Introduction

A primary component of an indirect (gravity fed) plumbing system is a cold water storage tank (or cistern). Normally located in the loft space, the cold water tank supplies water to the hot water tank (which sits directly below it, and feeds the hot water taps, shower etc) aswell as all of the cold water taps in the property, with the exception of the kitchen tap.

Before commencing work removing the old tank, make sure you have all the tools you need to successfully complete the job. Use the following link for a list of basic plumbing tools:

• Basic plumbing tools & materials

Replacing a cold water tank

Make sure your new cistern comes supplied with a byelaw 30 kit. This is a requirement of all water suppliers and in essence contains connectors and valves to prevent light and insects entering the tank - to thus keep the water clean. In addition there should also be a screened overflow pipe (to prevent insects), a reinforcing plate to stiffen the cistern wall around the float valve, and an insulating jacket to prevent freezing during winter.

Removing the old tank

Before commencing work dismantling the existing water tank, you must first isolate the tank in the plumbing system and drain it thoroughly.

• Turn off and/or extinguish your water heater/boiler
• Turn off the mains water at the stopcock, and also close the valve to the rising main
• Open the all the cold taps in the/a bathroom until the water stops
• Bail out any remaining water with a small bucket, or similar

Next you can dismantle all the connections to the old empty tank, and move it out of the way. Make a note of where the plumbing fixtures currently sit, as this will help considerably when connecting the new cistern. If necessary write a description on each of the pipes using a pen and masking tape.

• Using a spanner, first remove the connections to the float valve
• Then dismantle the distribution pipes and overflow to the tank
• If the joints are stiff use a little penetrating oil to loosen them (WD40 or similar)

If the tank is too large to lower through the hatchway (often in older houses the tank was fitted in place before the roof was finished) then you can place it to one side of the loft cavity, out of the way of the hatchway. If this is not an option the the tank can be dismantled using a angle grinder whilst still in the loft space.

• If you are using an angle grinder in the loft space, make sure you wear a mask, safety goggles, gloves and ear defenders.

Once the old tank has been removed, ensure that the base is firm enough and large enough for the new tank to sit on. If necessary construct a new platform by securely nailing planks across the joists and creating a flat base with 18mm (¾in) plywood.

Installing & plumbing a new storage tank

Place the new tank in position and ensure it sits firmly on it's base. Once you are happy with it's position you can set about connecting the pipes and fittings.

• Common metal compression fittings & adaptors

The new cistern will require holes to be drilled in it's sides, in the appropriate places, for the numerous pipes to pass into and out of it. For this it is advisable to use a hole saw, which is connected to a electric drill via an arbor in the same way as a standard drill bit. If you do not have one, they can easily be hired from your nearest tool hire shop.

Connecting the float valve

• Firstly cut at hole in the appropriate side of the new cistern, 75mm (3in) down from the top, to accomodate the float valve
• Slide a plastic washer onto the tail of the valve and pass it through the hole
• Slide the reinforcing plate onto the protruding valve thread, together with another plastic washer and the fixing nut
• Secure the float valve in place using two spanners, then screw a tap connector onto the final section of valve thread, ready to connect with the 15mm (½in) rising main

Plumbing the distribution pipes

• In the sides of the tank, approximately 50mm (2in) from its base, drill two holes to accomodate the outlet pipes to the hot water cylinder and the cold water taps (these were ideally labelled before the old tank was removed, as described above)
• The 22mm (¾in) outlet pipes are connected to the tank using tank connectors, slide a plastic washer over the shaft of each connector and pass through the holes from the inside of the tank
• Wrap a few turns of PTFE tape around the exposed connector threads, and slide on the other plastic washer
• Tighten the nut firmly (¼ turn beyond finger tight) whilst holding the connector inside the tank to stop it turning - DO NOT over tighten as you will damage the washer and this will cause the tank to leak

• This is now an ideal opportunity to add a gate valve to each outlet pipe to allow one to be able to easily isolate the cold water storage tank in the future, as opposed to having to drain it.

Attaching an overflow to the tank

• Drill a hole, on the opposite side of the tank to the float valve assembly, 25mm (1in) below the float valve inlet
• Slide a plastic washer over the overflow connector and pass through the sides of the tank from the inside. There should be a plastic nut on the inside as a part of this connector
• Offer the overflow pipe assembly from the outside of the tank to the connector on the inside, and once aligned connect the two parts together by tightening the nut on the inside of the tank
• The overflow assembly should contain a dip pipe and an insect filter to comply with byelaw 30. If not attach them to the assembly at this point
• If the existing overflow pipe to the outside is still functional, simply connect the new assembly to this pipe
• If a new overflow is required, run a length of 22mm (¾in) plastic overflow pipe from the assembly, across the floor to the outside of the house (this may mean making a hole in your roof to accomodate this)
• Clip the plastic piping to the roof timbers where appropriate

• The overflow pipe must maintain a continuous fall
• It must protrude from the property in a conspicuous location in order to allow an overflow of water to be spotted immediately

Connecting the new tank to the plumbing system

• Ensure the rising main lines up squarely with the tap connector attached to the float valve, when you are happy connect the two together using compression fittings
• Ensure the two outlet pipes (cold water taps and hot water cylinder) line up with their respective gate valve at the base of the tank, when you are happy connect each pipe using compression fittings
• Secure the pipework to the base and/or joists using plastic clips
• Adjust the vent pipe from the hot water cylinder so it can pass through the lid of the new cold water cistern

• Do not solder the joints near a polythene tank/cistern, as the heat will easily damage the new tank

Refilling the new storage tank

Once the new cold water cistern has been plumbed in, it can now be filled with water. This process should be done carefully, checking for leaks all the time, until one is satisfied that all is well.

• Turn on the mains water at the stopcock, and also open the valve to the rising main
• Ensure all the cold taps in the bathroom(s) are in the closed position
• Keep a close eye on tank as it fills, checking for leaks - if detected shut off the water again immediately
• As the water approaches the top of the cistern, adjust the float valve arm to maintain the water level approximately 25mm (1in) below the overflow outlet
• When you are happy that all the plumbing is watertight and sound, attach the lid, and insulate the tank and surrounding pipework. DO NOT insulate underneath the cistern as this will prevent warm air rising from below
• Finally turn on and fire up your water heater/boiler

Further information and useful links

• Find a Tool Hire Centre near you
• Find a Plumber near you