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Combi boilers are a lot more reliable than they were ten years ago. However, combi boilers are prone to the common problem of causing the hot water to go cold or lukewarm and then back to hot again. Symptoms like these are usually signs of a blockage in the hot water heat exchanger in the boiler.
Beside this, why does my hot water temperature fluctuate?
Adjust the thermostat brackets if necessary to keep the thermostats snug against the tank. A failed thermostat or heating element in an electric water heater can also cause hot water temperature fluctuations. Sediment build-up at the bottom of the tank can cause hot water temperature fluctuations.
Consequently, why does my combi boiler run out of hot water?
If your home runs on a combi-boiler, then low pressure may be the reason for your lack of hot water. The water pressure for a standard domestic boiler is usually around 1.5 bar and will be displayed on a gauge known as a 'Manometer', usually located where the water supply pipes are connected to the boiler.
To adjust the mixed outlet temperature of the valve, remove the cap to gain access to the adjusting spindle. The spindle should be rotated-clockwise to reduce the temperature, counter-clockwise to increase the temperature until the desired set point is reached.