How to Install a Water-Efficient Toilet Cistern

Replacing a thirsty old cistern is one of the cheapest, least disruptive water-saving jobs you can do in a British home. A toilet from the 1980s can use nine litres every flush; a modern dual-flush cistern uses six for a full flush and as little as three for a reduced one. In a family of four that adds up to tens of thousands of litres a year, and if you're on a water meter, real money off the bill.

But a water-efficient cistern only works if it suits the pan underneath it. Buy the wrong one and you either can't fit it or you find yourself flushing twice, which defeats the entire point.

Work out what you're replacing

Lift the cistern lid and look inside. You'll find one of two things.

• A syphon — a large bell-shaped unit with a diaphragm washer, often with a plunger or lever. Syphons need a decent volume of water to start the flush, which is why they're common in older 7.5, 9 and even 13 litre cisterns.
• A drop valve, the plastic column with a seal at the bottom and a button on top. These work happily with the lower volumes used in dual-flush cisterns.

While you're there, note where the cold feed enters (left, right, bottom or side), whether the cistern sits directly on the pan (close-coupled) or connects to a low-level pan by a flush pipe, and the width and depth of the space. If you're keeping the pan, measure the distance between the two bolt holes in its base, because close-coupled cisterns are not always interchangeable.

Check compatibility with your existing pan

This is where most DIY disappointments happen, so do it before you order anything.

• Wash-down pans. Older pans with a wash-down flush rely on volume and a bit of force. Fit a 6/4 cistern to a mid-1990s wash-down pan and you may get a weak, smeared flush, followed by a second flush. Modern pans are designed for reduced volumes, so if in doubt, budget for a new pan too.
• Bolt centres and the doughnut washer. Close-coupled kits are often maker-specific. A mismatched gasket will weep at the joint between cistern and pan.
• Water pressure. If your cold water comes from a tank in the loft, you have a low-pressure gravity supply. A high-pressure fill valve will be slow, noisy and prone to dribbling, so specify a low-pressure one instead.
• Height and space. A taller or deeper cistern can foul a window sill, shelf or boxing. Measure before you commit, and check the cistern is deep enough to take the flush valve you have in mind.

Pick your approach

• Displacement device. A save-a-flush bag or a sealed bottle of water inside the cistern. Free or nearly so, and it suits older syphon cisterns that already flush more water than they need. Don't bother if the flush is already marginal.
• Dual-flush conversion kit. A drop valve, button and seal that replaces the syphon inside your existing cistern. Modest cost, a couple of hours' work, and a good compromise when the cistern and pan are otherwise sound.
• Complete new cistern. The best result. A modern dual-flush cistern with an adjustable flush volume lets you tune it to the pan, and you get a new inlet valve, seal and button into the bargain.

Fitting it: the practical steps

Isolate the water first — the small isolation valve on the toilet feed if you have one, or the main stopcock. Open a cold tap to drain the pipe, flush the toilet to empty the cistern, then sponge out what's left.

Disconnect the feed, undo the wing nuts or bolts holding the cistern to the pan, and lift it clear. Scrape the old washer off the pan and clean the mating face.

On the new cistern, fit the flush valve and inlet valve to the maker's instructions. Hand-tighten plastic nuts only: a quarter turn past finger-tight is plenty, and overtightening cracks them. Set the inlet valve so the water line sits about 25mm below the top of the overflow, and fit a new isolation valve on the feed while you're at it.

Drop the new doughnut washer or foam gasket into place, lower the cistern onto the pan, and tighten the bolts evenly, alternating sides a little at a time. Connect the feed with a flexible connector or a short length of copper. Turn the water back on, watch the joint underneath for a minute, then flush.

Setting the flush volumes

Most dual-flush valves let you adjust both the full and reduced flush, usually via a clip, float or screw on the valve body. The full flush should clear the pan comfortably; the reduced flush should handle liquid waste without a second press. If it won't, raise it rather than living with double flushes.

Test properly: flush several times, then try a realistic load of paper. Listen for the inlet valve shutting off cleanly, and check that water isn't trickling into the pan, which means the flush valve seal isn't seated. A cistern that refills constantly wastes far more than the flush ever did.

Finally, if the cistern sits in a loft or the feed pipe runs through a cold space, lag the pipe, and make sure the overflow or warning pipe discharges somewhere you'd notice it outside. Then enjoy the quiet satisfaction of a loo that does the same job on half the water.