Electric Resistance Storage Water Heaters
Storage water heaters are the most common type of water heater used in both residential and commercial applications. Storage water heaters incorporate heating elements or a burner and tank into a single unit. Electric units are typically available in one- to 120-gallon capacity with input ratings of 1 kW to 54 kW. Most smaller units, including almost all units applied in residences, are dual-element units with an upper heating element and a lower element controlled to prevent simultaneous operation. A typical electric resistance storage water heater is shown here:
Commercial water heaters may have several banks of elements that are allowed to operate simultaneously. Virtually all commercial electric units are 208 V or higher, either single or three phase. With 120-V service, maximum input capacity is limited to 1.9 kW or less.
The most common commercial gas storage water heaters vary in size from 20 to 100 gallons. Input ratings range from 30 to 600 kBTUH.
In almost all code jurisdictions, storage water heaters or hot water tanks must be ASME labeled if their capacity is greater than 120 gallons or if the rated input is 200 kBTUH or more. Higher-cost ASME-rated water heaters are available in capacities up to 5000 gallons and input ratings of 2000 kW and 750 kBTUH. As an alternative to high-cost ASME-rated tanks, several smaller, non-ASME-rated units may be used to provide equivalent capacity.
In comparing gas and electric water heaters, it is not appropriate to simply compare input ratings. Electric storage water heaters are inherently more efficient than gas units. While typical gas water heaters are limited by a combustion and heat transfer efficiency of about 78 to 80%, electric water heater elements are immersed in the tank, so that the conversion efficiency is 100%. Typical gas water heater tanks also have greater heat loss because of the uninsulated flue through the center of the tank and the uninsulated tank bottom. Thus, for similar input ratings, an electric water heater delivers significantly more heat input capacity.
Most electric storage water heaters are controlled by surface-mounted adjustable thermostats, although larger units may use immersion thermostats. In electric units, the upper and lower elements are controlled by separate thermostats which are adjustable from about 110 to 180°F. Fuel-fired water heaters have a single immersion thermostat. On all water heaters, control resolution is coarse and accuracy is limited.
- All applications where simplicity, convenience, and safety are primary considerations.
- Where installation of flues and provision of combustion air for fuel-fired systems would be inconvenient, unattractive, difficult or impossible or where potentially dangerous backdrafting may occur due to indoor/outdoor pressure differentials.
- Where hot water usage is low, the low standby loss of electric water heaters can make them less expensive than natural gas systems.
- Point-of-use applications.
- Most residential and commercial facilities are good candidates for electric resistance storage water heaters.
Applications To Avoid
- Where high output capacity is required and there is limited room for increased storage volume.
Electric storage water heaters are inherently more efficient than gas units. While typical gas water heaters are limited by a combustion and heat transfer efficiency of about 77% or less (higher for pulse-combustion and condensing water heaters), electric water heaters have their elements immersed in the tank, so that the conversion efficiency is close to100%. Gas water heater tanks also have greater heat loss because of the open flue through the center of the tank.
Because they are well-insulated, heat loss from the tank is low and air temperature has little effect on the operation and performance of electric water heaters.
Minimum efficiency requirements for storage water heaters sold in the United States are specified by the National Appliance Energy Conservation Act or NAECA. A number of other codes and standards also apply. Storage water heaters larger than 120 gallons or with input ratings of 200 kBtuh (58.6 kW) or more must comply with ASME pressure vessel standards, which substantially increases their cost.
The National Appliance Energy Act of 1987 and the 1988 amendments established minimum energy factors for electric, gas-fired, and oil-fired water heaters manufactured after January 1, 1990. Minimum figures were revised for equipment manufactured after April 15, 1991; see table1. The standards apply to storage water heaters having capacities equal to or smaller than the following:Gas: 75 kBtuh input
- Oil: 105 kBtuh input
- Electric: 12 kW input
|Table 1 NAECA Water Heater Energy Factor Standards|
|Product Class||Energy Factor|
|Gas water heater||0.62 - (0.0019 x rated storage volume in gallons)|
|Oil water heater||0.59 - (0.0019 x rated storage volume in gallons)|
|Electric water heater||0.93 - (0.00132 x rated storage volume in gallons)|
Source: Federal Register: Energy Conservation Program for Consumer Products, October 17, 1990, .
For commercially available storage water heaters, energy factor is generally higher for units with smaller tanks, however it varies substantially depending on design and construction details. For typical electric storage water heaters, energy factor ranges from 0.77 to 0.95, with a typical value of about 0.86. For gas storage water heaters, energy factor ranges from 0.43 to 0.86, with 0.54 a typical value; recovery efficiency ranges from 75 to 94%. Table 2 lists minimum energy factors set by NAECA for various system types and tank volumes. Also included are the maximum values in each category for all models listed in the October 1997 edition of the GAMA directory.
|Table 2 NAECA-Required Minimum Energy Factor and Best Available Energy Factors|
|Nominal Volume (gal)||Electric Resistance||Gas and LP||Oil|
a. Discontinued model; highest current model is 0.66.
b. Typical values are much lower.
Source: Minimum energy factors are from NAECA ; maximum values are from GAMA
Consumers' Directory of Certified Efficiency Ratings, October 1997