If a radiator is cold at the bottom and hot at the top, it usually means there is sludge in your radiator. This problem is usually created by oxidation in your radiators, causing rust and corroded metal to break off into the rads, creating a nasty sludge. The best way of avoiding this problem in the future is to add inhibitor to your system, which helps to prevent the oxidation process.
To remove sludge, a professional plumber would normally use a power flusher to blast it all out, however this article is aimed at the average layperson, so we’ve assumed that you do not have a power flusher at home. In this case, you will need to take off the individual radiator, take it outside and hose it through to get all the sludge out.
Firstly, you’ll need to isolate the radiator and dissipate the pressure using a bleed key. Turn off both lockshield valves using a pair of adjustable grips, or, if you have a thermostatic radiator valve, make sure it’s turned off. Then go to the airbleed and use a radiator bleed key to let any air out of the system. Once this is done, tighten up the airbleed valve again.
Place a towel underneath your radiator and around the pipes then loosen the nuts connecting the valves to the main body of the radiator. Your towel should protect your floor from any water leaking out.
At this point, you need to lift the radiator off the brackets on the wall in preparation for taking it outside. A good way of doing this without spilling any water is to loosen the valve connector nuts just enough to give you the flexibility to move the radiator, but not so much that you have completely unscrewed it.
Lift the radiator upwards and off the brackets and then pull it forward until the top of the radiator is touching the floor. The rad should still be connected at the nuts and all the water will have flowed to the top of the radiator. You can then manually unscrew the nuts fully and pull the rad completely away from the wall, making sure to keep it upside down so the water doesn’t leak out. If it’s a particularly big radiator, you may need a second person to assist with this part of the process.
Take the upside down radiator outside and tip it over to remove all of the water. Then grab a garden hose attached to a tap and attach the other end to the radiator. Switch on the mains water tap and blast through water into the radiator until you see it spraying out the other side. You may need to hold the hose onto the rad to stop it coming off.
It’s recommended that you blast water through both ends of the radiator and even into the air bleed valves, as this will really ensure that you get all of the nasty sludge completely out of the rad. When you’re happy all the sludge is out, drain out any excess water, dry it off with a towel and take the rad back inside.
Replace the radiator back on your wall brackets, attach it to the valve nuts and tighten them up again. Open up the valves and then open the air bleed valve and vent all the air out. Once the air stops escaping and water starts to come out, you can be confident that the air is out of the rad and you can close the air bleed valve again. If you have any other radiators with similar hot and cold symptoms, you can repeat this process with those.
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Article by Benjamin Clarke