Central Cold Water Storage Tanks

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Central cold water tank

Central cold water tank

If you have a water tank on the roof that supplies water for your building, every drop of water used has an associated energy cost – and every wasted drop is also wasting energy.

After roughly the first four floors in a building, mains pressure water will typically be no longer suitable to move water around. Instead, water is pumped to the tank on your roof.

In theory, the tank then supplies most remaining floors via gravity - except for the four or so floors directly underneath it, where gravity would not provide sufficient water pressure, which need to be supplied with water from a pressure-boosted system via a pump.

In practice, many buildings with central water tanks do not have separate plumbing lines for the top floors, and all water used in the building is pumped from the tank. You’ll need to check your hydraulic diagrams to ascertain exactly how water moves around your building.

This means that every drop of water is pumped from ground level to the tank, and then from the tank to where it is used.

Options to consider in ensuring efficient operation of your building’s water supply are:

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    Water tank float valve

    Float valve properly turned off by the water level

    Check that the float valves that supply the tank are not worn – this can be a source of hidden leakage if the tanks silently overflow to drain at night.  Check the valve by pulling up on the arm, and checking to see if the water stops flowing at a level that would not allow water into the overflow drain.

     

     

     

     

     

     

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    Water pressure reduction station

    Pressure reduction station

    Water pressure is higher for the lower levels of a building than the higher levels – due to the higher ‘head’ of water pushing down.  High pressures can cause higher water use through higher flowrates, a higher likelihood of leaks, and increased wear and tear on washers, valves and flow regulators.  Some buildings have ‘pressure reduction stations’ installed on some levels to limit this effect, but many don’t.  Installing pressure reduction could help to reduce water use and future maintenance in the lower levels.

     

     

     

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    Water pressure pump

    Water pressure pump

    If you are replacing your pumps, consider smaller single speed pumps.  Larger pumps and variable speed pumps are often operating well outside their optimum ‘efficiency band’ when pumping to a tank.

     

  • Pressure boosting the water supply lines out of the roof tank is common, though it is only necessary for the top few levels. Well-specified pumps with ‘pressure modules’ will help to limit energy used by these units.

 

 

Acknowledgements: 
We'd like to thank Adam Jones from BMT WBM for sharing his knowledge, and some photos, for this topic.
Last Updated: 
Tue 07/12/2010

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