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Savings from tankless water heaters depend on application Editor's note:The Energy Services Bulletin features real answers to real questions posed to our staff at the Energy Services Power Line. We hope you find it useful. Question: Answer: Heat loss from hot water in storage does decrease the overall efficiency of the equipment, however. This is reflected in the unit's Energy Factor. A system with an EF of .88 loses 12 percent of the heat energy from the storage tank. A better-insulated storage tank may have an EF of .96, losing only 4 percent of the heat energy from the storage tank. Instant electric water heaters provide increased efficiency by eliminating the loss from water in storage. Typical loss from the storage tank ranges from 4 to 12 percent. This is the energy that would be saved if a 100-percent efficient instant water heater replaced an electric storage water heater. However, many instant water heaters do not perform at 100-percent efficiency. On some models, the heating coil is not submerged in the water it is heating. A closer look at electrical input versus hot water output shows that models are likely to deliver 94- to 96-percent efficiency. Comparing energy costs "Tankless vs. Tank Type Storage Water Heater Efficiency Comparison Testing," in the January 2005 PM Engineer is a recent and well-designed comparison study. Researchers found that a tankless heater has a surprisingly small energy and cost advantage over a storage heater in a typical home application for a family of four with two bathrooms. This actually makes sense, because the standby losses are not that great with four people using water every day. A better application for a tankless heater would be for a single person or a couple, or better yet, a secondary residence where water would be sitting in a tank heated for long periods of time. Then the payback for a tankless heater would look much more attractive. Impact on peak demand Widespread use of instant electric water heaters for whole house water heating may have a significant impact on peak load for the utility. To provide a flow rate of one gallon of water with a temperature increase of 61 degrees F requires an input rating of 9 kW. More than one instant water heater may be needed if larger volumes of hot water are required, and this could increase the peak kW even more. For that reason, a gas-fired instantaneous system might be a better option. A 50-gallon storage water heater typically has a peak input rating of 4.5 kW. While they cannot instantly recover all of the heat lost from a large volume draw, the storage capacity generally provides adequate service. Drawing one gallon per minute for 20 minutes from 50-gallon tank will drop the water's delivery temperature by about 10 degrees. Short-term, large-volume draws do not require additional electrical input. The water heater will provide serviceable results with only 4.5 kW of input. Articles and fact sheets: Going Tankless; Kelly Faloon, Plumbing and Mechanical, January 2004 Tankless Coil and Indirect Water Heaters; U.S. Department of Energy Energy Efficient Water Heating: Purchasing a New Electric Water Heater (481 kb pdf) Western Area Power Administration, 2004 Home
Energy Briefs -
Water
Heating (159 kb pdf) Water
Heating Residential
Water Heater Energy
Requirements Update
on Demand Water Heaters Water
Heating: Energy-Efficient
Strategies for Supplying
Hot Water in the
Home (835 kb pdf) Reports and documents: Domestic
Hot Water System
Modeling
for the Design of
Energy Efficiency
Systems (627 kb pdf) |
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