How To Bleed a Radiator Combi Boiler

Water-based heating systems will usually work at their best when water can completely fill the radiator. In some cases, air will rush into your radiator combi boiler, and cause inefficiency in heating.

Diagnosing this problem should be easy. You can feel that the upper part of your radiator combi boiler is cooler than the bottom part. In extreme cases, the entire radiator might not be heating properly. If this is the case, then you will need to bleed your system.

You will need to prepare your bleed key, which is a small tool with either a flat-head or an allen-type head at the end. Get some rags and a small pail (or any wide-mouthed water container) ready.

  • Turn off the switch. Before bleeding your boiler, turn it off at the main switch. This is usually clearly marked, and should be hard to miss. It’s best to work on bleeding your boiler while it’s turned off, to reduce your risk of being scalded or burned from steam or hot water.
  • Locate the bleed valve. Most radiators and boilers have a bleed valve located either behind the unit or at the sides. These are conveniently located near the uppermost portion of the boiler, for easy access, and to easily let air out. The bleed valve should be located within a knob, or it can be a screw located on a recessed portion of the boiler. The head should match your bleed key.
  • Open the valve. Insert the bleed key into the slot or head of the bleed valve. Use your rags to cover the key before twisting it, as hot air or steam can rush out and cause injury. Place your bowl or pail right under the bleed valve, to catch any water that spews or flows out. When ready, turn the valve counterclockwise by about a quarter-inch turn. At this point, air should start rushing out of the valve, causing a hissing sound. You will know your job is finished when water starts dripping, flowing or spewing out of the valve. Be sure you can catch the water with the rags or the pail, as this can cause damage to your floor or carpeting.
  • Close the valve. Once all the air has been forced out, turn the valve clockwise, taking care to stop twisting when the screw is tight enough (over-twisting might cause it to lose tread).
  • Refill and turn on the boiler. Check the boiler’s gauge if there is enough water. If the gauge shows less than a bar, then you will need to replenish the water in the boiler until the gauge shows it’s full. You can now turn on your radiator combi boiler. Check if the radiator is giving off enough heat.

After bleeding your radiator/boiler, keep your bleed key somewhere safe, so you can easily get it when you need to bleed your radiator anew. Be sure to check your radiator and boiler for leaks regularly. An efficient system will result in lower electricity bills, better heating, and more peace of mind.


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