Tankless or point-of-use water heaters have become increasingly popular in recent years for heating potable water in residential homes in the U.S. There are several major factors in the trend of installing tankless water heaters. One is an increasing demand for continuous, unlimited streams of hot water for simultaneous operation of hot water-consuming appliances and fixtures. Another is a desire to save floor space and to conserve energy by reducing standby losses. There are many different models of fuel-gas and electric tankless water heaters, each having a specific rating. Tankless water heaters are rated at gallons per minute (gpm) and degrees of water temperature rise. www.nachi.org

Since tankless water heaters can discharge water at an uncertain range of temperatures at any given time, depending on the use, a temperature-control device is needed to protect the user from scalding water being discharged. A tempering valve can be adjusted to deliver water at a maximum temperature of 140° F (60° C), or the heater can be equipped with a temperature-limiting device or thermostat that has the maximum setting. When a tankless water heater supplies a shower or tub-shower combination, the maximum temperature of the outlet control valve of the shower or tub-shower must be set at 120° F (49° C).

One challenge in sizing a tankless water heater is to determine the demand in gallons per minute. The second challenge is determining what temperature rise is required for that flow. Because tankless water heaters are designed to accommodate a given maximum flow, there is a pressure loss associated with flows in excess of a unit’s usable flow rating. In a house with a tankless water heater, it’s possible to have a fixture with almost no flow of water if too many faucets are opened simultaneously. Occupants may have to adjust their expectations concerning when and to what extent simultaneous demands for hot water can be made before temperature and flow of hot water are affected.