Solar Hot Water Recirculation Considerations: Page 2 of 2
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Balanced flow. When a single recirculator is used on several parallel loops to various parts of the building, the flows must be balanced just like the loops in a hydronic heating system. Even a well-controlled recirculation pump wastes energy pumping through an unbalanced piping system because most of the flow goes through the shortest—and hottest—loop. A balance valve placed on the recirculation-return pipe under each fixture can be well worth the extra effort.
Multiple circulators. In larger buildings, energy savings may be accomplished by using several circulators instead of a single one feeding parallel loops. You can save both heat and electricity when you control each pump to provide hot water recirculation only to the occupied parts of the building.
Spring check valve. Every recirculation-return pipe must have a one-way check valve to prevent cold water from flowing backward to the hot faucet when the recirculation pump is turned off. Some pump manufacturers, such as Taco and Grundfos, include built-in check valves (known as integrated flow check valves) in the pump body, making this part of the installation completely painless. Do not forget to include this detail. Double check that it is installed in the right direction, pumping into the bottom of the DHW tank.
The most common upgrades to any DHW recirculation system, new or existing, are electrical controls. The typical controls I have added in recent years are listed here in order of most common to least common.
Temperature setpoint switch. The easiest and cheapest way to limit the run time of the recirculation pump is to put a sensor on the recirculation-return pipe that turns the pump off when hot water comes back from the building. This can be a snap-disc, a cap-tube or an electronic setpoint controller. When the return pipe cools off, the pump runs only until hot water arrives back in the mechanical room and then pauses until the pipe cools again. For proper control, the circulation loops must be balanced.
Timer switch. A timer switch can be used to allow the recirculation pump to run only during critical occupancy hours. A timer switch is often used along with a setpoint control. The biggest drawback of the common timer switch is that the clock must be reset after a power failure.
Manual demand switch. A momentary contact switch, which resembles a doorbell button, can be placed at each hot water fixture. Homeowners who are energy conscious and do not mind asking for instant hot water can press this button. Both wired and wireless switch systems are now available.
Automatic demand switch. An automatic switch such as an infrared (IR) motion detector or IR beam switch can be installed near each hot water fixture. This type of sensor uses a relay to press the demand button whenever it senses a nearby presence. A timer or setpoint switch is used to turn off the circulation after a reasonable amount of time.
The manufacturers of hot water recirculation pumps now offer many of these controls and features either built into their pumps or as add-on control packages. Some are fairly sophisticated, with built-in sensors, timers and electrical connections for demand switches. If you have not seen this equipment at your local supplier, ask about it. For peak energy performance, good recirculation control is just as important for systems without solar heat input as those equipped with it.
When modifying these systems, sometimes the existing recirculation pump will not be suitable. This is most often the case when a continuously circulating system is modified to include a demand switch. With a demand switch system, the circulator runs only for a few minutes. In that short amount of time, the user expects the hot water to arrive without delay. If, as is typical of older retrofits, the pipe diameter is very narrow, a pipe run is long and/or the heat loss is high, a larger circulator pump may be needed to provide the flow and pressure to deliver the goods on time.
For a recent retrofit solar heating system, I modified a continuously circulating Taco 006 bronze pump with demand buttons and a timer switch to minimize its run time. The owner then reported that it took 7 minutes for the hot water to arrive at the far end of the house. Since it was a retrofit situation with all the piping under a concrete slab, the plumbing and balancing could not be changed easily. I substituted a Taco 009 bronze pump, and the hot water then arrived in less than 1 minute. Since the 006 pump originally ran for many hours with a 59.8 watt rating, and the 009 pump runs only minutes per day with a 161.0 watt rating, the electrical savings per day is substantial. The heat savings in the solar water heater tank is plainly evident with consistently higher water temperatures.
Remember that DHW piping is an open system that requires either bronze or stainless steel pump bodies to resist the corrosion caused by the high oxygen content of potable water. Cast-iron pump bodies are not recommended in this application.
—Bristol Stickney / SolarLogic / Santa Fe, New Mexico / solarlogicllc.com