Off-peak Thermal Storage Water Heaters | My Business | Mississippi Power | A Southern Company

Off-peak Thermal Storage Water Heaters

Thermal storage system configurations

Off-peak water heaters usually have greater storage capacity than would be used in a conventional system for the application.  Off-peak water heating features and functions are often combined with systems designed for off-peak space heating or process heating. Both custom-made dedicated storage systems and systems assembled in the field from conventional components are used. The common operating pressure and temperature combinations are described by the following three categories:

  • Direct
  • Indirect-vented
  • Indirect-pressurized

Direct storage systems

Direct storage systems heat water directly in the storage tank or by an open loop to a remote boiler or other heat source. They are essentially simple storage water heating systems designed with controls to accomplish off-peak operating functions. Maximum operating temperature is about 200¡F. Direct systems can be assembled from conventional water heaters and storage tanks or they may be built particularly for the purpose. Smaller, off-peak storage systems built from conventional water-heating components may be less expensive. Dedicated packaged systems may include added insulation, connections for external control, and tank baffles or inlet headers to minimize mixing of the tank when cold water is introduced.

Indirect-vented storage systems

Several manufacturers build indirect thermal storage systems using tanks vented to the atmosphere, which ensures operation at atmospheric pressure and allows savings in tank costs. A typical indirect-vented storage system is shown in figure 1. Alternatively, a closed system with an expansion tank may be used and the system can still avoid the expense of ASME rated construction if it operates at less than 15 psig.  Indirect systems use the water in the tank only for thermal storage; potable water is heated either by an external heat exchanger or by a heat exchanger located inside the storage tank. Indirect, vented storage systems also operate at a maximum temperature of about 200¡F.


Typical indirect, vented storage system

Potable water circulated through the heat exchanger does not contact the water in the storage tank. Consequently, the thermal storage tank can be unlined since it is not subject to corrosion caused by the frequent exchange of oxygenated fresh make-up water. Furthermore, only the heat exchanger must be pressure rated. Tanks are constructed of steel or lightweight aluminum with an EDPM liner. Tanks are typically insulated with 2 to 2.5 inches of foam.

The thermal storage tank is heated by electric resistance elements, usually flanged immersion-type elements, near the bottom of the tank. Multiple elements provide staged heating capacity and redundancy.  The elements are operated by electronic controllers through mercury switches or mechanical contactors. The controller can be interfaced to an energy management and controls system or to an electric utility control system for positive demand control.

Indirect-pressurized storage systems

Indirect-pressurized storage systems are supplied as packaged systems by several manufacturers. The systems are usually installed to meet both water heating and space heating loads. The elevated operating pressure and the volume of typical tanks requires that the tanks be built to ASME pressure vessel specifications, raising costs somewhat. However, other advantages may make them attractive for certain applications.

A typical pressurized off-peak water heating system consists of a well-insulated sealed pressure vessel containing water treated to remove corrosion. Heat is supplied by immersion electric resistance elements or a steam heat exchanger located near the bottom of the tank. See figure 2. Unlike conventional direct storage systems, potable water does not flow through the tank. Instead, water is heated in a water-to-water heat exchanger, located either inside the tank or externally. Since the storage tank is a closed system, the continuing introduction of oxygen, minerals, and particulates that occur in the incoming cold water for open systems is avoided. Scaling, liming, and corrosion of the tank and elements are reduced, extending the overall life of the system and reducing maintenance costs. This configuration also allows the storage units to be connected to a space heating or process heating loop without the need to design the heating loop to accommodate potable water.


Typical indirect, pressurized storage system

Typical packaged systems operate at a maximum temperature of 250 to 300¡F and maximum working pressure from 65 to 160 psig. Depending on the temperature approach of the heat exchanger, energy can be delivered at tank temperatures as low as about 10 to 20¡F above the hot delivery temperature. Using the direct relationship between pressure and temperature in a closed system, a pressure controller operates the heating elements. Pressure control provides more rapid response and better representation of average tank temperature. Leaving water temperature is regulated by a mixing valve. A high limit temperature control and low water cutoff are provided as safety controls.  The tanks are heavily insulated and available in capacities from 200 to 18,000 gallons in vertical and horizontal configurations. Electric heating elements are available in capacities from 20 to 1000 kW.  A variety of steam heat exchanger capacities are available, subject to the steam system supply pressures and flow rate. Dual-fuel systems are used to provide redundant heat sources and to allow optimization of electric and steam loads.

Since indirect-pressurized systems are provided as a package, design options usually involve only selection of thermal storage capacity, number and type of control of heating elements, operating voltage, control interface with the utility or an EMC or BAS, and selection of water heating and/or space heating capability.

Thermal storage water heaters are typically applied for off-peak water heating. 

In some commercial water heating applications, all water heating energy input can be shifted entirely to the electric utility's off-peak period by using additional thermal storage capacity and simple controls.  Figure 1 compares the operation of a conventional water-heating system and an off-peak system for a water heating load in a typical daytime commercial water heating load. Unlike off-peak space heating and air conditioning, off-peak water heating has the advantage of producing year-round benefits. Savings may also be available from reduced electric service entry and wiring size by keeping the water heating load off the facility peak.


Benefits of off-peak water heating

Indirect-pressurized systems

For common hot supply water temperatures of about 140¡F, indirect-pressurized systems store about 50% more energy than conventional systems by relying on elevated temperature and pressure in the storage tank.  However, the energy storage density of indirect-pressurized systems operating at high temperatures is not always appreciably greater than that of conventional direct storage systems.  In fact, where the hot water supply temperature is 180¡F or more, indirect-pressurized systems may store less energy per gallon of storage volume.  The difference is a result of the heat exchanger approach and the fact that the required hot water delivery temperature is close to the temperature of the water in the tank.


  • In facilities with central water heating systems where off-peak electric rates are available and facility operations allow off-peak heating and on-peak hot water usage.
  • To avoid increases in electric service capacity by limiting the demand of the water heating load during peak periods.

Possible applications

  • For meeting both hot water and space heating loads in facilities with central HVAC and water heating systems.


Thermal storage water heating systems using elevated storage temperature may consume slightly more energy than conventional systems because of the increased heat loss from the tank.  In good applications, such small increases in operating costs are more than offset by savings from the use of lower-priced off-peak electricity.

Operating cost savings for off-peak thermal storage water heaters are available only if the utility rate includes a provision for lower-cost off-peak service or where electric demand control incentives are available.

Contact us for a detailed list of manufacturers for this equipment.

Other information

For pressurized systems, the elevated operating pressure and the volume of typical tanks requires that the tanks be built to ASME pressure vessel specifications, raising costs somewhat.