The amount of hot water that a relatively small (115,000 to 125,000 Btu per hour) gas-fired tankless water heater can provide depends on the temperature rise, which can range from about 35°F to more than 90°F, depending on the incoming water temperature and the water heater setting. At a typical temperature rise, such a model may provide about 2.5 gallons per minute (gpm). That’s usually enough hot water for a small house with water-efficient fixtures and appliances—if the homeowners are willing to avoid operating multiple hot-water-consuming devices at the same time. To serve multiple devices simultaneously or to fill a bathtub quickly, a larger tankless water heater (for example, 180,000 Btu per hour) is required.
Even a fairly small gas-fired tankless water heater takes a lot of combustion air. A 125,000 Btu per hour model operated at full capacity consumes about 2 cubic feet per minute (cfm) of natural gas and 30 cfm of air. An 180,000 Btu per hour tankless water heater requires up to 45 cfm of air. Such large airflow requirements can limit the options for placement—and produce significant noise from the fan. To simplify air supply, some tankless water heaters are designed for outdoor installation and include built-in freeze protection.
Older gas-fired tankless water heaters all had pilot lights. While the heat from a pilot light in a storage water heater may help to replace standby heat loss from the tank, that can’t occur in a tankless water heater. The pilot light energy is simply lost, dropping the EF by 0.10 to 0.12 points. Fortunately, most manufacturers offer tankless water heaters with electronic ignition. These products have EFs between 0.78 and 0.85; a few condensing models have EFs as high as 0.98.
Along with considering the maximum hot water flow from a tankless water heater, also pay attention to the minimum flow rate. Many models won’t start if the flow is less than 0.5 to 0.6 gpm—which a low-flow bathroom faucet (and even some dishwashers) may not reach. Varying flow rates can also sometimes produce “cold water sandwich” problems, in which a hot shower is suddenly interrupted with a spurt of cold water.
Another consideration with tankless water heaters is whether the output temperature is thermostatically controlled. Some models are designed to raise the water temperature by a set amount. These “delta-T” models generally work fine with municipal or well water (though some seasonal adjustment in the temperature increase may be required). Such models are not appropriate as backup heat for solar water heaters, however, because the solar-preheated water temperature may vary considerably. More sophisticated tankless water heaters provide output at a set temperature, no matter the input temperature—but they also cost more.
With electric tankless water heaters, the power required to provide even a modest hot water flow is significant. One 28 kW model, for example, provides a maximum flow rate of 3.0 gpm at temperature rise of 63°F—and draws 116 amps at 240 volts to do so! (Most homes have 200-amp service; some only have 100 A.) Providing that much power requires very large (and expensive) breakers and large-diameter wire or space in the service panel for several smaller circuit breakers (for example, four 30 A two-pole breakers). Utility companies are rightly concerned about these units, since hot water draws coincide with peak electric demand, in the morning and early evening. In this respect, a tank water heater is better, because it can heat water off-peak for use during on-peak times.
Another significant downside to tankless water heaters in some locations is scale buildup. If your water is mineral-rich, manufacturers recommend periodically flushing the heat exchanger coils with vinegar or another descaling fluid. This can increase installation cost if a special drain has to be plumbed in. If this maintenance isn’t carried out, the scaling can significantly shorten the water heater’s life.