Commercial Hot Water Heating Systems
By Susan Ecker
Rumsey Engineers Inc.
The commercial water heaters in many facilities are major energy consumers, accounting for about 11% of the total energy load, on average. A hospitality facility may use up to 40% of its energy for heating water. There is a potential for improving energy efficiency by considering the facility's needs, including time of use, amount of hot water required, temperature range and flow rates. (1)
Energy efficiency should be a major consideration when choosing a water heating system. The design of the system is not based solely on the water heater. For example, tanks (if used), pipes, etc., should be insulated in order to enhance performance and, in most cases, to comply with energy codes, and such things as the type of circulation pump should also be considered. There is a range of water heater types to choose from. Choosing an appropriate water heater for a particular application is an important task, so here is an overview of four commonly specified water heaters. The four systems are:
- Conventional storage water heaters
- Demand (tankless or instantaneous) water heaters
- Standalone air source heat pump water heaters
- Solar water heaters
There are other options available, including indirect water heaters (unfired hot water storage tanks) but these will not be discussed here.
Conventional storage water heaters
A conventional storage water heater has a reservoir of available hot water. It operates by releasing hot water from the top of the tank when the hot water tap is opened. To replace that hot water, cold water enters at the bottom of the tank.
Fuel sources for conventional storage water heaters include natural gas, propane, fuel oil and electricity. Natural gas and propane water heaters operate by utilizing a gas burner under the tank to heat the water. A thermostat opens the gas valve as the water temperature falls. The valve closes when the temperature rises to the thermostat's setpoint. Oil-fired water heaters operate by using power burners that mix oil and air in a vaporizing mist that is ignited by an electric spark. Electric water heaters have one or two electric elements. Each element has its own thermostat. A standby element at the bottom of the tank maintains the minimum thermostat setting, while the upper demand element provides hot water recovery when there is an increased demand.
Standby heat losses occur, and, as a result, the water in the tank needs to be constantly heated. Only tankless (instantaneous) water heaters (discussed below) avoid standby heat losses. It is important for the storage tank to be heavily insulated to mitigate the heat loss, resulting in a decrease in energy consumption.
Demand (tankless or instantaneous) water heaters
Demand water heaters provide hot water on an as needed basis by heating water directly, without the use of a storage tank. Standby energy losses associated with storage water heaters are not incurred; this can result in decreased energy and operating costs.
When a hot water tap is turned on, cold water travels through a pipe into the unit. Either a gas burner or an electric element heats the water. Demand water heaters deliver a constant supply of hot water. There is no need to wait for a storage tank to fill up with hot water. A demand water heater's output limits the flow rate.
Typically, demand water heaters provide hot water at a rate of 2-5 gallons (7.6 -15.2 liters) per minute. Gas-fired demand water heaters produce higher flow rates than electric units. It may be necessary to install two or more demand water heaters connected in parallel for simultaneous demands of hot water. Another option to satisfy simultaneous hot water demands would be to install separate demand water heaters at various points of use where there is a high hot water demand. (2)
It is important to note that, if they have a constantly burning pilot light, gas units can waste energy. This can sometimes offset the elimination of standby energy losses when compared to a storage water heater. The cost of operating a pilot light in a demand water heater varies from model to model. Consult the manufacturer for additional information about pilot light gas usage. Some models use an intermittent ignition device (IID) instead of a standing pilot light; this would eliminate the energy wasted by having a constantly burning pilot light. This IID is similar to the spark ignition device on some gas kitchen ranges and ovens.
Standalone air source heat pump water heaters
Heat pumps can be used to heat water, either as a standalone water heating system or as a combination water heating and space conditioning system. Instead of generating heat directly, heat pump water heaters use electricity to move heat from one place to another. Heat pumps can be two to three times more energy efficient than conventional electric resistance water heaters.
A heat pump moves heat like a refrigerator does, but in reverse. (2) A refrigerator pulls heat from inside the unit and dumps it into the surrounding room. A standalone air source heat pump water heater pulls heat from the surrounding air and dumps it, at an elevated temperature, into a tank to heat water. A standalone heat pump water heating system can be supplied as an integrated unit with a built in water storage tank and backup resistance heating elements. Heat pumps require installation in locations that remain in the 40 - 90 F (4.4 - 32.2 C) range year round and provide at least 1,000 cubic feet (28.3 cubic meters) of air space around the water heater. Cool exhaust air can be exhausted to the room or outdoors. Heat pump water heaters will not operate efficiently in a cold space. They tend to cool the spaces where they are located. (2)
Solar domestic hot water heaters
Solar water heating systems can be a cost-effective way to generate hot water. Because they use the heat from the sun for water heating, they can be used in any climate. These systems include storage tanks and solar collectors. There are two types of solar water heating systems: active systems have circulating pumps and controls, passive systems do not have pumps and controls.
Solar water heaters require an insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one- tank systems, the backup heater is combined with the solar storage in one tank.
There are two types of active solar water heating systems:
- Direct circulation systems, in which pumps circulate water through the collectors and into the facility. These systems work well in climates where it rarely freezes.
- Indirect circulation systems, in which pumps circulate a nonfreezing, heat transfer fluid through the collectors and a heat exchanger. This heats the water that then flows into the facility. These systems work well in climates where freezing is common.
There are two basic types of passive solar systems:
- Integral collector storage passive systems work best in areas rarely subject to freezing temperatures. They are also better suited to locations where there are significant daytime and evening hot water needs.
- Thermosyphon systems, in which water flows through the system when warm water rises as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank.
Solar water heating systems require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup.
One final note on solar water heating: There may be tax incentives to offset the first cost of purchase and installation of a solar system. These incentives can vary by state so research should be done to determine whether tax incentives are available and whether your client qualifies. (3)
Conclusion
The greatest energy efficiency opportunity lies in the design of the entire water heating system of a facility, not just in the selection of the water heater, although Selection of a water heater is an important part of the process. Consideration should be given to the facility's needs, including time of use, amount of hot water required, temperature range and flow rates. After considering these factors, an appropriate selection of a hot water heater can be made that will provide hot water to the facility while conserving energy.
References:
1. California Flex Your Power - A partnership of California's utilities, residents, businesses, institutions, government agencies and nonprofit organizations working to save energy. (www.fypower.org)
2. United States Department of Energy (www.energy.gov)
3. Database of State Incentives for Renewable Energy - DSIRE contains information on what state tax credits or rebates are available for using renewable energy technology (www.dsireusa.org)