Tankless (Instantaneous) Water Heaters
Most houses have storage water heaters, which allow many users to draw hot water at once. But if you don't have many people in the house, if you're adding on faucets far from the current water heater, or if you are remodeling a guest cottage or summer home, a tankless water heater may be the best choice. They don't waste energy by storing hot water until it's needed. Instead, when a faucet draws hot water, the gas burner or electric heating elements turn on, heating the water in the pipe as needed. They are also sometimes called instantaneous water heaters, because you never have to wait for hot water to arrive.
Unlike storage water heaters, tankless ones have a theoretically endless supply of hot water. However, the maximum hot-water flow is limited by the size of the heating element. Gas units typically heat more gallons per minute than electric units, but in either case, the rate of flow is limited. This means you can't run the dishwasher and take a shower at the same time, and it takes a long time to fill a bathtub.
There is no industry standard for testing and rating the efficiency of tankless water heaters. Electric tankless heaters should save energy compared to electric storage systems. But gas-fired tankless heaters are only available with standing pilot lights, which lower their efficiency. In fact, the pilot light can waste as much energy as is saved by eliminating the storage tank.
Tankless systems can be installed centrally, but they are more valuable in a remote part of the house. Having one at the point of use will minimize pipe runs from the main storage water heater to the tap, saving water and reducing plumbing costs. Also, their small size lets you fit them into tight spaces. A tankless water heater is a good option for a one- or two-person household, in a vacation home, or as a backup system for a solar water heater. In addition, it can sometimes be useful to install one as a booster heater for a dishwasher, so you can keep your storage water heater set at a lower temperature.
Special Features
Tankless heaters have either modulating output control or fixed output control. The modulating type delivers water at a constant temperature regardless of flow rate. The fixed type adds the same amount of heat, regardless of flow rate and inlet temperature. Avoid fixed output; with these systems, the temperature at the taps can fluctuate wildly. In fact, the more you open the hot water tap, the cooler the water will become.
Precautions
The heating elements and gas requirements for tankless water heaters are much larger than those for storage water heaters. Typical storage water heaters have a gas input of 40,000 Btu/h, while tankless ones go up to 170,000 Btu/h. Similarly, electric storage water heaters draw at most 6,000 watts, but electric tankless heaters can draw as much as 28,000 watts. So a tankless unit calls for bigger gas lines and vents, or for a large power supply.
If a tankless heater breaks down, it may be hard to find a contractor who can fix it. And if it needs parts, they may not be readily available.
You may also have difficulty finding an installer who has experience with tankless heaters. You will need to hire a second qualified technician (either a pipe fitter in the case of a gas heater or an electrician in the case of an electric heater) to upgrade the energy supply lines to the heater. This can be expensive.
In some cases (such as installing a heater just for a single sink), it may be better to install a small (3? gallon) tank heater. These units don't need such high powered hook-ups, so installation costs are lower, although the little tanks can cost almost as much as a 40-gallon tank.
Sizing
Tankless water heaters are sized in gallons per minute (gpm) of flow. Most provide 2 to 3 gpm of hot water. Size your unit to meet the peak demand that will be placed on it. For example, a low-flow showerhead will require about 2 to 3 gpm of mixed hot and cold water. Since a shower is usually partly cold water, a 2 gpm tankless heater will probably provide enough hot water for that. However, 3 gpm would allow a little slack in case someone turned on a hot-water tap while the shower was in use. It's unwise to serve many appliances with one tankless heater. It's better to provide a separate heater at each tap, especially if you're only providing hot water to two or three widely spaced faucets.
The other thing to consider with tankless heaters is the minimum flow rate required to activate them. Most need a flow rate of about 1/2 to 3/4 gpm to power up; but some are as high as 2 gpm, so they won't heat water unless you turn the water up high. This means you want to make sure that the flow rates at specific fixtures and appliances exceed the minimum requirements of the tankless heater you are considering.
Choose a Tankless Water Heater?