Domestic hot water should never be above 120F. Above that temperature the excessive heat tends to lead to lime scale and corrosion of pipes and apparatus. Lime scale increases heating costs by insulating the water from the heating source, and therefore more energy is needed to heat the water. Moreover, a badly maintained boiler can also lead to air pollution within the home.

To check whether your apparently harmless boiler is poinoning the air in your home or contributing excessively to warming the planet, have it serviced once every year.