Why Your Radiators Are Cold at the Top (And How to Bleed Them Properly)
If you’ve turned your heating on and noticed that your radiators are warm at the bottom but stone cold at the top, you have trapped air in your central heating system. We’ve created this handy guide on how to bleed a radiator and the likely causes / impact.
Leaving air in your radiators isn’t just an annoyance that leaves your rooms feeling chilly. Over time, it forces your boiler to work much harder to heat your home, driving up your gas bills and putting unnecessary strain on your system’s pump and components. In severe cases, trapped air can create an “airlock” that completely blocks the flow of hot water, stopping your heating dead in its tracks.
As Grimsby’s local Gas Safe experts, we at iWarm see this issue constantly. Fortunately, bleeding a radiator is a straightforward job you can do yourself. Here is how to do it safely.
Why Do Radiators Get Air in Them?
Air can enter your heating system in a few ways:
- New water entering the system: When you top up the pressure, fresh water carries dissolved air that eventually separates.
- Micro-leaks: Tiny drops in pressure can pull air in over time.
- Corrosion: If your system’s chemical inhibitor levels are low, the water reacts with the steel inside your radiators, creating a buildup of hydrogen gas. (If the water you bleed out is black or smells like rotten eggs, this is likely what’s happening).
- Blockages: If your central heating pipework becomes blocked with magnetite, the systems pump can apply too much fluid pressure and cause the central heating water to “pump over” into the feed & expansion tank.
The iWarm Step-by-Step Guide to Bleeding Radiators
What you need: A radiator bleed key (available at any hardware store) and an old cloth or rag.
Step 1: Turn your heating completely off. This is the step most people miss. Never bleed a radiator with the heating on. If the system pump is running, it can actually drag more air into the system or move the air bubble somewhere worse. Wait for the radiators to cool down so you don’t scald yourself.
Step 2: Start with the right radiator. If you live in a two-story house, start with the radiators on the ground floor, furthest away from the boiler, and work your way up. Air naturally rises to the highest point in your system.
Step 3: Open the bleed valve. Locate the bleed valve (usually a small square peg at the top corner of the radiator). Wrap your cloth around it to catch any drips. Insert the key and turn it slowly anti-clockwise—usually, a quarter to a half-turn is enough.
Step 4: Listen for the hiss. You will hear a hissing sound as the trapped air escapes. Keep the key in place and wait. As soon as the hissing stops and water begins to trickle out, immediately turn the key clockwise to close the valve tightly. Wipe up any drips.
Step 5: Check your boiler pressure. Because you have let air out of a sealed system, your boiler pressure will have dropped. Go to your boiler and check the pressure gauge. It should typically sit between 1 and 1.5 bar when cold. If it has dropped below 1, you will need to use your filling loop to top the system back up.
When to Call a Professional
Bleeding your radiators should be an occasional maintenance task, usually done once a year before winter. However, if you find yourself facing any of the following, there is a deeper fault in your system:
- You have to bleed them constantly: This indicates a leak, a failing expansion vessel, or severe internal corrosion.
- The water won’t stop running black: Your system is full of sludge and desperately needs a chemical flush and fresh inhibitor to protect your boiler’s heat exchanger.
- The pressure keeps dropping to zero: You have a leak somewhere on the circuit.
If you are dealing with persistent heating issues, don’t let a small fault turn into a costly boiler repair. If you’re based in Grimsby, Cleethorpes, or the wider North East Lincolnshire area, get in touch with iWarm today to book a system health check or service.
