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Drain Off Cock - Faulty

So you've come to drain down the water and guess what the drain off cock isn't working or you've found that its leaking. Well in either case their is nothing to worry about. We will cover what to do here in this section. As usual it is advised that you put down protective sheets before attempting any work.

If the drain off cock itsn't working you will have to drain from another suitable point. Then replace the faulty valve once the systems empty. In this case we will assume that the faulty valve is on pipework directly below the radiator.


Replacing a faulty valve on heating;


  • Turn off power to the heating (at boiler controls).

  • Turn off water supply.

  • Turn off the radiator at both ends.

  • Drain the radiator by undoing the nut at the tailpiece (part when the radiator valve connects to the radiator), leaving the other side connected (leave the side that has the drain off cock on connected), and empting into a suitable container.

  • Once the radiator is empty take the nut and olive off the tailpiece you've undone.

  • Connect a hose pipe with jubilee clip to the tailpiece (make sure the other end of hose is going to a suitable drain point) and open up the valve that is still connected to the radiator.

  • Once drained fully you will need to place a suitable container under the valve to catch any leftover water and then slowly undo. If there seems to be a lot of water allow to drain off before removing completely.

  • Now its empty replace the drain off cock (having wrapped the thread with ptfe tape).

  • Remove hose pipe and put tailpiece nut and a new olive back on the tail piece and tighten back up to valve.

  • Check the drain off valve is closed, tailpiece is tight and then turn the radaitor valves back on to radiator.

  • Fill back up and check for leaks.


Job Done!

If its on the hot water this is also easy. In this example we will assume the drain off cock that is faulty is on the bottom of the cylinder. we will assume that it is a cylinder like the one pictured.


Replacing a faulty valve on the hot water;


  • Turn off power to the boiler (at controls).

  • Turn off water supply.

  • Open all hot water taps.

  • Put suitable covers around the cylinder to catch excess water.

  • Undo the hot water draw off point, located at the top of the cylinder.

  • Insert your hose pipe into the cylider at the top point mentioned.

  • Siphon the water off to a suitable drain pointuntil empty.

  • Slowly undo the drain off cock catching any water spilt into a suitable container.

  • Replace the valve.

  • Remove hose pipe, reconnect union on cylinder and check the drain off valve is closed.

  • Fill up and check for leaks.


Job Done!



  • If the tailpiece you've disconnected is ground faced (in other words a big nut) you will need to undo this tailpiece and replace it with something that you can fit your hose onto. For example a compression couplin with a piece of 15mm pipe.

  • You could always just change the inside of the valve either with a new washer or complete new insides.

  • Leaking drain off cocks can often be fixed by simple tightening up the valve.

Example of a half inch drain off cock - or d.o.c for short.
Example of a radiator valve with a ground face tailpiece.
Example of a radiator valve with a drain off valve intergrated.

Related Topics

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