Drain the hot water cylinder: how-to guide

Need to drain the hot water cylinder? Whether you need to fit a new immersion heater or replace the cylinder altogether, it’s not a difficult task.

However, unlike draining a cold water storage cistern, it cannot be done as easily as shutting off the supply and running the taps until the cistern is empty.

That’s because shutting off the supply of cold water to the hot water cylinder will stop the flow of hot water in the home, but it will still leave the cylinder full of water – even when the hot taps run dry.

This is because of how a gravity-fed system works. It’s gravity acting on the water in the storage cistern which effectively pushes hot water out of the top of the cylinder and off to the taps. Isolating the hot water cylinder from its supply prevents this from happening, but that’s all.

This arrangement is actually a safety precaution. It means that even if the supply to the cold water storage cistern is interrupted and the cistern runs dry, the cylinder will still be full of water. It protects the immersion heater(s) from inadvertently heating an empty cylinder. This could be dangerous.

How to drain the hot water cylinder

  1. Turn off the immersion heater and the boiler.
  2. Isolate the cylinder from its supply. In order to do this, close the gate valve on the cold feed pipe by turning it clockwise.
    • If there is no gate valve on the supply pipe: the cold water storage cistern must be emptied, or a bung must be fitted. Isolate the cistern via the isolation valve adjacent to the ball valve or by shutting off the mains, and drain the cistern down. Alternatively, tie up the ball valve to a batten of wood placed across the cistern. Another option is to bung the supply pipe from inside the cistern with a tank bung.
  3. Run the kitchen hot tap until it stops. This will drain all of the water out of the pipework above the cylinder, such as water in the expansion pipe.
  4. Locate the draincock at the bottom of the cylinder. The draincock is likely to be on the elbow of the cold feed. Alternatively, it may be on the cylinder itself from a small spur of pipe.
  5. Place an old towel underneath the draincock. This will catch any drips of water weeping from the spindle.
  6. Attach a hose to the draincock with a jubilee clip, and run the hose to an exit point such as a sink or drain that is lower than the base of the cylinder.
  7. Open the draincock using a draincock key or a pair of small adjustable grips. You should hear water start to flow.

Things to consider

Note that while this process will empty a direct cylinder (where the water is heated on an Economy 7 tariff), the heat exchanger or coil in an indirect cylinder will still contain water from the central heating circuit. In order to drain the coil, you will need to isolate the feed and expansion cistern, switch off the boiler, and drain the system down.

Don’t forget that an empty hot water cylinder is a great opportunity to remove any internal limescale. You should definitely consider doing this if you’re replacing a lower entry immersion heater.

Refilling the hot water cylinder

Refilling the cylinder is simply a case of reversing the above instructions:

  1. Close the draincock and remove the attached hosepipe. Use a towel or cloth to catch any drips from the draincock.
  2. Close any hot taps you used to drain the pipework.
  3. Open the gate valve on the supply pipe. If there was no gate valve, open the supply to the ball valve. If you used a tank bung instead, remove it.
  4. Refill the central heating circuit (for indirect cylinders).
  5. Turn on the immersion heater and the boiler.