Drainback Solar Hot Water Systems


Inside this Article

Drainback reservoir with integrated heat exchanger.
A cutaway diagram of a drainback reservoir with an integrated heat exchanger.
The DBHX 6000 Drainback Reservoir from Radco
The DBHX 6000 drainback reservoir from Radco comes with an internal heat exchanger, heat-transfer fluid pump, domestic hot water pump, and differential controller.
Sloped Pipe Run
Pipe runs, like this insulated return for the heat-transfer fluid, should slope a minimum of 1/4 inch per foot of horizontal run.
Drainback Reservoir Tank
Mounting drainback reservoirs as high as possible within the conditioned space reduces total head, and thus the load on the HTF pump. Proper support is required, as the reservoir and other components may weigh 100 pounds or more, depending on the volume.
Drainback Reservoirs with or without Integrated Heat Exchangers
Heat Transfer Products offers drainback reservoirs with or without integrated heat exchangers.
The Taco 009-F Pump
The Taco 009-F is a typical high-head pump suitable for heat transfer fluid circulation to the solar thermal collectors in a residential DHW drainback system.
The Taco 006-BW Bronze Pump
The Taco 006-BW is a bronze pump. It is well-suited for domestic hot water loops.
The Grundfos 15-18 SU Stainless-Steel Pump
The Grundfos 15-18 SU is a stainless-steel pump. It is well-suited for domestic hot water loops.
Drainback reservoir with integrated heat exchanger.
The DBHX 6000 Drainback Reservoir from Radco
Sloped Pipe Run
Drainback Reservoir Tank
Drainback Reservoirs with or without Integrated Heat Exchangers
The Taco 009-F Pump
The Taco 006-BW Bronze Pump
The Grundfos 15-18 SU Stainless-Steel Pump

These popular, proven performers offer simplicity, reliability, and flexibility in system design.

Most locations in North America are subject to at least one freeze per year, making freeze protection necessary for SHW systems. Some systems are pressurized, using glycol or other types of antifreeze to prevent freezing; others, such as the failure-prone draindown and recirculation systems, use motorized valves and additional controls. But the beauty of a drainback system is that freeze protection is passively provided by gravity’s ever-reliable pull.

Drainback Details

Drainback systems are closed-loop, indirect, active systems. A heat-transfer fluid (HTF, usually water) contained in an unpressurized, closed loop is pumped through the collectors and is separate from the end-use water being heated through a heat exchanger. When the pump is off, the HTF drains out of the properly sloped collectors and pipe, leaving them empty and protected from freezing. 

While this article will primarily focus on residential domestic hot water systems, many options exist for system configurations, ranging from pool heating to space heating and combined domestic hot water and space heating. Regardless of what the heat is being used for, the basic components of a drainback system are:

  • A storage tank, to hold the end-use water being heated. This could be a domestic hot water tank, a several-thousand-gallon tank for a space-heating system, or, in the case of a pool system, the pool itself.
  • One or more solar hot water collectors.
  • A differential controller, which monitors the water temperature in the tank and the collector temperature. When the collector temperature exceeds the storage temperature by a set differential (number of degrees), the controller activates the HTF pump. In some cases, a DC pump, powered by a PV module, acts as the controller—when appropriately sized, the PV module receives enough sunlight to operate the pump when there is enough heat in the thermal collector for it to be worthwhile to circulate the HTF.
  • A reservoir—a tank in the HTF loop plumbing—which holds the HTF in the drainback/collector loop. The draining of the HTF into the reservoir creates the characteristic gurgling sound of these systems, due to the air space required in the unpressurized reservoir.
  • A heat exchanger to transfer heat between the drainback loop and the end-use water (unless it is a pool system). The heat exchanger is often inside the drainback reservoir, and a second pump circulates domestic water through the exchanger, which is immersed in the HTF. Most pool-heating systems circulate the pool water through collectors, eliminating the need for a heat exchanger.


Drainback systems have many advantages compared to other types of SHW systems. Because it needs an air space in order to drain, the HTF loop is not pressurized. Less stress is placed on solder joints, threaded fittings, and gaskets. If a break occurs in the HTF loop plumbing, it will leak more slowly than if it was pressurized. Furthermore, there are no motorized valves to fail, and the system does not rely on electricity to maintain freeze protection. If the power goes out, the pump shuts off and the HTF drains from the collectors back into the reservoir.

