If your water is too hot you can adjust the thermostat on the front of a combi or use the tank thermostat to change the temperature of stored hot water.

Two types of hot water

The two most common types of hot water system are instantaneous (mostly combi boilers) and stored (mostly hot water tanks).

Combi boilers

Combi boilers have in most cases a dial on the front to adjust the temperature of the hot water. This is designed to be used by the end user so adjust it as much as you want.

Stored hot water

Stored hot water 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 hot water temperature above this ensures the water is safe.

The other reason to keep your hot water hot is the storage cylinder is relatively small compared to the amount of water you need. The volume of the hot water 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 added to hot water systems to reduce the temperature of the water at 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. The temperature can be adjusted but only within the limits set by the valve. The valve won’t allow very hot water to flow to your taps.

You can buy and fit your own replacements as it is just a plumbing job. A replacement is available on this link.

Hot water controls don’t work

If you have turned the thermostats down and the water is still too hot 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.

Summary

The temperature of your hot water can be decided by you by using the thermostats that your boiler system has built-in.

If you turn the thermostats down and the water is still too hot you may have a fault.