No Hot Water to the Shower Only

Prove it is only the shower

Run the hot tap on the bathroom basin, the nearest outlet on the same pipe, for a minute. If it gets properly hot, the heat source is working and you can ignore the boiler. If the basin is also cold or lukewarm, this is a whole-house problem and why have I got no hot water is the better starting point.

Then work out what kind of shower you have, because each fails differently, whether it runs cold or stops giving any water at all.

• Electric shower: a white box on the wall fed only by cold mains, with its own pull cord or isolator switch. It heats its own water, so the boiler has nothing to do with it.
• Mixer shower: one or two handles on a valve, either a bar on the wall or a valve built into it, fed by both hot and cold pipes. Most are thermostatic.
• Power shower: a mixer with a built-in pump, or a separate pump in the airing cupboard, on a gravity system with a loft tank.

Mixer shower: the thermostatic cartridge

A thermostatic mixer holds the temperature steady with a cartridge containing a wax element. The wax expands and contracts with the water temperature and moves a shuttle that blends hot and cold. Over time scale jams the shuttle or the wax element weakens, and the cartridge stops letting enough hot water through. The symptom is a shower that gets steadily cooler over weeks, or will only go lukewarm with the handle turned right round.

Many thermostatic valves also have a temperature stop, a button or collar on the handle that limits how far it turns. If someone has reset the handle after cleaning, or a child has fiddled with it, the stop can end up set too low. Check you can press the override button and turn past it before assuming a fault.

The cartridge is a replaceable part. It is specific to the make and model of valve, so the maker name on the valve body or the handle matters. Descaling sometimes brings an old one back, but a new cartridge is usually the lasting fix.

Isolate the valve before taking the cartridge out. Many shower valves have small screwdriver isolating valves behind the handle plate or on the inlet elbows; if yours has none, the job means turning off the water to the house.

Mixer shower: check valves and crossflow

Most mixer showers have small check valves (non-return valves) in the hot and cold inlets, often built into the wall elbows of a bar mixer. They stop the higher-pressure side pushing water into the lower-pressure side. When the one on the hot side sticks open, mains cold forces its way back into the hot pipe and the shower never gets hot, even with the cartridge working.

A simple sign of crossflow is a basin hot tap that runs cold or lukewarm for longer than usual after the shower has been used. On a gravity system with mains cold, it is worse, because the hot side is at a far lower pressure than the cold.

That pressure mismatch is also why a shower valve sometimes fails straight after new work. A mixer designed for balanced pressures, fitted where the hot comes from a loft tank and the cold from the mains, will struggle to get hot at all. The engineer will check the valve is suitable for your system before replacing parts.

Combi boiler: the shower is not drawing enough flow

On a combi, the boiler only fires when enough water runs through it. The minimum is often in the region of 2 litres a minute, but it varies by model and the installation manual gives the figure. A shower with a water-saving head, a flow restrictor, a scaled head or a partly closed isolating valve can fall below that. The boiler fires, cuts out, fires again, and the shower swings between warm and cold.

Take the head off and run the hose into the tray. If the water gets hot without the head, clean the head in descaler or replace it. If it still stays cold, go back to the cartridge and check valves. When the taps are also lukewarm, the fault is in the boiler and combi boiler hot water not working takes over.

Electric and power showers

An electric shower runs cold when its heating element fails, its thermal cut-out has tripped, or its pressure relief device has operated. The thermal cut-out trips when flow drops too low and the water inside overheats, often because of a blocked head or kinked hose. Switch the shower off at the isolator, leave it to cool, clean the head, and try again; if it cuts out again, stop using it. A pressure relief device that has operated is normally replaced rather than reset, and that means opening the unit, which is a job for an electrician or engineer. More on this in electric shower stopped working.

Anything inside an electric shower is mains electrical work on a high-current circuit. If the pull cord or isolator feels hot, smells of burning or the breaker keeps tripping, switch it off at the consumer unit and leave it off.

A power shower with a pump can lose its hot side if the pump impeller is airlocked or failing on the hot inlet, or if the loft tank is running low. The water may come through strong but cold, or cut in and out. Bleeding the hot side of the pump or checking the loft tank level usually shows which.

What the engineer does and when to call

A cold shower is rarely an emergency, but it is miserable, and in homes with small children or someone who cannot use a bath it is worth fixing the same day. Call when the cartridge and check valves need taking apart and you have no local isolating valves, when an electric shower shows any sign of heat damage, or when the fault started right after other plumbing work.

The engineer measures the hot temperature and flow at the valve inlets, checks for crossflow, tests or swaps the cartridge, and confirms the valve suits your system. If a separate thermostatic mixing valve sits on the hot supply to the bathroom, it is checked too: Part G of the Building Regulations limits the hot water to a bath in new homes to 48°C, so many newer bathrooms have one, and if it also serves the shower, a scaled one can starve it. Our no hot water repair visit starts with the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts.