If your hot water keeps going hot and cold, first check whether the problem affects every hot tap or only one shower or tap.
If all outlets fluctuate, the boiler or hot-water supply may be responsible. If only one outlet is affected, the problem is more likely to be with that tap, shower or its water supply.
With a combi boiler, fluctuating hot water can be caused by changing water flow, a restricted plate heat exchanger, faulty sensors or another problem controlling domestic hot water. The pattern of the temperature changes can help narrow down the cause.
Does every hot tap go hot and cold?
Before investigating the boiler, try several hot-water outlets around the property.
If every hot tap alternates between hot and cold, the problem is likely to be somewhere they share, such as the boiler or incoming water supply.
If only one shower or tap is affected while the others maintain a steady temperature, start with that outlet instead. A faulty thermostatic mixer, restricted shower head or local flow problem could be responsible.
You can also check whether the problem changes when other people use water elsewhere in the property.
| What happens? | Where to investigate first |
| Every hot tap goes hot and cold | Boiler or hot-water supply |
| Only one shower fluctuates | Shower mixer, cartridge or flow |
| Only one tap fluctuates | Tap/mixer or local restriction |
| Temperature changes when another tap opens | Water flow and simultaneous demand |
| Combi repeatedly fires and stops | Boiler or heat-transfer problem |
| Hot water repeatedly cycles hot-cold-hot | Heat exchanger, sensor or another DHW fault |
These patterns provide clues rather than a definitive diagnosis.
Why can a combi boiler produce fluctuating hot water?
A combi boiler heats mains water on demand instead of storing a cylinder of hot water. When you open a hot tap, the boiler detects the demand, fires and transfers heat to the water passing through it.
To maintain a reasonably stable outlet temperature, the boiler needs to respond to changes in water flow and temperature.
If water flow changes significantly or a component responsible for detecting, heating or controlling the water isn’t working correctly, the temperature at the tap can fluctuate.
A combi’s ability to provide stable hot water therefore depends partly on its hot-water flow rate, as well as the incoming mains supply, boiler output and temperature rise.
Could the plate heat exchanger be restricted?
A restricted plate heat exchanger is one possible cause of hot water repeatedly going hot and cold on a combi boiler.
The plate heat exchanger transfers heat from the boiler’s primary heating circuit to the mains water that eventually comes out of your taps.
Its internal waterways are relatively narrow. Limescale or debris can restrict water movement or interfere with heat transfer, particularly over time.
One possible result is a repeating cycle. The boiler heats the water, temperatures rise too far inside the appliance and the burner shuts down. Once things cool sufficiently, it fires again.
At the tap, this can feel like:
Hot → cold → hot → cold
However, this pattern doesn’t prove that the plate heat exchanger is responsible. Flow, sensor and other boiler problems can produce similar symptoms.
Cleaning or replacing an internal heat exchanger isn’t a homeowner repair and should be diagnosed by a suitably qualified engineer.
Can low water flow make hot water go cold?
Yes. Water flow is particularly important with a combi because the boiler needs to recognise a hot-water demand and then heat the water as it passes through.
Individual combis have minimum flow requirements. If the flow falls too low, the appliance may stop recognising a continuous hot-water demand or operate inconsistently.
Changing flow can also affect outlet temperature. A boiler has a limited amount of heating power, so the amount of water passing through it and the required temperature rise both influence hot-water performance.
If the problem affects several outlets, check whether the cold mains flow also seems unusually weak. If the flow problem is limited to one tap or shower, a restriction at that particular outlet becomes more likely.
Why does the shower go cold when another tap is opened?
If the problem occurs when someone uses another tap, it doesn’t necessarily mean the boiler is faulty.
A combi has a maximum amount of hot water it can supply in a given period. Opening a second outlet increases the demand, while the available mains water supply also has to be shared.
For example, if a boiler can provide 12 L/min of hot water under particular conditions, that doesn’t mean it can provide 12 L/min independently to two showers at once. Both outlets are drawing from the available supply.
Changes in mains pressure and flow can also affect the balance between hot and cold water at a mixer shower.
Try the affected shower or tap while no other water is being used. If the temperature becomes stable, simultaneous demand may be contributing to the problem rather than an intermittent boiler fault.
Could a flow or temperature sensor be faulty?
A combi uses sensors to monitor what’s happening while producing hot water.
The boiler first needs to recognise that you’ve opened a tap. If the component responsible for detecting water flow isn’t working reliably, the boiler may intermittently lose the hot-water demand.
