The most popular replacement is the circular 227 litre (50 gallon) polyethylene cistern. This is because the cold water storage cistern is normally situated in the loft and this type of replacement cistern can be folded to pass through the narrow hatchway to the roofspace. The cistern supplies all the cold water taps in the house apart from the drinking water tap as well as the hot water cylinder.
Removing the old cistern
All water heating appliances should be switched off. Shut off the stopcock on the rising main. Turn on the cold taps in the bath and bathroom sink as this will help to empty the cistern by draining. There will be a little water left at the bottom of the cistern, and this should be removed using a small container to bail out the remainder. As the fitting could prove difficult to remove, a little oil will often help, at the fitting you are attempting to dismantle. Using a spanner, dismantle the distribution pipes, the overflow and the float valve. Move the old cistern out. If it is too big to remove from the roofspace, pull it to one side.
Prepare the location
Check the seating of the new cistern. The base should be firm and supported by a platform built across the joists. The platform should be nailed securely to the joists.
Commence plumbing in the new cistern
75mm (3 inches) below the top of the cistern cut a hole for the float valve to pass through. Put a plastic washer onto the tail of the float valve and pass it through the hole. Place the reinforcing plate onto the tail, then another washer and a fixing nut. Now tighten the fitting with your fingers until tight. Then use two spanners to tighten the fitting. A tap connector should now be screwed onto the valve, ready for the connection to the 15mm (0.5 inches) rising main.
Drill a second hole 25mm (1 inch) below the inlet for the float valve in order to connect the overflow pipe. Pass the connector through the hole. Now fit a washer and tighten its fixing nut on the inside of the cistern. Fit the insect filter and the dip pipe. A length of plastic overflow pipe needs to be attached to the assembly, run to the floor and then to the outside of the house, where it must be positioned in a conspicuous location. It is important to check the positioning of this pipe as continuous fall must be maintained.
Drill holes 50mm (2 inches) above the bottom of the cistern for each tank connector, to the hot water cylinder and the cold taps. Push the tank connectors through each hole with one polyethylene washer on the inside. Wrap a couple of turns of PTFE tape around the threads and fit the other washer. To stop the tank connector from turning, screw on the nut, but be careful not to overtighten as you could damage the washer and this could cause leakage. At this stage it is prudent to fit a gate valve to each distribution pipe. This will enable the you to cut off the water supply without having to empty the cistern.
Check the alignment of both the rising main and the distribution pipes to the system before connecting with compression fittings. Remember that if the cistern is plastic, soldered joints are unsuitable.
Final steps.
Clip all pipework to the joists to avoid movement as water passes through the cistern. Open the main stopcock, checking for leakage as the system fills. The float arm needs to be adjusted as the level of water rises, to maintain a water level 25mm (1 inch) below the overflow outlet. The vent pipe from the hot water cylinder should be adjusted to pass through the hole in the lid.
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