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Quick answer
When a combi’s hot water swings between hot and cold, the boiler is struggling to hold a steady output for the flow of water passing through it. Common causes are a flow rate too low for the boiler to stay lit reliably, a scaled or blocked plate heat exchanger, a sticking diverter valve, a faulty temperature or flow sensor, or changes in mains pressure when other taps are used. A thermostatic shower valve that is failing can also cause swings. Some causes are simple to check; most boiler-side causes need a Gas Safe engineer.
Key takeaways
- Combis modulate their output to match flow; very low flow can make them cycle on and off.
- Hard water scale in the plate heat exchanger is a common cause of fluctuating temperature.
- A diverter valve not fully in hot water position reduces and destabilises hot water output.
- Temperature and flow sensors tell the boiler what to do; faults cause erratic behaviour.
- Mains pressure changes from other taps affect combi showers.
Low flow causing cycling
A combi needs a minimum flow of water to fire and stay firing. If you run the tap or shower at a trickle, the boiler may light, overheat quickly, shut off, then relight as the water cools. The result is hot, cold, hot. Try opening the tap more and mixing with cold at the tap rather than reducing flow.
Scale in the plate heat exchanger
Hard water leaves limescale inside the plate heat exchanger, reducing its ability to transfer heat and causing uneven temperatures. It is common in hard water areas. A descale or replacement of the plate heat exchanger usually cures it. A scale reducer on the cold mains feed helps prevent it coming back; our boiler repair team can advise.
Diverter valve
If the diverter valve is not fully moving to hot water, some heat goes to the radiators and output to the tap is weaker and less stable. See diverter valve problems and radiators hot when the heating is off.
Sensors
Combis use a flow sensor to detect demand and temperature sensors (thermistors) to control output. A faulty sensor can make the boiler over- or under-react. An engineer can test sensor readings.
Mains pressure and other taps
When someone flushes a toilet or runs a tap, the pressure and flow to the shower change. With a combi, that changes the temperature too. See shower goes cold when a tap is turned on.
Shower valve
A thermostatic shower valve with a sticking cartridge can cause swings regardless of the boiler. If all hot taps are stable but the shower is not, suspect the shower valve.
What you can check
- Does it happen at every hot tap, or only one?
- Does opening the tap fully help?
- Does it happen when other water is being used?
- Is the boiler’s hot water temperature set sensibly? See combi hot water temperature setting.
Book with Gasmaster
Hot water refusing to stay hot? Gasmaster Plumbing & Heating, Gas Safe registered (220932), repairs combi boilers across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our boiler repair service.
Frequently asked questions
Why does my combi shower go hot and cold?
Low flow, scale, the diverter valve, sensors or mains pressure changes are the usual causes.
Can I descale my combi myself?
Descaling the plate heat exchanger means opening the boiler and should be done by a Gas Safe engineer.
Is fluctuating hot water a sign my boiler is failing?
Not necessarily. Most causes are repairable parts or settings.
Sources
- Manufacturer servicing manuals for combi boilers
- Energy Saving Trust, water heating guidance (energysavingtrust.org.uk)
- Gas Safe Register (gassaferegister.co.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
