Drain the cold water tank: how-to guide

Need to drain the cold water tank? It’s not a difficult task, and in most cases you shouldn’t need to go into the loft. Simply do the following:

  1. Turn off the mains stopcock, either on the rising main or outside the property.
  2. Turn on the hot taps.

And that’s it – when they stop running, the tank should be empty. Well, almost. There will still be a small amount of water below the outlet points. If you need the tank to be completely empty, you’ll have to bail the rest out or use a wet vac.

Drain the cold water tank without turning off the mains

If you can’t find the stopcock or you can’t get to it, you’ll need to go into the loft and close the isolation valve on the supply to the ball valve with a screwdriver. No isolation valve? No problem. You can lay a piece of wood across the tank and tie the ballcock up with string or garden twine.

An empty tank is a great opportunity to remove any limescale or sediment which may have gathered in the bottom – or dead rodents and insects. Buy a lid to prevent any animals or loft debris from getting in the tank in future. Ideally, you should fit a Byelaw 30 kit (or Byelaw 60 kit if you live in Scotland). These kits consist of a number of fittings which protect the water from external contamination.

Draining the cold water tank will also stop the flow of hot water in your house. That’s because it’s gravity acting on the water in the loft which effectively pushes hot water out of the top of the hot water cylinder. Consequently, this means that draining the cold tank will not empty the hot water cylinder. If you want to drain the hot water cylinder, you’ll need to shut off the supply from the cold water tank and drain the cylinder via its draincock.

If you have an indirect plumbing system, i.e., the cold water tank supplies the cold bathroom taps, you should open these taps until the tank is empty. This will save you from having to waste all of the hot water in the hot water cylinder.


Refilling the tank is easy: simply ensure that all taps and drain-off points are closed. Untie the ball valve, or open the isolation valve or stopcock. Check the cistern for any leaks while it’s refilling.