Temperature sensors also provide information the boiler uses to control its operation. Incorrect readings could cause the burner to reduce its output or stop when it shouldn’t.
Sensor faults can therefore cause unstable hot-water performance, but they aren’t something that can reliably be diagnosed from tap temperature alone.
If the boiler display shows a fault code when the water turns cold, note the code and check the manufacturer’s instructions.
Could the diverter valve be responsible?
The diverter valve is another possible cause on a combi boiler.
When you request hot water, the boiler needs to direct heat towards the domestic-hot-water side of the system. If the valve sticks or doesn’t move properly, heat may not be directed where it’s needed consistently.
One clue can be radiators becoming warm while you’re running hot water even though the central heating isn’t supposed to be on.
That doesn’t confirm a faulty diverter valve, however. Several boiler faults can cause unstable hot-water temperatures, so an engineer should diagnose the component before it’s repaired or replaced.
What if only the shower goes hot and cold?
If the taps maintain a steady temperature but your shower doesn’t, the boiler becomes a less likely starting point.
A thermostatic mixer shower continuously balances hot and cold water to maintain the selected temperature. A worn or faulty thermostatic cartridge can interfere with that process.
Other possibilities include a restricted shower head, changing water pressure or another outlet being used at the same time.
Try other hot taps while the shower is fluctuating. If they’re consistently hot, investigate the shower before assuming there’s a boiler problem.
Cleaning a removable shower head according to its manufacturer’s instructions may also help if scale has significantly restricted the flow.
What if you have a hot-water cylinder?
Fluctuating water isn’t limited to combi boilers, although the causes can be different with stored hot water.
If you have a system or conventional boiler, hot water is normally stored in a cylinder rather than heated instantly each time you open a tap.
Check whether the cylinder actually contains sufficient hot water. If several people have recently used showers or baths, the stored supply may simply have been depleted and need time to reheat.
If the cylinder contains hot water but only one shower fluctuates, the shower or mixer remains a more likely place to start investigating.
What can you safely check yourself?
You can gather useful information without opening the boiler or dismantling components.
Check:
- Whether every hot outlet is affected.
- Whether the temperature fluctuates when no other water is being used.
- Whether cold-water flow also appears unusually weak.
- Whether the boiler repeatedly fires and stops while a hot tap remains open.
- Whether a fault code appears when the water turns cold.
- Whether radiators become warm when you’re requesting only hot water.
- Whether a cylinder system has had enough time to reheat after heavy hot-water use.
These observations can also make it easier for an engineer or plumber to diagnose the problem.
When should you call a plumber or heating engineer?
Who you call depends partly on where the problem appears to be.
If only one tap or shower is affected while every other hot outlet works normally, a plumber may be the appropriate starting point.
If every hot tap fluctuates and you have a combi boiler, particularly if the boiler repeatedly stops firing, displays fault codes or behaves unusually when a tap is opened, have the boiler investigated.
Internal faults involving heat exchangers, sensors, diverter valves and other boiler components aren’t suitable for DIY repair. Work on a domestic gas appliance should be carried out by a Gas Safe registered engineer who is competent for the work involved.
Frequently asked questions
Why does my hot water keep going cold and then hot again?
With a combi boiler, possible causes include changing water flow, a restricted plate heat exchanger, faulty sensors or another domestic-hot-water fault. Check whether every outlet is affected and whether the boiler stops firing when the water turns cold.
Why does my shower go hot and cold but the taps are fine?
If other hot taps remain at a stable temperature, the problem is more likely to be associated with the shower. A thermostatic cartridge, restricted shower head or changing hot/cold pressure could be responsible.
Why does my shower turn cold when someone opens a tap?
The additional outlet changes the demand on the water supply. With a combi, both outlets also have to share the boiler’s available hot-water capacity. This can affect flow and temperature without necessarily indicating a boiler fault.
Can a blocked heat exchanger cause fluctuating hot water?
A restricted plate heat exchanger can interfere with heat transfer and may cause unstable hot-water temperatures. However, fluctuating water isn’t enough on its own to confirm that the heat exchanger is blocked.
Can limescale make hot water go hot and cold?
Limescale can restrict components involved in producing or delivering hot water, particularly in hard-water areas. The effect depends on where the scale has accumulated, so don’t assume limescale is responsible without investigating the wider symptoms.
Sources
HSE, Gas safety – Home owners (https://www.hse.gov.uk/gas/domestic/faqownerocc.htm)
Accurate as of: 24 September 2026