Storage Water Heaters vs. Tankless Units
Tankless water heaters are becoming increasingly popular in New York homes as efficient gas-fired water heating systems. As of 2009, there were 400,000 units installed in New York, accounting for about 10% of all gas-fired water heaters. Compared to standard gas-fired tanks that have an energy factor (EF) below 0.60, EF for tankless units often exceeds 0.80. Efficiency is higher in part because storage losses associated with the tank are eliminated.
While tankless units are efficient, they provide water in a way that some homeowners may not find acceptable. A very low-flow water draw at a sink, which could easily be provided by a storage tank, might not be possible to satisfy with a tankless unit. The burner on a tankless unit normally needs a minimum flow rate to activate and provide hot water. Therefore, homeowners may have to modify their behavior to draw enough water flow to activate the burner. The inevitable delay times for burner operation can also result in a “cold water sandwich” where a draw at the fixture provides alternating cold and hot water.
Tankless water heater products have greatly improved in recent years to reduce delay times on startup and to operate at lower water flow rates (as low as 0.5 gallons per minute). Many homeowners easily adapt to the way these systems operate. At one field test site in this research study, the homeowners had a new tankless unit installed. The new tankless unit did not change the amount of hot water they used, but they did decrease the number of water use draw events by a factor of two, they adapted their behavior to the capability of the tankless unit and stopped making small draws. The homeowners found this new tankless unit to be acceptable and the minor inconvenience to be worth the increased efficiency.
New hybrid systems are now available to combine the efficiency benefits of the tankless unit with the convenience of a small water tank to provide small hot water draws on demand without waiting. Only a few manufacturers make these hybrid units. Current versions of these units are expensive and the one tested at the Syracuse COE laboratory at least was not especially efficient.