Dr Deane Hutton on Water Heaters

WATER HEATERS

Hot water is needed in homes and commercial buildings. What is the best way to heat water - electricity, gas, solar or heat pump? I'm glad you asked! Traditionally, hot water systems have used electricity or gas to heat the water. Solar water heaters and heat pumps are newer systems which have claims of greater energy efficiency.

ELECTRIC HOT WATER SYSTEM

Electric hot water systems are like giant kettles. They have an electric heating element which heats the water to a high temperature and stores the hot water in a tank. These systems are not very energy efficient, .since they heat water to a temperature which is higher than needed to shower, bathe, or wash hands. For these activities, we add cold water to lower the temperature. Some of the newer electric hot water systems have a thermostatic control which heats water to the desired temperature on demand. They do not waste energy by storing a large volume of very hot water in a tank. Consequently, on-demand systems are more energy efficient.

GAS HOT WATER SYSTEM

Many gas hot water systems use a gas flame to heat water to a high temperature and then store it in a tank. Not very energy efficient! Newer gas hot water systems are on-demand devices with a thermostat to select the desired temperature. Much more energy efficient!

SOLAR HOT WATER SYSTEM

Solar hot water systems use energy from the sun to heat water flowing through tubes in special panels on the roof. In the summer, solar systems produce more hot water than needed by the average household. And the running cost for the solar hot water system is zero. However, in the winter there are two problems - less sunshine and more energy required to raise the temperature of the very cold tap water. So, solar hot water systems are usually supplemented by an electrical or gas booster system which kicks in when needed. But even in winter, running costs are lower than that of an all-electric or all-gas system. Solar hot water systems are very energy efficient. The limitations of solar hot water systems are high installation costs and the necessity of having a suitable room at the correct angle for solar heating panels.

HEAT PUMP

To understand how a heat pump hot water system works, we need to understand the workings of a refrigerator. A refrigerator has a closed system of tubes through which a refrigerant circulates. When the liquid refrigerant is pumped through a fine nozzle it expands into a gas or vapour and becomes very cold. The cold vapour passes through coils in the freezer section of the refrigerator, absorbing heat from the air and food inside the fridge. At the back of the fridge, the vapour passes through a compressor which changes the refrigerant vapour back into a liquid. The compressor increases the temperature of the refrigerant, which then flows through coils at the back of the fridge. Air surrounding those coils at the back of the fridge becomes warm. So the refrigerator has a cold region inside the cabinet and a hot region outside the cabinet at the back. Heat energy is transferred from inside the refrigerator to outside. It is a heat pump! Reverse-cycle refrigerated air conditioning is like a giant fridge - it is a heat pump with cold coils and hot coils. Reverse-cycle air conditioning can cool a home when air is forced into the house around the cold coils. And it can be used to heat the home when air is forced into the house around the hot coils. A dedicated heat pump hot water system forces water around the hot coils. This hot water can then be used for all the normal functions in a home or business. .

Heat pump hot water systems are useful in large-scale commercial applications and in buildings which do not have a suitable roof for solar heating panels. Heat pumps achieve approximately 80% of the cost savings of a comparable solar system - but they are cheaper to install. So heat pumps are very energy efficient.