Residential Cold Water Systems



Residential/domestic cold water systems are overlooked by home owners (not really considered) as pipework is generally concealed, storage cisterns/tanks are mainly kept in lofts (roof cavities) and hot water cylinders stored in cupboards.

Apart from knowing where all the taps/faucets are most people do not know what type of cold water system is installed in the property.

It is good to know what type of water system you have in your home and to be aware of how to isolate the system or sections for repair or in an emergency.

Direct System

In a direct system water is supplied at mains pressure to all cold water taps/faucets, WC (toilets) cisterns and a cold water storage cistern/tank if hot water is to be supplied from an open vented (low pressure) hot water cylinder.

This is an 'unbalanced' cold water system because the cold water outlet pressure at taps/faucets is higher than the hot water from the open vented cylinder.

To have a balanced cold water system the cold water storage cistern must be removed and the open vented hot water cylinder replaced with a mains pressure supplied unvented hot water cylinder.

The pipe circuit for cold water distribution in the home branches off after the pressure reducing valve on the supply pipe thereby balancing the system enabling equal cold and hot water pressure at all draw-offs (outlets).

However, the trade off with the use of an unvented cylinder is that you no longer have stored cold water for toilet flushing in the event of a mains water failure.

With a direct cold water system you have the advantage of being able to draw drinking water from any cold water taps/faucets in the house.


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Indirect System

An indirect cold water system is when water is supplied to the house at mains pressure, this water is fed directly to a cold water storage cistern via the supply pipe called the 'rising main'.

A branch pipe off the rising main delivers drinking water to the kitchen and garden tap/faucet, cold water to all other taps/faucets and appliances is provided indirectly from the cold water storage cistern (not for drinking) under gravity pressure not mains pressure.

The hot water storage cylinder is also supplied with cold water from the same cistern.

With an indirect cold water system there is always a temporary back up of stored water in the event of a mains failure. Also, because it is a low pressure system it is generally quieter therefore eliminating noise like 'water hammer' which can occur when high pressure water tries to negotiate tight bends in the pipework.

Indirect cold water systems do slightly reduce the risk of impure water being siphoned back into the mains water supply by having fewer outlets (taps/faucets and appliances) connected to the mains supply.

However, this can easily be protected against in both the direct and indirect cold water system by installing a non-return valve or check valve immediately after the main stop-valve supplying water to the house. This would be good practice.

A non-return or check valve only permits water to flow through it in one direction

Note: Fitting a drain valve after (downstream) the non-return valve after the main stop-valve will enable draining of the rising main pipe.

Garden taps/faucets should also have a non-return valve to prevent back siphoning which can contaminate the distributed water within the house and the mains supply.

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Isolating The System

The entire water system in both direct and indirect cold water system can be isolated by closing off the main stop-valve. This stop-valve can be located inside or outside the property. If located outside it is generally below ground.

Water to any cold water storage cisterns/tanks can be closed off by the stop-valve on the rising main just before connecting to the cold water storage cistern/tank.

Water to the WC cistern, cold water storage cistern/tank and a feed and expansion cistern/tank for central heating must be isolated with a stop-valve or service valve prior to connecting to the cistern as water is allowed to enter these cisterns through a 'ball float valve'.

If the ball or valve failed then there would be considerable water wastage and possible water damage to the property.

All water cisterns/tanks must have an overflow or warning pipe designed to discharge water in a conspicuous external location so quickly alerting you to the problem. Most modern close-coupled WC cisterns will overflow directly into the toilet bowl, however, the high and low level wash down WC cisterns overflow pipe discharges externally.

The water supply from the storage cistern/tank feeding the hot water cylinder can be isolated by closing off the gate valve. This is a 'full bore' valve designed to allow full water flow through it, and should ideally be installed in the vertical section of pipe before connecting to the hot water cylinder.

Because this cold water feed connection is made near the base of the hot water cylinder a drain valve should be located before connecting to the cylinder to enable the cylinder to be drained.

Unvented hot water cylinders depending on type and building regulations are isolated by the main stop-valve on the supply pipe before the cold water control valves, or a stop-valve before the cylinder connection, or an integrated stop-valve if using a composite valve set-up.

