Fix a shower that runs hot and cold

Find out why your shower's temperature swings between hot and cold, then fix the cause safely and test it properly.

SH-01Work out what type of shower you haveLook at the unit on the wall. An electric shower is a sealed box with a power cable running to it, and heats water instantly with an element inside. A mixer shower blends your existing hot and cold supply at a valve, either with a plain lever (no thermostat) or a thermostatic mixer valve, often marked TMV, that holds the temperature steady on its own. Knowing which one you have decides which parts you need and how you open it safely.

SH-02Rule out the easy causes firstAsk whether the swings happen when someone else uses a tap, flushes the loo, or runs the washing machine - that's classic for a mixer shower with no thermostat, caused by a sudden drop in cold water pressure. Check your stopcock, usually under the kitchen sink, is fully open, and that any isolating valves near the bath or behind the shower panel are open too. Clean the shower head and hose in descaler, since limescale restricts flow and can make an electric shower's thermostat misbehave. If this sorts it, you're done - if not, move on to the valve or unit itself.

1 each · Limescale remover

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SH-03Turn off the water and power before opening anythingFor a mixer shower, close the isolating valves feeding the valve, usually small slotted screws under the bath panel or in an airing cupboard; if there aren't any, turn off at the stopcock and check it turns freely before you start. For an electric shower, switch off its isolating switch (a pull-cord or wall switch near the bathroom door) and turn the circuit off at the consumer unit - never open the case with power still on. If you're not sure which breaker feeds it, turn off the whole consumer unit while you work.

RULES

KNOW THE RULES

Fixed wiring inside a bathroom is a 'special location' under Part P of the Building Regulations. Like-for-like part swaps are generally fine for a competent DIYer, but any new wiring or circuit changes need a registered electrician or building control sign-off.

SH-04Open a mixer valve and check the cartridgePrise off the temperature handle's cap, undo the small fixing screw underneath, and pull the handle off. Unscrew the retaining collar or nut to lift out the cartridge - on a thermostatic mixer this is usually a chunky cylindrical unit with a wax or piston core inside. Look for scale, a cracked core, or a stiff, gritty action. If it doesn't turn smoothly, or the shower still swings temperature once it's clean, fit a new cartridge that exactly matches the make and model.

1 each · Thermostatic shower cartridge (matched to your valve)

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SH-05Check an electric shower's filter, element and thermostatUndo the screws holding the cover plate, usually 2 to 4 Philips screws, and lift it away from the base plate, which stays fixed to the wall. Find the small mesh inlet filter where the pipe enters the unit, unclip it, and rinse off any limescale under a tap. Check the element and thermostat for visible scale or scorching. If the shower still cuts between hot and cold once the filter is clean, the thermostat or element has likely failed and needs a like-for-like replacement.

RULES

1 each · Replacement shower element/thermostat kit (matched to model)

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KNOW THE RULES

You are handling the internal wiring of a bathroom electrical fixture, a Part P special location. If in doubt, or if the fault turns out to be wiring rather than a simple part, get a registered electrician to check it.

SH-06Fit the new part and put it back togetherFit the new cartridge or element the same way the old one came out, using any new seals or O-rings supplied with it. A light smear of silicone grease helps seals go in without tearing. Tighten the retaining nut or screws by hand first, then snug up gently with a spanner - overtightening cracks plastic parts. Refit the cover plate or handle and check nothing is trapped or twisted before you turn anything back on.

1 tube · Silicone grease (plumbers' grade)

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SH-07Turn the water and power back on and bleed the air outOpen the isolating valves or stopcock slowly, then switch the circuit back on at the consumer unit if you turned it off. Run the shower on cold first for a minute to clear air and grit from the pipework, then bring it up through the full temperature range slowly and check it holds steady. If it still swings, the fault may sit further up the system - low mains pressure, a shared cold feed, or a failing main valve - and it's worth calling a plumber.

SH-08Check for leaks and tidy upRun the shower for five minutes and check around the valve, cover plate and any joints you touched for drips. Dry the area and check again after a few hours, since small leaks often only show once the unit has been used properly a couple of times. Wipe the shower down, and set aside the old cartridge or element for recycling. Note the exact part number you used in case you need it again.

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