Boilers work by heating cold water inside your radiators. They are unvented, so over time, air will get into the radiators, become trapped, and that prevents the entire radiator from heating.
The end result is your boiler is firing energy to every radiator in your home, but not every heater rising to the temperatures you’re paying for. If you don’t periodically bleed your radiators, you’re burning money for nothing.
A tell-tale sign that a radiator needs bled is when there are cold spots toward the top of the heater. Cold spots on the bottom of a radiator are entirely different as that indicates a power flush is more likely needed to rid accumulated debris from the system. Cold spots at the top of the heater are indicative of trapped air, and that’s a job you can do yourself easily.
Best tip: Start from the ground floor and work your way up.
Why? Because air rises.
What You’ll Need
A Radiator Key
Some radiators can be bled with a flat-head screwdriver. It is preferable to use a key though because they have more control. Using a screwdriver is more finicky and you will be slower to close the valve if there’s a sudden rush of water when you open it.
An old towel to catch the water
There are radiator keys that are attached to water collectors. Those are handy for collecting the excess water that drips out your air valves. Even if you are using one of those, it’s safer to put an old rag over your floor and behind the air valve to prevent dirty water splashing on your paintwork or a light coloured carpet causing staining. The water inside radiators can be mucky. Not what you want splashing your painted plaster walls or staining wallpapers or carpets.
How to Bleed Your Radiators: Step-By-Step
1. Turn your boiler on
Two things happen when you fire your boiler up. It builds pressure in the system and lets you identify which heaters have cold spots, indicating they have trapped air preventing the radiator from reaching the temperatures it should.
2. Turn your heating off after 20 minutes
20 minutes is sufficient time for all your heaters to be up to a decent temperature. You never want to bleed your radiators when the central heating is on. The water can be scalding hot, taking you by surprise, and if you’re using a flat-head screwdriver to open the valve, that’s when you’re more likely to jump back leaving the valve open with dirty hot water spraying down the walls and over the floor. Disastrous!
Let the heaters reach temperature, check for cold spots, turn it off, then bleed the radiators when the water isn’t so hot. Remember: The water inside the radiator will be hotter than the radiator.
3. Protect surfaces around the air valve before opening it
For best protection, an old rag like a piece of cloth, face towel or even a duster can be placed against the wall behind the air valve, and an old towel on the floor to keep the dirty water from staining carpets or walls.
4. Opening the air valve
The air valve is located at the top of the radiator. They usually have a white round cap over a square valve which is what the radiator key locks onto. (You can get these in hardware stores like B & Q, and even the Pound Shops are stocking radiator keys now). Lock the key in place, turn it gently anti-clockwise for up to a quarter turn. If there is trapped air, you’ll hear it hissing. Don’t put your hand to the valve to try to feel it because it will be hot. So will the water that comes out after the air has escaped. When water starts trickling (or pouring) out, close the valve, being careful not to overtighten it.
5. Re-pressurise your boiler
No matter how quick you are to close the air valve, you’ll always lose some water. The more radiators you have in your home, the more water will be lost from the system. Once you’re done bleeding your radiators, check your boiler’s water pressure. Most systems should have pressures between 1.0 and 1.5 bar pressure. After bleeding your radiator, they can drop below the recommended 1.0 bar minimum required for safe operation.
Under the boiler will be a central filling loop that looks like two small taps with a connector hose. If the connector hose isn’t attached, the taps will be capped and the hose could be stored on top of the boiler. Remove the caps, and attach the hose by screwing them over the taps like you would a regular hose.
The valves for the filling loop are like the ones under your sink for your washing machine and there will be two handles. Both are at a 90-degree angle of the pipe direction indicating they’re closed.
The pipes you’re opening top up the system are connected to your main water supply, which does mean the water flow is extremely high pressure. Turning both taps simultaneously opens the valves to top up the system. Only open the valves slightly to avoid a rush of water filling the system up too fast, which can over-pressurise it.
When you’re topping up your boiler water pressure, you should have a clear view of the pressure gauge throughout. If that’s not possible, it’s best to have someone else tell you when it’s reached pressure rather than continuously opening and closing the valves.
If, on the off chance, you do add too much water, open the air valve on a radiator and let water escape being sure there’s a basin there to collect excess water. It will be time-consuming to drain water from the system, but it’s essential that your boiler operates at the proper water pressure levels.
About Us
Edgware Plumbing has years of experience servicing and repairing leading brands of boilers across Edgware and surrounding areas including Elstree and Borehamwood. We provide annual boiler serving, emergency plumbing and heating repairs, and strive to help our customers save as much on their energy bills as possible by keeping heating systems operating efficiently and safely. Find out more about our services here, or contact us here to find out how we can best be of service to you.