Gas water heaters
What is a gas water heater?
Gas water heaters burn either natural gas, delivered via a piped gas (reticulated) network, or bottled gas (LPG) to heat water. There are two main types of gas water heaters: storage and instantaneous.
Storage systems heat the water stored in a tank.
Instantaneous heaters, sometimes called continuous flow, don't have a storage tank and only heat the water when it is required.
Using gas instead of electricity to heat your water can help to reduce your greenhouse gas emissions, and may lower your water heating costs.
Types of gas water heaters
Storage
Gas storage systems are all very similar in the way that they work. The main variations are in the tank material, the burner and flue technology, and whether they are designed for indoor or outdoor installation.
The most common tank material is enamelled steel, but stainless steel is also used. Stainless steel is more expensive, but the tank will usually last longer.
Some units use a special flue system to recirculate the hot flue gases around the outside of the tank to increase heat transfer into the water and increase their efficiency.
Units installed indoors need a flue to exhaust the combustion gases outdoors.
Instantaneous
There are two different types of instantaneous heaters. One type is very simple and has a fixed burner flame so the water temperature will vary with the rate of water flow from the tap. They usually have a pilot flame or battery ignition and do not need an electricity supply. They generally have limited flow rates of up to about 16 litres per minute and are not suitable for houses with more than one bathroom, however, they are usually cheaper to buy and install.
The other type of instantaneous heater has electronic controls that vary the flame size to deliver water at a constant preset temperature and fans that improve the combustion efficiency. They can be fitted with controls to allow you to set the water temperature you want. This means you can preset the water temperature so you don't need to mix the hot water with cold water to ensure a safe bathing temperature.
Electronic units can be preset to deliver hot water at no more than 50°C as required by plumbing regulations for bathrooms.
There are also very efficient condensing instantaneous units available. These use the heat from the exhaust gas to pre-heat the cold water before it enters the heat exchanger, thereby reducing energy use. Although rated at 6 stars, as this is currently the maximum rating, they use less energy than non-condensing units so look for the MJ per year energy consumption figure on the label as well as the star rating to compare products.
High-efficiency gas instantaneous water heater with condensing technology
Some of the non-electronic instantaneous models may not be suitable for use with water efficient 3 star showerheads as the hot water flow rate will be too low. Check with your plumber or water heater supplier or manufacturer.
How do they work?
Storage heaters
Storage heaters use a gas burner located underneath the water storage tank to heat the water. They usually have a continuously burning pilot flame that ignites the main burner when it is needed. Heat is then transferred from the gas burner through the bottom of the cylinder. Some heat also flows from the hot flue pipe into the water. When hot water is drawn off at the top of the tank, cold water enters the tank at the bottom. Most units have an adjustable thermostat to set the water temperature. When the temperature at the bottom of the tank drops below the level set on the thermostat the pilot flame ignites the main burner so the water is kept constantly hot. The energy from the pilot flame is not wasted as it helps to heat the water in the tank.
Storage systems should be set to at least 60°C to kill any pathogens and avoid legionella, so you might like to consider a temperature control valve to limit the bathroom water temperature. The hot water from the water heater needs to be mixed with cold water to ensure a safe bathing temperature.
Storage systems come in a range of tank sizes from about 90 to 300 litres.
Storage heaters lose heat through the walls of the tank (called standing heat loss), so they need to burn gas regularly to keep the water at the desired temperature. These losses can be a significant proportion of your hot water energy use and your gas bill. All tanks have some insulation to reduce standing heat losses, but a better insulated tank will have lower storage losses, so look for one with a high energy star rating.
Gas water heaters (except gas boosted solar) have an energy star label to tell you how efficient they are. This is similar to the energy label on electrical appliances. The more stars the better. Once you have decided on the type and size of water heater you need, use the label to select the most efficient model by choosing one with the highest number of stars and the lowest energy consumption. More stars mean lower running costs for you and less greenhouse gas emissions.
Storage systems have a range of efficiencies from about 2 to 5 stars. The Australian Government plans to introduce a minimum performance standard for gas water heaters that will require them to be at least 4 stars.
