MIL-HDBK-1003/1
Federal Energy Management Program analysis of residential-size single-family electric resistance versus natural gas fired SWH indicates a lifetime energy costs of
$2600 for electricity at $0.06 per kW, and $900 for natural gas at $0.40 per therm.
Evaluate SWH storage tank capacity and electric heater element sizing to minimize electric demand charges to the government. Larger storage tanks with smaller heater elements may be cost effective due to reduced demand charges, although the longer storage recovery time and the additional costs for providing non-standard tank and element selections should also be considered. Do not exceed the ASHRAE recommended 8-hour maximum recovery time. Also consider the incremental cost of additional electric service capacity
versus the incremental costs of providing steam or gas service or providing fuel oil storage and delivery. Include the costs of providing combustion air and flue gas exhaust for fuel- fired water heaters.
Do not consider the cost of the tempering valve in the life cycle costs analysis, it is required by the 140 degrees F storage temperature to avoid legionella. However, since it is required, consider the life cycle cost advantages of storing hot water at higher temperatures, if the source is electric, steam, or natural gas, and the tank insulation is increased to maintain equal total heat loss. A smaller SWH storage tank may reduce the size of the mechanical room and the building, which will reduce the energy and capital costs of the facility.
Consider a pre-heat tank upstream of the SHW storage tank for applications recovering heat from refrigerant hot
gas, steam condensate, process waste cooling, solar
collectors, and diesel engines. Provide a tempering valve for
all heat recovery SWH systems to limit the supply temperature,
because the recovered heat can heat the storage tank above the
normal storage temperature. A pre-heat tank may increase the
amount of heat usably recovered prior to allowing the water to
be heated by prime energy.
Consider sealed combustion chambers for natural gas- fired SWH's, with combustion air ducted directly from and flue gases ducted directly to the outside air. This may reduce the
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