No hot water in the shower but the taps are fine

Which kind of shower do you have?

An electric shower is a box on the bathroom wall with its own power switch, usually a pull cord or a wall switch outside the bathroom. It takes only cold water and heats it itself, so the boiler has nothing to do with it. A mixer shower takes hot and cold water from the house supply and blends them, either in a bar valve on the wall or in a valve concealed behind the tiles with a separate control. A power shower is a mixer with a built-in pump, and some homes have a separate shower pump in the airing cupboard.

If you are not sure, look at whether the shower has its own electrical switch and a power setting on the unit. If it does, it is electric. If it has just one or two controls for flow and temperature and no power switch, it is a mixer.

Mixer and thermostatic showers

A thermostatic mixer has a cartridge that constantly adjusts the blend of hot and cold to hold a steady temperature. When the cartridge wears or limescale builds up inside it, it can stop letting enough hot water through, so the shower tops out at lukewarm however far you turn the dial. Much of London has hard water, so this is a common shower fault here.

Most thermostatic showers also have small filters and check valves where the hot and cold pipes enter the valve. If the filter on the hot side clogs with scale or debris, the hot supply to the shower is starved while the basin taps, which have their own feed, still run hot. Many thermostatic valves are also designed to shut off or reduce the hot water if the cold supply fails, as an anti-scald safety feature, so a problem on one side can show up as a temperature problem.

If the shower went cold straight after a cartridge was replaced, check the temperature limit stop. This is a small stop behind the handle that sets the maximum temperature, and it may not have been reset correctly. Our guide to thermostatic shower temperature problems goes into more detail on diagnosing the valve.

Electric showers

An electric shower that runs cold usually has a failed heating element or a tripped thermal cut-out. The cut-out is a safety device that switches the heat off if the unit overheats, often because the water flow has dropped. A scaled shower head or a kinked hose can reduce the flow enough to cause this. Check the unit is on its hottest power setting, not a cold, eco or low setting, and try a clean head.

If the shower runs cold and the unit is old, or it cuts out and comes back, see our guide for when an electric shower keeps cutting out. Replacing an element or the unit itself involves mains electrics in a bathroom and needs a qualified electrician. Do not open the cover of an electric shower yourself.

If an electric shower shows scorch marks, smells of burning, or trips the electrics, switch it off at the isolating switch and do not use it until it has been checked.

Pumped showers and gravity systems

In homes with a hot water cylinder and a cold water tank in the loft, the shower may rely on gravity or on a pump. Air trapped in the hot pipe or in the hot side of a pump is a common reason for a shower that suddenly runs cold, splutters or runs weakly, especially after the water has been turned off for work elsewhere. Our article on airlocked pipes explains how that happens.

A shower pump that has failed may still let cold water through while the hot side struggles, and the cold tank running low, for example because its float valve has stuck, will starve the hot cylinder and therefore the shower. If the pump is noisy, cycles on and off or runs without the shower being on, have it checked. Our guide to shower pump suitability explains which systems a pump suits.

Showers on a combi boiler

A combi only fires when it detects enough water flowing through it. Some water-saving shower heads, or a head badly blocked with scale, reduce the flow so much that the boiler never switches on for the shower, even though a fully open hot tap brings it to life. Try the shower with the head removed, pointing the hose into the tray. If it runs hot, clean or change the head.

If the shower is hot for a minute and then goes cold, the combi may be struggling. That can be a scaled plate heat exchanger or a boiler fault, and you may notice the kitchen tap doing the same when run hard. In that case the boiler needs looking at rather than the shower; our Worcester boiler no hot water guide covers the checks that apply to most combis.

What you can safely check

A few minutes of checking often narrows the fault down, and none of these steps involve taking the valve apart or turning off the water.

• Run the basin and kitchen hot taps. If they are hot, the problem is local to the shower.
• Check the shower is on its hottest setting and, on an electric shower, on full power.
• Unscrew and descale the shower head, then test again.
• Check any isolation valves on the shower pipework are fully open. They are often a slotted screw in the pipe.
• Note whether the problem came on suddenly, after a cold spell, or after other plumbing work.

Stop there if the valve is concealed behind tiles, if the shower is electric and needs opening, or if you would need to turn off the water to take anything apart. Concealed valves are easy to damage, and a leak behind the tiles costs far more to put right than a cartridge.

Booking a shower repair

A cold shower is usually a straightforward repair once the cause is found, and it often comes down to a cartridge, a filter or a head. What slows jobs down is not knowing the make and model of the valve, so take a photo of the shower controls and, if you can see it, the brand name on the valve or the unit. Tell us whether the hot taps elsewhere are fine and what type of hot water system you have.

Loyal Plumbers has 10 vans covering London and everywhere inside the M25, 24 hours a day. Call or send an enquiry with the details above and we will diagnose the fault before replacing anything. If the valve is concealed, we will explain the access needed before we start.