Why hot water matters
After heating and cooling, heating water is often one of the larger uses of energy in a home. Water takes a lot of energy to warm: raising a bathtub’s worth of water by a few dozen degrees uses far more energy than running a light for the same time. The share varies with household size, climate and habits, so a family of five with daily laundry will differ from a single person who showers briefly.
Common ways homes heat water
Storage tank water heaters. A tank holds a volume of hot water, kept at a set temperature by a gas burner or an electric heating element. Hot water is ready when needed, but the tank loses heat to the surrounding air all day, which is called standby loss. Better tank insulation reduces it.
Tankless (on-demand) heaters. These heat water as it flows through, with no stored volume, so they avoid standby loss. They need a high rate of heat while running, so gas models need adequate burner capacity and venting, and electric models may need a heavy electrical supply. Very large simultaneous demands, such as two showers at once, can exceed a unit’s capacity. Whether the savings are substantial depends on how much hot water the household uses, because homes with low use save the most from avoiding standby loss.
Heat pump water heaters. These use the same principle as a space-heating heat pump, moving heat from the surrounding air into the water instead of generating it directly. They can use substantially less electricity than a resistance electric tank for the same hot water. They need suitable space, adequate airflow and a location where cooling the nearby air is acceptable. They also make some noise, and performance depends on the room’s temperature.
Solar thermal systems. Collectors on the roof heat water or a fluid, often supplemented by a tank with a backup heater. They suit sunny locations and households with steady hot water demand. This is different from photovoltaic panels, which produce electricity, and its relation to solar power more broadly is only that both use sunlight.
Combination boilers and indirect cylinders. In some regions, hot water comes from the same boiler as space heating, either on demand (combi) or by heating a storage cylinder.
Point-of-use heaters. Small electric heaters at a single tap can avoid long pipe runs for a remote sink, though they are not suited to whole-house demand.
What affects the energy used
- Volume of hot water. Showers, baths, laundry, dishwashing and taps all count. Low-flow showerheads and aerators reduce the volume without necessarily reducing the experience, though preferences differ.
- Temperature setting. A higher stored temperature means more standby loss and more energy to heat the water. A lower setting reduces both, but there is a safety consideration (see below).
- Incoming water temperature. Water arrives colder in winter in many places, so more energy is needed to heat it.
- Heater efficiency. Fuel-burning heaters and electric tanks have efficiency ratings, and heat pump models can show much higher figures because they move heat. Compare ratings on the same basis, as units differ between types.
- Insulation and pipe layout. Long runs of uninsulated pipe lose heat, and you may wait a while for hot water to reach the tap, wasting both water and energy. Insulating tanks and accessible pipes can help.
- Fuel type and price. Natural gas, electricity, oil, propane and others all have different prices and emissions, and the comparison depends on local rates and on how the electricity is generated.
- Timing. Heating water is a flexible load, because a well-insulated tank can store heat. Under time-of-use tariffs, heating when electricity is cheaper may lower costs, and some utilities use controllable water heaters to balance demand on the grid, a form of thermal storage.
Safety points
Water stored too cool can allow bacteria such as Legionella to grow in some conditions, while water that is too hot causes scalds, especially for children and older adults. Guidance on safe storage and delivery temperatures differs between countries, so follow local recommendations and the manufacturer’s instructions. Thermostatic mixing valves are used in some systems to store water hotter while delivering it at a safer temperature.
Water heaters involve gas, high-temperature water, pressure relief valves, combustion venting and mains-voltage electricity. Installation, replacement and repairs should be done by qualified, licensed people following local codes. Do not remove or tamper with pressure or temperature relief valves. If you smell gas, leave the building and contact the emergency service for your area.
Practical steps with modest effort
- Check the thermostat setting, and lower it if it is higher than needed and local guidance allows.
- Insulate the tank (if the manufacturer permits) and the first stretch of pipe.
- Fix dripping hot taps, which waste both water and energy.
- Use full loads in dishwashers and washing machines, and cooler wash settings where they work.
- Consider low-flow fixtures.
- When replacing a heater, compare lifetime cost rather than only price, and consider whether household size is changing.
Replacement decisions
A water heater generally lasts for a number of years, and replacement is often forced by a failure, which leads to rushed decisions. Thinking about options before that happens gives time to compare. Check the installation requirements for any new type, such as space, electrical capacity or venting, and look into available incentives, which differ widely and change. A home energy audit may include the water heater in its review.