Before you touch the shower, run the hot tap at the kitchen sink or the bathroom basin for a minute. That one test splits the problem in half. If the tap runs hot and the shower does not, the hot water is reaching the bathroom and something inside the shower valve is stopping it. If nothing in the house runs hot, the shower is the wrong place to look, and hot water failure: what to check first is the page you want.
What the first test tells you
| What you see | Where the fault usually is |
|---|
| Kitchen tap hot, shower cold or lukewarm | The shower valve: cartridge, inlet filter, check valve or temperature stop |
| Shower hot for a few seconds, then cold | A combi that stops firing because the flow is too low, or a thermostatic cartridge shutting the hot side |
| Shower goes hot and cold on its own | A cartridge that can no longer balance the two supplies, or another outlet stealing flow |
| Basin hot, bath and shower both cold | An isolation valve on the hot feed to that bathroom, or an airlock on a gravity system |
| No tap in the house runs hot | The boiler or cylinder, not the shower |
| Electric shower runs cold, taps fine | The shower unit itself. It heats its own water |
Mixer showers: the cartridge does most of the failing
A mixer shower takes a hot and a cold supply and blends them inside the valve. On a thermostatic mixer, a cartridge with a wax or bimetal element opens and closes the hot and cold ports to hold the temperature you set. If the cold supply drops, it cuts the hot so nobody is scalded, which is why a scaled or worn cartridge tends to fail towards cold.
London water is hard, and scale is the reason most shower cartridges fail here. The element sticks, the ports fur up, and you get a shower that will only go lukewarm however far the handle turns. On an exposed bar valve this is usually a straightforward cartridge change. On a valve buried in the wall it is the same part, but getting to it is the job. Hard water scale in London explains why the same deposits turn up in cylinders and immersion heaters too.
Three smaller parts on the mixer are worth knowing about:
- Inlet filters. Most mixers have a small mesh filter where each supply enters the valve. They catch debris and scale, especially after pipework has been disturbed, and choke the hot side.
- Check valves. Mixers are normally fitted with non-return valves on the inlets so hot cannot push into cold or the other way round. A check valve that sticks shut starves the valve of hot. One that fails open lets the stronger supply push into the weaker one, which is how cold water ends up coming out of hot taps elsewhere in the house. No hot water when it is actually a leak covers that crossover in more detail.
- The temperature stop. Most thermostatic valves have a limit stop that caps how far the handle turns towards hot. If the handle has been taken off and refitted one spline out, the valve simply cannot reach hot. It looks like a fault and is not one.
Combi boilers: the shower may not be asking for enough water
A combi only heats water while it senses water flowing through it, and it needs a minimum flow before it will fire. A shower head full of scale, a flow restrictor, or a thermostatic valve throttling the hot side can drop the flow below that point. The boiler fires, the flow dips, the boiler stops, and you get the familiar pattern of hot for a few seconds and then cold. Descale or swap the shower head first. If the kitchen tap on full stays hot but the shower still cycles, it is the valve or the flow to it. Hot water pressure versus flow rate explains why a weak shower on a combi is a flow question, not a pressure one.
If the combi will not give hot water at any tap, that is a diverter valve, a sensor or the heat exchanger, and it is gas appliance work for a Gas Safe registered engineer, not a shower repair.
Gravity systems and pumped showers
With a cold tank in the loft and a cylinder in the airing cupboard, hot water reaches the shower by falling from the tank. Two things go wrong there that show up only at the shower. The first is air: after the water has been off or the tank has run low, an airlock in the hot feed can leave the shower spluttering or dead while taps lower down still run. Airlock or leak in a hot water system shows how to tell the two apart, because a system that keeps getting airlocks is usually drawing air in somewhere. The second is the pump. A shower pump that has failed on its hot side runs cold, and one that is leaking usually shows it on the floor of the airing cupboard first; see shower pump leaking.
Electric showers are a different appliance
An electric shower heats cold water from the mains inside the unit, so the boiler and the cylinder have nothing to do with it. A cold electric shower is usually a failed heating element, a tripped thermal cut-out, or a pressure switch that thinks there is not enough water. That is for the manufacturer or an electrician. A unit weeping from its case or inlet is a plumbing problem too, and should be isolated at the fused switch before anyone touches it.
Checks you can do safely
- Run the kitchen hot tap for a minute and note whether it is hot. That decides everything else.
- Take the shower head off and run the hose into a bucket. If the flow is strong and hot without the head, the head is scaled. Soak it in descaler or replace it.
- Check any isolation valves on the hot feed to the shower. A slotted valve turned across the pipe is closed. If a plumber has recently worked in the bathroom, this is the first suspect.
- Turn the temperature handle through its whole range slowly and listen. A valve that does not change at all, or changes only at one end, points at the cartridge.
- Look below the bathroom. A fresh mark on the ceiling under the shower, or on the wall behind a concealed valve, changes the job. More on that below.
Stop there. Taking a cartridge out means isolating both supplies properly, and on a valve with no isolation of its own that means the stopcock and a drained-down hot side. A cartridge refitted without its seals seated is a leak inside the wall.
When a shower fault is also a leak
Most showers that have lost their hot water are not leaking. The ones that are tend to be concealed valves. The same worn seals and scaled bodies that stop a valve mixing properly also let it weep, and a concealed valve sits at full supply pressure all day inside a stud or masonry wall. The signs are a stain on the ceiling below that grows whether or not anyone has showered, grout that stays dark behind the valve, a shower head that drips steadily when the shower is off, or a water bill that has crept up. Shower leaking through the ceiling has a simple timing test that tells a valve leak from a tray or tile leak in a few hours. If the drip at the head is the main symptom, shower head dripping explains which part of the valve has stopped shutting off.
What the repair involves
On an exposed valve, the supplies are isolated, the cartridge, filters and check valves are taken out, and the parts are cleaned or replaced to match the make. The valve is reassembled, the temperature stop is reset against a thermometer at the outlet, and the joints are checked with the water back on.
On a concealed valve, the question is how to get to it. Many are serviced from the front through the trim plate. Where the body itself has failed or is weeping, the wall has to be opened. We map the wet area first so the tiling is opened at one point rather than stripped, repair or replace the valve, pressure test it, and then make good: tanking, tiles and grout on the shower side, and the ceiling below if it has been wet. That is bathroom and shower leak repair, and it is the part of this problem we are set up for.
We charge £150 per hour, agreed before we attend, with no call-out fee. If the job turns out to be a hidden leak and we do not find it, you do not pay for the time spent looking. The full basis is on the pricing page.
When it is not us
- No tap in the house runs hot. That is the boiler, the cylinder or their controls. Start with what to check first.
- A cartridge change on an exposed bar valve with nothing leaking. Any competent plumber can do it, and plenty of homeowners do it themselves with the right part.
- An electric shower that has gone cold. The manufacturer or an electrician.
- A combi that will not heat water anywhere. A Gas Safe registered engineer.
- You rent. Report it to your landlord in writing. Hot water is part of the repairing obligation, and landlord obligations for hot water and leaks sets out what that covers.
Call us when the valve is buried in the wall, when there is water where it should not be, or when a shower has already been "fixed" once and the ceiling below is still marking.