Gas storage water heater
Instantaneous heaters
In an instantaneous system, the water is heated by a gas burner as it flows through a coiled pipe called a heat exchanger. The gas burner starts when a hot water tap is turned on. It only heats the amount of water that is required instead of continuously heating a full tank. As they don't store any hot water they do not have heat losses from a tank and so can have energy and cost savings.
Standard gas instantaneous water heater
To allow enough time for heating, the water is slowed down as it flows through the heat exchanger. This means that instantaneous units supply hot water at a lower pressure than storage systems. They can typically deliver hot water at flow rates of between 10 to 30 litres per minute depending on the model. Some instantaneous systems have a pilot flame to light the main burner and some have electronic ignition that uses a spark to light the gas, the same as those used on gas stoves. Instantaneous units with a pilot light are less efficient than those with electronic ignition, because, unlike in a storage system, the pilot light energy is wasted. Look for models with electronic ignition.
Instantaneous units have an efficiency rating in the range of 4 to 6 stars. The energy label assumes that you use 200 litres of hot water per day, which is about what a typical 4 or 5 person household would use. If you use that much hot water, then a 5 star storage system and a 5 star instantaneous system will use about the same amount of energy each year. However, if you use less hot water, an instantaneous system might use less energy as it has no storage losses.
Comparative energy consumption of gas water heaters
Getting the most out of your gas water heaters
Saving Energy
If you are going away on holidays you can safely switch your heater off to save energy. If you have a system with a pilot light make sure you know how to relight it. This information can usually be found on the heater unit or in the owner's guide.
Another way to improve the efficiency of your hot water system is to insulate the hot water pipes, if you can, to reduce energy wastage through heat losses from the pipes. This is especially important if you have hot pipes longer than a few metres. The normal green lagging that some plumbers use is usually not enough. Pipe insulation should have an R rating of at least 0.3, which is about a 10mm thickness of plastic foam insulation. You should be able to buy suitable pipe insulation from a hardware store. In colder areas a higher insulation (R) rating is desirable.
Make sure your storage heater is set to the correct temperature and not overheating the water, as this wastes energy.
Maintenance
A well maintained gas water heater can give you years of trouble-free service. Have your system installed by a licensed tradesperson and have it serviced in accordance with the manufacturer's instructions.
Storage tanks made of enamelled steel have a sacrificial anode, which is usually a rod of magnesium, placed inside the tank that protects the steel in the tank from corrosion. The anode needs to be checked and replaced about every five years by a licensed service person. The actual time interval will depend on your water quality and the type of heater. Follow the manufacturer's recommendation. The service person should also inspect other parts such as the pressure relief valve and make sure the burner is working efficiently. Most storage systems should have a life of at least 10 years if properly maintained.
Instantaneous units require no routine maintenance apart from having the burner operation checked every few years. The heat exchanger usually comes with at least a 10 year warranty and can be easily replaced. Most systems will have a life of at least 15 to 20 years.
Reducing water wastage
Instantaneous units waste some water in the time it takes for the heat exchanger to get hot. This can be 1 to 2 litres each time the hot tap is turned on. If you turn hot taps on frequently this can amount to a lot of wasted water each day. This can be reduced by installing a water recirculation system. You simply press a button before turning on the hot tap and the cold water in the pipe is pumped back to the heater until the water at the tap warms up. You then turn the tap on. They can be used in new installations or easily fitted to the existing plumbing. They can also be used with storage systems.
There are several systems available, and some hot water system manufacturers produce their own. Avoid those that use electrical elements to keep the water hot between uses as they use more energy.
Water recirculation system to reduce water wastage
You can also reduce water wastage by turning the tap on at a low flow rate until the water gets hot. To avoid water wastage, you may like to collect the water to use for your plants.
Is it right for you?
Whether a gas water heater is the best available choice for you depends on your particular circumstances.
Do you have access to natural gas?