A composite valve is comprised of a line strainer, a pressure reducing valve, a non-return/check valve, an expansion release valve and a isolation valve designed to speed up installation of unvented cylinders

All water pipes servicing taps/faucets, baths, basins, sinks and appliances such as dish washers and washing machines etc should ideally be fitted with service valves on both the hot and cold service pipes. This will enable easy isolation for repair or upgrade without having to isolate the entire house or property.

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Draining Cold Water Taps/Faucets and pipes

In a direct cold water system close off the main stop-valve and open all cold taps to drain, in multi level properties the kitchen tap/faucet will be the last to drain. Further draining can be done through a drain valve if fitted.

With an indirect cold water system to isolate the bathroom taps/faucets close off the gate valve on the appropriate cold feed pipe from the cold water storage cistern/tank, then open all bathroom cold taps/faucet to drain.

If you can't find the appropriate cold feed isolating valve then close off the stop-valve before the cold water storage cistern/tank. Failing that you can place a wooden batten across the top of the cistern/tank and tie the float valve to it preventing it from opening then open bathroom taps/faucets to drain. However, if you can't access the loft then close off the main stop-valve.

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Draining The WC Cistern

To drain a WC cistern simply close off the isolation valve (this can be either a stop-valve, wheel head/gate valve, or a service ball valve) which should be located on the cold feed just before it connects to the WC cistern.

If you can't locate this isolation valve, or worse it isn't installed, simply tie up the float valve within the WC cistern to prevent it opening and flush to empty.

In a direct system, to empty the cold feed pipe that supplies the WC cistern, close off the main stop-valve and open drain valve (if installed) to empty pipe.

To empty the cold feed pipe in a indirect cold water system close off the gate valve on the cold feed from the cold water storage cistern/tank. If you can't locate this valve, close off the stop-valve on the supply to the cold water storage cistern/tank, then open the bathroom cold taps/faucets to drain the cold water storage cistern/tank and flush toilet until the WC cistern is empty.

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Draining The Cold water Storage Cistern/Tank

If you want to drain the cold water storage cistern/tank simply close off the stop-valve on the rising main to the cistern/tank (or the main stop-valve on the supply pipe if you can't access the loft) then for indirect systems open the cold taps/faucets to drain and empty the storage cistern. In a direct cold water system open the hot taps/faucets to drain the cold water storage cistern/tank.

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Draining The Hot Water Cylinder

Before draining the hot water storage cylinder make sure that the heat generator such as the boiler or electric immersion heater is turned off. For open vented cylinders close off the gate valve on the cold feed to the hot water cylinder, for unvented cylinders close off the main stop-valve or isolation vale.

If you can't locate the gate valve then close off either the stop-valve on the rising main or the main stop-valve on the supply pipe then follow the cold water storage draining procedure in the above section.

With an open vented cylinder open the hot taps/faucets to partially drain the cylinder, open the drain valve to finish the draining the cylinder. This valve is generally near the base of the cylinder on the cold feed pipe to it. However, if you can't locate it you can drain the cylinder from the boiler's drain valve.

There are different types of drain valves, however, they are generally designed so a hose can be connected to it for draining. Some cylinder drain points are plumbed directly into a discharge pipe eliminating the need for a hose.

Because an unvented hot water cylinder is not open to the atmosphere it is not possible to drain it by simply opening the taps/faucets when the main stop-valve or isolating valve is closed.

Most new unvented hot water cylinders come with built-in drain valves (part of the unit). Once the cold water feed to the unvented cylinder has been closed off, simply connect a hose to the drain valve and then open the hot taps/faucets. This will open the cylinder to the atmosphere allowing it to drain and empty.

Some older unvented cylinders are drained by syphonic action, e.g, keep the mains water supply to the cylinder open then open hot taps/faucets to create a flow. Then close off the water supply to cylinder with the taps/faucets still open - this will setup a syphonic action that will drain and empty the cylinder.

Note: All indirect hot water cylinders which are heated by a boiler that also provides hydronic central heating (both vented and unvented), have inside a coil pipe heat exchanger that is part of the primary heating circuit.

  • With an open vented cylinder this can be drained by first closing off the supply to the 'feed and expansion cistern/tank' and then open the boilers drain valve to drain as the open vented cylinder is generally a floor or two above the boiler.
  • The primary circuit on an unvented hot water cylinder is a 'sealed circuit' which means the expansion of hot water is taken up in an expansion vessel and not a 'feed and expansion cistern/tank'.
  • Because the unvented hot water cylinder can be installed anywhere in the home as it is mains pressured and not gravity fed they are sometimes placed in basements and may be below the location of the boiler.
  • In these situations an additional drain valve can be installed on the primary circuit to facilitate any draining

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Draining Hot water Taps/Faucets and Pipes

To drain the hot taps/faucets and pipes (not the hot water cylinder) first turn off the heat source, e.g, the boiler or immersion heater.

For open vented cylinders close off the gate valve on the cold feed pipe to the cylinder then open hot taps/faucets to drain the pipe and a small portion from the hot water cylinder (the cylinder will still be reasonably full)

If you cannot locate this valve then close off the stop-valve to the cold water storage cistern/tank to the open vented cylinder, or tie off its float valve, or close off the main stop-valve on the supply pipe.

In a direct cold water system open the hot taps/faucets to drain the cold water storage cistern/tank, the hot water cylinder, pipes and taps/faucets (you may want to collect the hot water in a bath before it runs cold and then continue draining from the basin's hot tap/faucet).

In a indirect cold water system open the cold taps/faucets to drain the storage cistern/tank. If you open the hot taps/faucets first you will flush away all the hot water stored in the cylinder (you may want to collect this hot water in a bath).

Once the cold taps/faucets have run dry open the hot taps/faucets to drain pipes and partially drain the hot water cylinder.

For unvented cylinders see 'draining the hot water cylinder' above for details but only partially drain cylinder then close off taps/faucets and drain valves. Open hot taps/faucets to again confirm that the pipe is empty.

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Draining and Refilling The Entire System

Normally you wouldn't drain the entire system but if you were going to be away from your house for a long period of time in cold winter months then you may want to do it to prevent any risk of the pipes freezing due to cold temperatures and no water circulation.


  • Switch off your heat source, the boiler or immersion heater (rake out solid fuel boilers and allow to cool)
  • Close off the main stop-valve
  • Open taps/faucets to drain
  • Drain cold water storage cistern/tank and hot water cylinder
  • Flush the WC
  • Open drain valve after the main stop-valve to drain rising main
  • Hydronic central heating systems can remain full if it has been treated with antifreeze
  • Antifreeze can be added to water traps or even salt to prevent freezing

When refilling an indirect cold water system close all taps/faucets and drain valves then open main stop-valve to refill the cold water storage cistern/tank, hot water cylinder and WC cistern.

Check also that the ball float valves opens and closes smoothly.

To refill a direct cold water storage system open all cold taps/faucets, close off all hot taps/faucets and any drain valves then open main stop-valve. Once the water is running freely from the cold taps/faucets close them off one by one and continue filling the cold water storage cistern/tank and hot water cylinder.

Once again check the ball float valves for smooth operation in opening and closing.


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Fixing An Airlock

If air is trapped in the system if can cause the taps/faucets to splutter or even to fail completely by air blockage particularly in indirect cold water systems where both hot and cold taps/faucets may be affected. With a direct cold water system only the hot taps/faucets may be affected.

To cure the airlock in a indirect system attach a hose to the kitchen cold water tap/faucet (any cold water tap/faucet in a direct system) then connect the other end of the hose to the problem tap/faucet. Open both taps/faucets and leave running for a short while. The mains pressure will be higher than the low pressure water stored in the cold water storage cistern/tank and the open vented hot water cylinder.

The greater water pressure will force the air back up through the pipes and into the cold water storage cistern/tank and feed and expansion cistern/tank where the air can naturally escape to the atmosphere.

If necessary repeat this until the water is running free completely cleared of air.

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