No Hot Water in the Shower but the Taps Are Hot? Here Is Why

If the taps run hot but the shower does not, the boiler is usually working and the fault is in the shower. On a mixer shower it is most often a scaled or worn thermostatic cartridge. On an electric shower, the shower heats its own water, so the boiler is not involved at all. On a combi, a shower head that restricts the flow can stop the boiler firing. A quick test at the basin tells you which one you are dealing with.

First, What Kind of Shower Do You Have?

The type of shower decides where the fault can be. Check this before anything else, because it can save you booking the wrong trade.

• Electric shower: a box on the wall with its own pull-cord switch and a cold water feed only. It heats water itself.
• Mixer shower: a valve that blends hot and cold from your boiler or cylinder. It may be a bar valve on the wall, a concealed valve with one or two handles, or a bath-shower mixer.
• Power shower: a mixer with a pump built into the unit, usually on a system with a cylinder and a loft tank.

Electric Shower: The Boiler Is Not the Problem

If you have an electric shower, no heating engineer can fix it through the boiler, because the boiler does not heat that water. The usual causes are a failed heating element, a tripped thermal cut-out, a faulty flow switch or a supply problem at the isolator.

Check the pull-cord switch is on, the fuse board has not tripped, and the shower head is not blocked with scale, which can trip the safety cut-out. If it still runs cold, you need an electrician or a shower specialist. Booking a boiler engineer for an electric shower fault is paying for the wrong visit.

Mixer Shower: Usually the Cartridge

On a mixer shower, a worn or scaled thermostatic cartridge is the most common reason the shower runs cold while every tap runs hot. The cartridge blends hot and cold to a set temperature, and when it sticks it can let through far more cold than hot.

London's hard water makes this common. Scale builds on the cartridge until it can no longer move freely. Signs include water that starts warm and turns cold, a temperature handle that makes little difference, or a shower that is fine on some days and not others. Other causes are a maximum temperature stop set too low, a blocked strainer or filter on the hot inlet, or a failed non-return valve letting cold water push into the hot side. Replacing a cartridge is plumbing work on the water side, not gas work. Most manufacturers sell the cartridge as a separate part, so a stuck cartridge rarely means replacing the whole valve. If the valve is old and parts are no longer made, the plumber will say so.

Combi Boiler: Not Enough Flow to Fire

A combi only fires for hot water when it detects enough water flowing through it. A low-flow or eco shower head, a scaled head, or a flow restrictor can drop the flow below that point, so the boiler never lights for the shower.

The test is simple. Turn the shower on to hot, then look at the boiler. If the display shows no flame or hot water symbol, run the hot tap at the basin at the same time. If the boiler then fires and the shower warms up, the shower is not pulling enough flow on its own. Cleaning or changing the shower head is often the fix. If the boiler will not fire for any outlet, the fault is in the boiler, and our guide to no hot water from the boiler is the better starting point.

Cylinder Systems: Pressure Imbalance and Pumps

On a system with a hot water cylinder, a shower that runs cold while taps run hot usually comes down to unequal pressures or a failing pump. Hot water from a tank in the loft is at low pressure, and mixing it with mains-pressure cold lets the cold overpower the hot.

This is common after a new shower valve is fitted on an older gravity system without checking the supply. On a power shower, a pump that has started to fail can run the cold side and struggle on the hot side. If the cylinder has only a small amount of hot water left, the shower may also be the first place you notice it, because it draws more than a basin tap. Our guide to hot water running cold then hot again covers the related pattern where temperature swings rather than disappears.

When Is It a Boiler Repair?

It is a boiler repair when no hot outlet gets hot, when the boiler shows a fault code, or when a combi will not fire even with full flow at the basin. If the taps are all hot and only the shower is cold, a boiler engineer is usually the wrong call.

A few patterns do point back to the boiler: hot water that runs lukewarm everywhere, a combi that fires and then cuts out after a few seconds, or hot water that only works when the heating is off. Those are boiler-side faults, often in the diverter valve, flow sensor or plate heat exchanger. If that sounds like yours, our no heating and no hot water repair service covers London, and we will tell you if the fault turns out to be in the shower rather than the boiler.