LPG systems are typically 2 to 3 times more expensive to run than natural gas. If you don't use much hot water this may not be an important consideration. For example, if you have a clothes washer and dishwasher that heat the water themselves, most of your hot water will be used for showering. If there are only one or two people in your household and you have a water efficient showerhead, then your hot water use will probably be low.
Instantaneous models, particularly high power electronic control models, use a much larger gas burner than storage systems, and will generally need a larger gas supply pipe. So, where possible, decide what you are going to buy before having the gas supply pipe installed. If you already have gas connected to supply a stove or room heater, the supply pipe may have to be replaced with a larger one. LPG units will generally require 2 x 45 Kg capacity gas cylinders to avoid frequent replacement of gas bottles.
Instantaneous systems with electronic controls also require electric power. If the electricity supply fails they won't provide hot water. If you live somewhere with frequent power outages this may be important.
If you have access to both gas and sunshine you could also consider a gas boosted solar system to reduce your energy use and greenhouse gas emissions even further.
Location
Every 10 metres of 15mm copper pipe contains about 1 litre of water which is wasted every time you turn the hot tap on, and the energy used to heat the water left in the hot pipe is wasted when it goes cold. Whichever hot water system you choose, try to locate it as close as possible to the most frequently used outlet. When designing a new home, it's ideal to put all the rooms that use hot water close together. If you have long pipe runs consider installing a recirculation system.
If space is limited for a storage tank then an instantaneous system might be the answer as they take up very little room. It is possible to locate some instantaneous units indoors if the correct flue is fitted.
What size system do you need?
Storage units are usually sized according to the number of people in the house and instantaneous units according to the maximum number of hot water outlets likely to be used at the same time.
Gas storage systems can have a smaller tank size than electric off-peak systems as they reheat the water whenever it cools down.
The table below gives an indication of what size heater you might need, but you should always ask the supplier or manufacturer for specific size guidelines for the products you are considering.
| Storage | Instantaneous |
|---|
| Number of people | Size (litres) | Number of hot taps used
at the same time | Flow rate
(litres per minute) |
|---|
| 1-2 |
90 |
1 |
16 |
| 2-4 |
130 |
2 |
20 |
| 3-5 |
170 |
3 |
24 |
| 4-6 |
200 |
3+ |
32 |
| 5-9 |
270 |
|
|
Climate
Storage heaters are effective in any climate. However, in a very cold climate you may want to consider a larger storage tank as hot water use tends to be higher. You may also consider installing the heater indoors, such as in a laundry, to reduce the heat loss from the tank.
If everyone in the household tends to shower at the same time of day, such as first thing in the morning, you may want a bigger storage tank to ensure you don't run out of hot water. But remember, the bigger the tank the greater the standing heat loss, so aim for the smallest tank that will meet your needs.
In a climate with very cold winters, such as alpine areas, you should choose a system with a higher capacity than is normally recommended for your needs because instantaneous heaters are limited in how much they can increase the temperature of the cold water. They can raise the temperature of the cold water by 25 degrees at their nominal maximum rated flow. Reducing the flow rate will also allow the water to be heated to a higher temperature.
How do they compare with solar water heaters?
If you do not have good solar access, or live somewhere with limited sunshine, gas water heaters can produce lower greenhouse gas emissions than electric-boosted solar systems.
Further information
CHOICE publishes information on hot water options, manufacturers and government rebates. In particular see their Buying Guide: Hot Water Options.
For more information on water efficiency and hot water systems see the Your Home website, in particular the information on hot water services.
The energy star rating of any gas water heater on the market can be found on the website of the Australian Gas Association.
Contact your state or territory government or local council for information on hot water systems including rebates for replacing electric storage systems.
For specifications and installation details contact your local supplier. The manufacturers' websites also have a lot of useful information.
Acknowledgements
This information drew on the following references:
Your Home Technical Manual (viewed December 2009). Hot Water Service fact sheet
Sustainable Energy Authority of Victoria Choosing a hot water system fact sheet (2002)
Sustainable Energy Development Office WA Choosing a hot water system fact sheet (2003) Rheem Hot Water Manual (2001)