Plumbing & water
Shower running cold, weak or scalding?
Shower faults sound identical from inside the cubicle, but an electric shower, a mixer and a power shower fail in completely different ways. Identify which you own, descale the head before blaming anything, and the symptom usually names the part.
By the Tickover team · Published 27 May 2026 · 4 min read
How urgent is this?
This month, unless someone is being scalded
A weak or lukewarm shower can wait for the right fix. A thermostatic mixer that no longer holds its temperature is urgent in a house with children or older people.
First, know which shower you own
Electric showers have their own cold feed and heat water on demand: a pull-cord or wall switch, a thick cable, and a box on the wall doing the work. Mixer showers blend the home’s existing hot and cold, so they inherit every quirk of your boiler or cylinder. Power showers are mixers with a built-in pump, found on gravity-fed systems. Digital showers are mixers with better manners.
| Shower type | Usual fault | First move |
|---|---|---|
| Electric | Scale-blocked head, tired heater unit | Descale the head, then electrician |
| Mixer | Worn or scaled thermostatic cartridge | Descale, then match a new cartridge |
| Power | Pump losing pressure, or airlocked | Check and bleed the pump, then plumber |
| Combi-fed mixer | Winter flow-rate drop | Lower the flow, nudge hot water up |
Descale the head before blaming anything
Much of Britain showers in hard water, and a scaled-up head causes weak flow, odd spray and even overheating: an electric shower cuts the heat when a blocked head throttles the flow, because its safety cut-out senses the water moving too slowly. Ten minutes with vinegar rules all of that out.
The vinegar bag descale
Mix warm water and white vinegar
Half and half, in a freezer bag or a bowl deep enough to submerge the head.
Soak the head for 30–60 minutes
Unscrew it, or bag it in place with a rubber band around the neck. Go shorter on plated or brass finishes, and check the maker’s advice first.
Scrub the nozzles
An old toothbrush across the jets, and a thumb-rub over rubber nozzles to crack off the last of the scale.
Rinse and run it hot
A minute of full flow flushes loosened scale out of the head instead of back into your eyes.
If descaling was not the answer
Electric showers that run cold, cycle hot and cold, or trip the pull-cord have usually worn out a microswitch, thermal cut-out or the heater unit itself. These are sealed, mains-fed boxes running at cooker-level current: do not open the casing, ever. Replacement or repair is an electrician’s job, and if the unit trips the circuit at the consumer unit, leave that circuit off until they arrive.
Mixer showers that drift scalding or cold have a tired thermostatic cartridge, the part whose whole purpose is holding your chosen temperature as pressures wobble. Cartridges are model-specific, so photograph the shower and source the exact part before anything comes apart. Power showers that fade or surge have a pump problem: airlocks after the water has been off, or an impeller wearing out noisily.
Combi-fed mixers get feebler every winter, by design: the incoming mains is colder, so the boiler can only reach your set temperature by slowing the flow. Nudge the boiler’s hot water dial up or run the shower a touch below full open; anything beyond the dials, inside the boiler, is work for a Gas Safe registered engineer. Poor flow in July too? The season is innocent: see low water pressure and, on cylinder systems, the hot water cylinder guide.
Getting it fixed
Plumber call-out, first hour
parts on top
Plumber, per extra hour
Electrician, per hour
Typical UK ranges, mid-2026, as a guide only. Your home, your area and the job itself all move the number — always compare real quotes.
Not sure what you’re dealing with?
Tell Self-Diagnose your shower type and symptom, cold, cycling, weak or scalding, and it will point at the likely part and line up the right vetted trade, plumber or electrician, first time.
While you are thinking about the bathroom: a shower that works hard deserves an extractor fan that actually extracts, or the room pays in mould. And for everything upstream of the shower valve, the home plumbing guide has the map.
Quick answers
Why does my shower keep going hot and cold?
On a mixer, a worn thermostatic cartridge has stopped compensating as pressures shift, and a toilet flush or washing machine fill swings the temperature. On an electric shower, a scale-blocked head or failing thermal cut-out makes it cycle. On a combi, too little flow makes the boiler start and stop. Descale first, then pursue the part.
Why is my electric shower running cold?
The usual suspects are the microswitch that starts heating, the thermal cut-out, or the heater unit itself, and they fail with age and scale. Do not open the casing: an electric shower is a sealed, high-current appliance. Have an electrician test and replace it, and leave the circuit off if the shower has tripped it.
How do I descale a shower head?
Soak it for 30 to 60 minutes in a half-and-half mix of warm water and white vinegar, either unscrewed into a bowl or bagged in place with a rubber band. Scrub the nozzles with an old toothbrush, rub rubber jets with a thumb, then rinse and run the shower hot for a minute. Go gently on plated finishes.
Why is my shower weaker in winter?
On combi systems this is physics, not failure: winter mains water arrives several degrees colder, and the boiler can only hit your set temperature by slowing the flow rate. Turning the flow down slightly or raising the boiler’s hot water setting a notch restores comfort. Weak flow in summer as well points to a pressure or scale problem instead.
This guide is general information for UK homes, not advice about your specific property. Anything involving gas must be done by a Gas Safe registered engineer, and notifiable electrical work by a qualified electrician. If you smell gas, call the National Gas Emergency line on 0800 111 999. If you’re ever unsure, stop and get a professional in — that’s exactly what Tickover is for.
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