats evenly again. It’s a simple process that doesn’t require draining the system or calling a technician.
How to Bleed a Radiator: Step by Step
Here’s how to bleed your radiator from start to finish.
Turn on the Heating and Identify the Problem Radiators
Let your heat run for 15 to 20 minutes first. Then carefully feel each radiator in your home and make a note of any that have uneven cold patches. Any radiator that feels evenly warm is fine, skip it and focus on ones that aren’t heating properly.
Turn Off the Heating and Let the System Cool
Once you’ve identified which radiators need bleeding, turn off your heat completely to prevent hot water from spraying when you open the bleed valve. Give the radiators 10 to 15 minutes to cool down before you start.
Gather a Few Supplies
You’ll need a radiator bleed key (available at any hardware store at a low cost) or a flat-head screwdriver if your radiator has a slotted valve. Grab an old towel or rag and a small container to catch drips. The process shouldn’t take anything more than these few items.
Locate the Bleed Valve
The bleed valve is a small square-shaped fitting at the top corner of the radiator. In older radiators, it might be tucked behind layers of paint, so check both top corners carefully.
Open the Valve and Release the Air
Place your towel and container beneath the valve. Insert the radiator key (or screwdriver) and slowly turn it counterclockwise about a quarter turn. You’ll hear a hissing sound as trapped air escapes.
Keep the valve open until the hissing stops and a steady trickle of water appears. For most radiators this takes just a few seconds, but some larger units can take closer to a minute.
Close the Valve
As soon as water flows steadily, turn the key clockwise to close the valve. Make it snug enough to stop drips, but don’t overtighten it. Wipe away any water and move to the next radiator using the same process.
Check Your Boiler Pressure
After bleeding all the radiators, check the pressure gauge on each one to ensure it matches what the system calls for. Most radiators should read between 12 and 18 PSI when cold. If it’s dropped below that range, you can refill it with water using the filling loop valve on your boiler.
Turn the Heating Back On and Test
Turn your system back on at the thermostat and give it about 20 minutes to reach the set temperature. Check all the radiators that you bled, they should feel evenly warm from top to bottom. If any still have cold spots, bleed them one more time, as it can sometimes take a second pass to get all the air out.
If you still feel cold spots, just repeat this simple process where needed. And if your radiators feel evenly warm again, you’re all set.
How Often Should You Bleed Your Radiators?
Bleeding your radiators at least once a year before the heating season starts will set your home up for efficient comfort. If there are any lingering issues that require a professional, calling a technician out before peak heating season will give you the chance to catch them before you need the heat daily.
And if you notice symptoms mid-winter, don’t wait to take action. The longer air sits in the system, the harder your boiler works, and the less evenly your home heats.
When to Call a Professional
Bleeding your radiators solves the problem most of the time. But there are a few situations that call for professional help:
- You’re bleeding your radiators frequently. If you’re doing this every few months, air is entering the system faster than it should. This could point to a small leak, a faulty valve, or an issue with the expansion tank that a technician can diagnose.
- The radiator is still cold after bleeding. If bleeding doesn’t fix it, the problem may be sludge buildup rather than air. This is common in older systems and usually requires a professional to clear out the sediment.
- Your boiler pressure keeps dropping. If the pressure won’t hold even after you top it up, there’s likely a leak somewhere in the system that needs professional attention.
Need Help With Your Heating System?
Bleeding your radiators is often a DIY fix that makes a real difference in your comfort and energy bills. But if the problem keeps coming back, or if something about your system doesn’t seem right, Mattioni is here to help.
We’ve been taking care of Greater Philadelphia’s plumbing, heating, and cooling systems since 1948, and we’ll always give you straight answers and honest recommendations.
Schedule your appointment today and let one of our technicians take a look at your heating system. Or give us a call at (610) 400-8510 to take the first step in regaining control of your boiler.
