Gas Storage Heaters - These systems store heated water at a constant temperature in an isulated tank, ready for use. The water is reheated by a gas flame when the temperature falls below the thermostat setting. Internal and external models require a flue. These systems are rated for their energy efficency with an energy star rating system. The more stars the more energy efficient. Their storage capacity and recovery rate determines how much hot water they can deliver. Because gas hot water systems heat much faster than electric systems, they usually require a smaller tank to produce the same amount of hot water per hour.

Gas Continuous Flow Systems - Continuous flow hot water units (also reffered to as instantaneous) heat water as required, so they never run out. They are a sensible method of water heating, since you only pay to heat the water you use. These units are much smaller in size than storage systems plus they can be installed externally or internally with a flue. They are rated by a volume of hot water they can produce per minute, which is usually 10 to 26 litres. The more hot water you use the higher capacity unit you will require. Gas units generally require a large gas supply line, potentially adding tothe cost of installation. Continuous flow systems are often popular for units or homes with limited space.