Solar Hot Water Recirculation Considerations
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Instant domestic hot water (DHW) recirculation has been popular for many decades. In buildings where the DHW source is a long way from the hot water fixtures, a circulator pump is frequently used to force hot water from the water heater through the hot supply pipes to the base of the fixtures and then back to the water heater through a recirculation-return pipe. A recirculation pump returns the cold water stranded in the pipes to the bottom of the water heater tank, filling the hot supply pipes with fresh hot water. Originally this was done mostly for convenience, so that the user did not have to wait for all the cold water stranded in the hot supply pipe to empty down the drain before the hot water finally arrived. Over the years, this became a standard method for saving water in arid climates and is now required by local codes in some locations for that reason.
In the simplest applications, it is common practice to install a continuous-duty circulator and run it 24/7. However, it has recently become obvious that by using this method, owners save water in a very clumsy way by throwing energy at it—both heat and electricity. This typically results in large amounts of energy being wasted with minimal benefit.
Misconceptions about Recirculation
The idea that we can save water by using more electricity can be a serious blunder if the electrical consumption is not carefully controlled. The backup heating element or burner in any domestic hot water tank must work harder to make up for the heat lost as hot water circulates constantly around a building, resulting in higher fuel bills. In a conventional grid-tied house, the circulator and other elements require electricity from the local power plant—which typically generates this energy by burning fuel and consuming water. In New Mexico, for example, the majority of that fuel is coal, and the water consumed at the power plant is close to 0.75 gallon for every kilowatt hour generated. This means that the water is not really saved; it is being consumed elsewhere.
New solar water heaters generally include highly insulated heat-storage tanks. In standby at normal DHW temperatures, the tank loses heat very slowly. Tanks that we have monitored remotely at SolarLogic show typical temperature drops of about 0.5–0.7°F per hour at the top of the tank when the water is around 125°F. When the solar storage temperature is near its maximum of 180°F, the standby heat loss is faster, reaching 1.5°F per hour and tapering off as the tank cools down. When a recirculation pump is running, the heat loss can increase dramatically. In fact, a poorly controlled recirculation system can draw the entire day’s worth of solar hot water out of a storage tank in a matter of hours.
Figure 1 shows a solar DHW tank in a new house in Santa Fe, New Mexico, with a recirculation pump that causes more than ten times the heat loss when it is running. When solar heat seems to disappear from the tank, causing the backup heater to run even during sunny weather, it is always a surprise and a disappointment to both the owner and the installer.
Rather than hoping this will not happen, it is better to avert this situation by including an upgrade to the DHW recirculation control system as part of the solar DHW package. At least offering this as an option lets your clients know that you are energy aware and have their best interests at heart. There are several possible upgrades in a DHW recirculation system to improve energy performance and eliminate this unintended waste.
With new construction, the hot water supply and recirculation-return plumbing details can be designed to minimize heat loss and maximize pumping efficiency. In a retrofit, the improvements may be more difficult to implement, but you should always carefully consider them.
Pipe insulation. Both the hot water supply and the recirculation return must be well insulated, especially in locations with ground contact or cold air temperatures. I have seen many installations where the recirculation-return line has no pipe insulation. Many experienced plumbers think of recirculation returns as cold lines, but they are not. When in use, the recirculation-return line is definitely a hot water pipe.
Pipe size and length. It is common practice in residential construction to connect all the hot faucet returns to a single half-inch recirculationreturn line. Often, this is done without regard for the length of the return line, the pumping power required or the control method intended for the pump. When designing a new recirculation system for high-energy performance, upsizing the return pipe may be required in order to downsize the pump wattage.