If your hot water is too hot you can adjust the thermostat on your combi or adjust the tank thermostat of your tank/cylinder. If they don’t work you may have a fault on your system.
Two types of hot water
The two most common types of system are instantaneous (mostly combi boilers) and stored (mostly tanks or cylinders).
Combi boilers
Combi boilers have in most cases a dial on the front for temperature adjustment. This is designed to be used by the end user so adjust it as much as you want.
Stored hot water
A cylinder or tank is heated by a gas boiler or by an electric immersion heater. The temperature is controlled by a thermostat on the tank or on the immersion heater.
Stored water must be heated to above 60ºC at least once per day to avoid health problems.
Lukewarm water is an amazing breeding ground for all sorts of unpleasant microbes such as legionnaires disease. Legionnaires disease doesn’t survive above 60ºC so keeping the temperature above this ensures the water is safe.
The other reason to keep the temperature high is the storage cylinder is relatively small compared to the amount of water you need. The volume of the storage cylinder is probably just enough to fill a bath. But if you use half the water from the tank and dilute it with cold from the main you could get two baths from the same small tank. This would save time heating the water again before use.
Thermostatic mixing valve
Thermostatic mixing valves (also known as blending valves) are installed to reduce the temperature of the water coming from the taps. This allows the water to be stored at a high temperature (above 60ºC) in the tank but still delivered to the taps without the risk of scalding.
They tend to be used in situations where scalding is very likely such as in schools or in sheltered housing.
The blue cap on the blending valve in the picture can be taken off to reveal a nut to adjust the temperature.
Hot water controls don’t work
If you have turned the thermostats down and the temperature is still too high then there may be a fault with your boiler or controls. This may be a valve to the hot water tank not fully closing or maybe even a faulty thermostat.
Faulty control thermostat
The temperature of the water coming from your combi boiler is regulated by a control thermostat. The thermostat can give the boiler a false reading, causing it to produce incorrect temperatures. This fault requires a gas-safe engineer to replace it.
Faulty PCB
A similar fault can be caused by the control board (PCB) recieving the right information from the control thermostat but reading it wrong. This also needs a gas safe engineer to repair it.
Faulty diverter valve
The boiler controls the temperature of the stored water through the use of a zone valve or diverter valve attached to the thermostat. When the correct temperature is reached the valve closes stopping the flow from the boiler.
If the zone valve can’t close properly or sends the wrong signal to the boiler the tank can continue heating past the point where the water is hot enough.
This requires a qualified plumber and possibly an electrician to replace the valve.
Faulty thermostat
The thermostat is designed to turn off the heat supply when the correct temperature is reached. If it has gone out of calibration the incorrect temperature can be set.
This requires a replacement thermostat which should be installed by a qualified heating engineer or electrician.
Summary
The temperature of your hot can be decided by you by using the thermostats that your boiler system has built in.
If adjusting the thermostat doesn’t reduce the temperature you may have a fault.