Hot water that goes hot, then cold, then hot again from a combi boiler is usually the boiler struggling to keep up with the flow of water through it. Combis heat water on demand, and if the flow is too low the boiler overheats and cuts out for a moment, then relights, which you feel as a cycle in the shower. The causes are a scaled plate heat exchanger, a partly blocked filter or flow sensor, a sticking diverter valve, low flow through a modern shower valve, or a boiler that is undersized for the outlets running. A few of these you can check yourself; most need a Gas Safe engineer and are routine repairs.
This guide works through the causes from the outlet back to the boiler, explains what each one feels like and tells you what to do.
What is happening inside the boiler
When a hot tap opens, a flow sensor in the combi detects water moving and the boiler fires, sending heat through a plate heat exchanger that warms the mains water on its way to the tap. The boiler modulates its output to hold the set temperature. If the flow is low or erratic, the water passes the exchanger too slowly, overheats, and the boiler cuts out on its overheat stat until the temperature drops. Then it fires again. That on-off cycle is the hot-and-cold you feel.
Causes at the outlet
Low flow at the shower
Modern thermostatic shower valves and water-saving heads can restrict flow below the level the combi needs to run steadily. If the problem is only in the shower and the kitchen tap runs steady and hot, try removing the flow restrictor from the head or opening the flow control fully. Our guide to boiler flow rate explains the minimum flows.
A mixer tap or shower valve passing cold
A worn thermostatic cartridge in a shower valve can let cold water bleed into the hot side, which the boiler cannot compensate for. Test another hot outlet; if the kitchen tap is fine, the shower valve is the suspect.
Causes inside the boiler
Scaled plate heat exchanger
In hard water areas, scale builds up inside the plate heat exchanger, narrowing the passages and making the water overheat in places. The classic sign is hot water that runs warm rather than hot, or cycles, while the heating is fine. The exchanger is descaled or replaced, and a scale reducer on the incoming main slows it recurring.
Blocked flow sensor or filter
The flow sensor, often a small turbine, and its filter collect debris from the mains. A partly blocked sensor reports erratic flow and the boiler modulates up and down in response. Cleaning is a routine repair.
Sticking diverter valve
The diverter switches the boiler between heating and hot water. A valve that does not fully move to the hot water position sends some heat to the radiators instead, and the tap water fluctuates. Diverter valves and their diaphragms are common combi repairs. Our guide to heating not working but hot water is covers the opposite symptom from the same part.
Faulty temperature sensor
The hot water thermistor tells the boiler how hot the water is. A failing one gives erratic readings and the boiler overshoots and undershoots.
Low system pressure or air
Some combis reduce hot water performance when system pressure is low or there is air in the primary circuit. Check the gauge and top up to around 1.3 bar.
The boiler is too small for the demand
A compact combi running two showers, or a shower and a running bath, simply cannot deliver the flow at temperature, and the output swings. That is not a fault, it is a sizing issue, and the answer is either a higher-output combi with a mains supply to feed it, or a system boiler with a cylinder. Our guide to what size boiler you need explains the sums.
What you can check before calling
- Does it happen at every hot outlet, or only the shower?
- Is the boiler pressure above 1 bar?
- Is the hot water temperature on the boiler set sensibly, around 50 to 55 degrees, rather than at maximum?
- Does it happen with only one outlet running?
- Is your water hard, and when was the boiler last serviced?
How DCI Boilers can help
DCI Boilers diagnoses hot water faults on combis across Cornwall and Devon, from plate heat exchangers and diverter valves to flow sensors, with Gas Safe registered engineers who carry the common parts and quote a fixed price before starting. Book boiler repairs, or if the boiler is undersized for your home, our gas boiler replacement survey is free. Regular boiler servicing in a hard water area keeps scale in check.
Need help now? Call DCI Boilers on 07888 862 603 or get a free quote.
Frequently asked questions
Why does my shower go hot and cold with a combi boiler?
Usually low flow through the shower valve, a scaled plate heat exchanger or a blocked flow sensor. Test another tap to narrow it down.
Can I fix a combi going hot and cold myself?
You can check the pressure, the temperature setting and the shower head restrictor. Anything inside the boiler needs a Gas Safe engineer.
Does hard water cause hot water problems?
Yes. Scale in the plate heat exchanger is one of the commonest causes, and a scale reducer on the main slows it down.
Is a combi going hot and cold dangerous?
No, it is a comfort and performance issue, though it is a sign the boiler is working harder than it should and worth fixing before winter.