How do I drain down the cold water system in my house?

Draining down is the step before almost any real plumbing job: replacing a stopcock, cutting into a pipe, changing a tank, or leaving a house empty over a hard winter. It is straightforward but the order matters, and the two things people get wrong — leaving the immersion on, and expecting the hot taps to drain from the mains — are the two that cost money.

Safety

What you'll need

• A bucket and towels
• A length of hose (optional)
• String or wire to tie up a ball valve (optional)

Step by step

1. Switch off the water heating and leave it off.Boiler off at its own switch or programmer, and the immersion heater off at its double pole switch. Do this first, before any water moves. An element that ends up above the water line burns out in seconds.
2. Close the internal stopcock.Clockwise, fully. If it will not close, or closes but still passes water, sort that out before going further — you cannot drain a system you cannot isolate.
3. Open the lowest cold tap in the house.The kitchen cold tap on a combi or unvented house, or an outside tap if you have one, or the lowest basin. Gravity does the work; the water above that point runs out through it.
4. Work upwards, opening every cold tap.Opening the upper taps lets air in behind the water, which is what allows the lower ones to run properly. Include the outside tap, the utility, the cloakroom and the bathroom. Flush the toilets and leave them empty.
5. On a tank-fed house, deal with the loft cistern.Tie the ball valve arm up to a batten across the tank so it cannot refill, then let the cistern drain through the cold taps it feeds. Remember the cistern also feeds the cylinder, so the cold side of the loft tank drains into the hot system unless you close the cylinder's cold feed valve.
6. Drain the hot side separately, if you need to.Only with the heat off and the cylinder cold. Open all the hot taps; on a vented cylinder that only lowers the water to the level of the highest draw-off, and fully emptying the cylinder needs its drain cock. On an unvented cylinder, stop here — draining it involves the safety devices and is ticketed work.
7. Leave a tap open while you work.An open tap is your proof that the system is not repressurising behind you, and it gives trapped air somewhere to go. Leave the lowest one open throughout the job.
8. Refill from the bottom up.Close all the taps, open the stopcock slowly — a quarter turn, not wide open — and then open taps one at a time from the lowest upwards, letting each run until it stops spitting. Filling fast is how you push air into places that then need clearing.

Tips

• Put a note over the immersion switch and the boiler so nobody switches them back on while the system is empty.
• Fit a hose to the lowest drain point and run it outside rather than filling buckets — much faster and much drier.
• Expect discoloured water and some spluttering for a few minutes after refilling. Run the taps until it clears before drinking.
• If a tap runs air but no water after refilling, you have an airlock — much more likely on tank-fed hot pipework than on the mains cold.

Common mistakes

• Leaving the immersion heater or boiler on — The element or heat exchanger ends up dry and either burns out or overheats. It is the single most expensive mistake in this job and it is entirely avoidable.
• Expecting the hot taps to run dry from closing the stopcock — On a tank-fed house the cylinder is fed from the loft, not the main, so the hot side keeps running long after the stopcock is shut. Two different feeds, two separate jobs.
• Opening the stopcock fully to refill — A sudden surge drives air pockets into high points and slams the pipework. Open it slowly and let the system fill quietly.
• Forgetting the outside tap — It is often the lowest point in the system and holds water that will freeze if the house is being drained for winter.

Questions people ask