How to Drain Down a Plumbing System


Even if you do not intend to carry out major plumbing works such as putting in a new sink or laying a drain, the chances are that at some time you will have a plumbing emergency -perhaps a leaking tap, an overflowing cistern or a blocked waste. The advice here is designed to help you to cope, either by fixing the problem or at least by minimising the damage until the plumber comes.

The onset of an emergency is the wrong time to start learning about your plumbing system – you should know how to isolate and drain various parts of your plumbing system before trouble strikes so that you can take immediate action if an emergency arises.

Flush Drain

To carry out repairs to taps, ballvalvcs and leaking pipes, the water supply to the offending part has to be cut off and the pipes drained of water.

Before trouble strikes

Learn the position of all stopcocks, gate valves and any drain-cocks. Make sure you known which parts of the system they each control. Regularly turn stopcocks and gate valves on and off to ensure that they are free if seized, apply penetrating oil and care­fully try again.

In an emergency

Cut off the water supply to the leaking tap (or whatever) and open all taps on this part of the plumbing system. Turn off any immersion heater or boiler. Attach hoses to any draincocks on the affected part of the system and open them.

If you are unsure which part of the system is affected, turn off the water at the main stopcock in the house. If the leak is in the section of pipe before this, also turn off the water at the water under­taking’s stopcock in the road if you can. This will isolate the leaking pipe which can then be repaired. Then:

  • with a direct plumbing system, open all the cold taps, drain off at any draincocks on cold water pipes, and flush WCs. If the leak is in the cold water system, it should stop when the laps run dry. If the leak is in the hot water side, isolate the storage cistern supplying the hot water cylinder by turning off the gate valve in the pipe connecting the two. Turn off any immersion heater or boiler supplying the cylinder. Open all the hot taps. If there are no valves, simply turn off any water heaters and open the hot taps – the system will take longer to drain, because the cistern will have to empty as well. Note the hot water cylinder will remain full of water
  • with an indirect plumbing system, open any taps or valves connected directly to the rising main probably only the tap over the kitchen sink, but possibly taps feeding washing machines, outside taps and perhaps an electric shower as well. Drain off at the drain cock immediately above the stopcock on the rising main. If the leak is in the rising main, or a pipe fed from it, it should stop when the taps run dry. If the leak is further on, isolate the cold water storage cistern. The cistern will have at least two pipes leading from it – one to feed the cold taps and one to feed the hot water cylinder which supplies the hot taps. Trace and close off the valves on both of these pipes and turn off any water heaters. The gale valve for the cold taps is likely to be in the loft: the one for the pipe feeding the hot water cylinder may be close to the cylinder. If both pipes have valves, turn on all the hot and cold taps, and flush WCs. The leak should slop when the taps run dry. If only one pipe has a valve, try turning on the appropriate taps first. If this does not work, turn on the other sets of taps – the system will take longer to drain because the cistern will have to empty as well as the pipes. If there are no valves, open all taps together -again, the cistern will have to drain as well.

Plumbing System

Note the hot water cylinder will remain full of water.

  • if the leak is in a hot water cylinder, this will have to be drained. Turn off water heaters. With solid-fuel heaters, rake out the fire and allow it to go cold before draining. Isolate the cylinder from the cold water cistern if possible: if not, cut off the supply to the cylinder by turning off at the main stopcock. Drain the cylin­der at the draincock usually located on the cold feed of an indirect hot water system or at the boiler on a direct hot water system.
  • if the leak is in the cold water storage cistern, isolate it by turning off the water at the main stopcock. Drain the cistern by turning on all the hot and cold taps (except those fed directly from the rising main). The last bit of water will have to be bailed out by hand.
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  3. How to Fit a New Cistern
  4. How to Replace Taps
  5. How to Overflow Cisterns

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About the Author: Jason Prickett loves to write about home maintenance and stuff you can do yourself instead of hiring any professional. His step by step guides will assist you in completing your home maintenance tasks.

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