Energy management opportunities (EMOs) can help increase a building system’s performance and decrease its operational costs. Numerous EMOs exist for Service Water Heating (SWH) systems. These include:
- Reducing hot water loads
- Reducing distribution losses
- Improving heater performance with energy-efficient heating systems
- Minimizing SWH system corrosion
Although it may seem basic, turning the thermostat to the lowest allowable temperature setting can also create substantial savings.
Reduce Hot Water Loads
A commercial building’s hot water load is a combination of two factors:
- The hot water consumed by building occupants.
- The amount of energy needed to heat the water, which depends on the temperature difference between SWH feedwater and supply water.
A building’s hot water load can be reduced by decreasing the amount of hot water used in a building, ensuring that hot water is the proper temperature, and raising the temperature of the water fed to water heaters.
Building occupants primarily determine how hot water is used.
Fixing drips in hot water faucets or leaks around pipe joints are obvious ways to reduce hot water consumption. A leak of one drop per second adds up to about 1.8 gallons per day, or 650 gallons per year. Small leaks in a hot water distribution system can waste a considerable amount of water, energy, and money.
If a substantial amount of hot water is used for hand washing, it may be worthwhile to install spring-loaded hot water taps, which will help reduce hot water consumption. Another way to reduce hot water consumption is to install flow-restricting orifices in the water line.
Some buildings use tempering valves (adding hot water to the cold) to toilets to avoid excess sweating of the fixtures, which can add to your costs. Many newer systems have made these tempering valves obsolete. It is possible to realize considerable savings if your systems have been upgraded or modified and no longer need these valves. If your systems still require them, you should check them as part of your routine maintenance plan to ensure they are functioning at their maximum efficiency.
Check Temperature Settings
Check the temperature setting of your SWH system. The setting most commonly found is the original factory setting of 140°F. If your system is primarily used for hand washing, for example, a storage tank temperature setting of 105°F or less is usually adequate.
A lower temperature setting on a central storage tank means that less energy is consumed for water heating. Lower settings also reduce the transmission losses that occur when heat from the water moves to the ambient air surrounding the storage tank and piping. When water temperature is reduced from 180°F to 125°F, for example, heat losses from uninsulated piping in a 68°F room are cut in half.
When a building is using a central SWH system, different hot water temperature requirements can be met by mixing hot water with cooler water. This method is inefficient, however, since water must usually be generated and stored to meet the highest temperature requirements. If higher temperatures are needed for short durations, it is usually more efficient to use separate, instantaneous hot water heaters.
Reduce Distribution Losses
Thermal losses from a hot water system can occur in either the storage tank or the distribution piping system. These losses are proportional to the temperature difference between hot water and its surroundings and the resistance of pipes and storage tanks to heat flow. These losses can be reduced by lowering the hot water temperature and adding insulation.
Insulate SWH System Piping
Piping insulation is the most effective way to reduce losses in hot water piping systems. It is common to insulate all hot water supply and return piping. Even a minimum quantity of insulation can reduce heat losses for various pipe sizes.
Bare pipes should be insulated, and damaged insulation should be repaired or replaced if the hot water system has a circulation pump. If no circulator is present, insulate the first 15 to 20 feet of pipe exiting the storage tank to reduce thermosiphon losses.
Small pipes should be insulated with cylindrical half-sections of insulation with flexible cell material. Large pipes can also be insulated with flexible material. If access to pipes for servicing is required, rigid insulation offers an advantage.
Fittings such as elbows, valves, and tees should be insulated with preformed fitting insulation, fabricated fitting insulation, individual pieces cut from sectional, straight pipe insulation, or insulating cements.
Insulate Storage Tanks and Water Heaters
The heat loss from hot water storage tanks is a significant portion of the total water heating load. This heat loss must be continuously offset by a supply of heat to maintain the hot water at its temperature setpoint.
To reduce these losses and save money, cover bare storage tanks and external heaters with at least 3 inches of insulation (thermal resistance R-10). Apply additional insulation up to 3 inches to tanks that are not sufficiently insulated. Replace or repair all torn or missing insulation as required. In the case of storage-type heaters with factory insulation jackets, add an additional layer of insulation.
Install Energy-Efficient Water Heating Systems
Decentralized, local heaters can be used to minimize distribution losses. Using separate water heaters may allow for the central plant to be shut down when not in use. Ensuring proper selection and operation can minimize energy waste from recirculation pumps. Finally, heat pump water heaters and solar water heating systems can further improve efficiency by reducing water heating costs where appropriate. In buildings using a central water-cooled chiller, it is possible to use the cooling tower to recapture heat to preheat supply water.
Install Energy-Efficient Recirculation Pumps
Select a hot-water recirculation pump with the minimum horsepower needed. A common practice in controlling the circulating pump is to let it run all the time or to have a temperature-sensing thermostat start and stop the pump according to the return water temperature. In either case, it is a good idea to provide a 24-hour or seven-day time clock or a manual switch to shut off the circulator when SWH is not needed. This saves pump energy and reduces storage tank and piping heat losses.
When using a thermostat to control the pump, locate the temperature-sensing element near the last or most remote fixture to take maximum advantage of the water in the supply piping system. A setting of 110° to 120°F is usually adequate.
Install Desuperheaters and Heat Pump Water Heaters
The efficiency of a SWH system can also be improved by adding a desuperheater or by replacing an existing electric water heater with a heat pump water heater. A desuperheater is made up of a heat exchanger, a small water circulating pump, and controls contained in a cabinet on the wall or floor. Its main component is a doublewalled refrigerant-to-water heat exchanger. This component is used to recover waste heat for SWH from the hot refrigerant vapor in the condenser of an air conditioner or a heat pump.
A heat pump water heater can perform the dual function of heating water and providing space cooling and dehumidification. Dedicated heat pump water heaters are devoted to producing hot water and are not integrated with space conditioning equipment. Commercial heat pump water heaters have a COP of about 2 to 4 when heating water to 140°F and of 4 to 6 when heating water to 85°F. They are generally moderate in cost and easy to install.
This article is adapted from BOMI International’s course Energy Management and Controls, part of the SMA and SMT designation programs. More information regarding this course or the new High-Performance certificate courses is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.