Having an unpressurized HTF loop means that numerous components required in pressurized systems are not needed. An expansion tank, check valve, pressure gauge, and an air vent are not required, though a pressure-relief valve may still be installed. Though any equipment cost savings, might be negated by the expense of the drainback reservoir, installation costs can be lower because the installation is simplified to some degree.

Pressurized, glycol antifreeze systems offer extremely reliable freeze protection. However, these systems are susceptible to overheating. If the pump fails or the system shuts off because it has reached its high limit, the system pressure will increase and can actuate the pressure-relief valve. This can result in the system operating at reduced pressure, requiring additional HTF to be added. Furthermore, glycol can become acidic when repeatedly overheated, risking corrosion to the piping and other components in the system. Drainback systems simply drain the collector because the pumps shut off when the system’s high limit is reached.

Drainback systems are often superior for space heating because of that same potential for overheating. Space-heating systems require enough collector surface area to generate useful heat during the coldest, shortest days of the year. But a large system is prone to overheating during the warmer months of the year. During the non-heating season, glycol systems may require covers for some or most of the collectors, a heat dump, a second heat load (such as a pool), some combination of these options, or even draining the system entirely for the season. A drainback space-heating system can safely exceed the recommended collector-to-storage ratio—usually 1:1 or 1:2, depending on the area climate—which means more square footage of collector space capable of producing more Btu in the winter, without the problem of overheating during summer months. The system simply shuts off when the high limit has been reached. Large, combined space-heating and DHW drainback systems can heat water very quickly in the summer when there is no space-heating load and all of the production is focused on the domestic water.

Although some drainback systems use a blend of antifreeze and water for enhanced freeze protection, many use only distilled water as the HTF. Water has superior heat-transfer abilities compared to glycol, adding a slight boost to system performance. Using water as the HTF also allows for the use of single-wall heat exchangers, which are slightly more efficient and usually less expensive than double-wall varieties. The 2009 Uniform Solar Energy Code now allows single-wall exchangers to be used in some propylene-glycol-based systems—but single-wall exchangers are always allowed in drainback systems using water as the HTF. 

Drainback Considerations

Because the HTF is in the reservoir and the collector is empty  when the pump starts, the pump has to overcome more static head to lift the HTF from its lowest level to the highest point in the system. This requires a larger pump, using more electricity. For systems with very high head, large-enough pumps may not be available. Some installers will plumb multiple pumps in series, but if one pump fails, the HTF may not make it all the way to the top of the collector and out the return piping, leaving it vulnerable to freezing.

Many drainback systems use a second pump to circulate the end-use water through the heat exchanger in the reservoir, adding to the amount of energy required. A standard, two-pump, residential domestic drainback system can easily use three or more times the amount of power to operate the pumps compared to a single-pump, pressurized glycol system. For example, a pump in a glycol system may use about 85 watts, while the total for the two pumps in a comparably sized drainback system may be 260 watts or more. In some cases, the additional energy required—which could be nearly 2 kWh per day for a domestic system during high-production days—makes it prohibitive to install drainback systems in off-grid applications.

To facilitate HTF drainage from all the plumbing that is exposed to potential freezing—including the collector, exterior piping, and plumbing in unconditioned spaces such as an attic—it is critical that they slope downward to drain. Any low points, whether due to improperly sloped or sagging piping, can trap water, defeating the freeze protection. If the slope is insufficient, a vacuum can result in the HTF loop, which will also prevent it from fully draining. It is recommended that larger piping be used to reduce the head caused by friction, which, for the same flow rate, decreases as pipe size increases. For example, if a pressurized, single-collector system would be plumbed with 1/2-inch pipe; a comparable drainback system should use 3/4-inch pipe. 

Even though it is theoretically a closed loop, HTF will evaporate. The drainback reservoir volume should be checked periodically, and topped off if fluid is low. If the volume of HTF is too low, it may result in reduced or even no output, and possibly damage the pump.

Drainback system freeze protection is not fail-safe. Though rare, relays in the differential controller that operates the pump(s) can fail, causing the pump to continue to run and circulate fluid through the piping and collectors. Pipes can sag over time, or may have been poorly installed and improperly sloped in the first place; low spots in the plumbing can trap water and prevent it from fully draining, defeating the freeze protection of the system.

During cold conditions, this puts the system in jeopardy of freezing. Although the actual amount of water released by a freeze-caused leak will be limited to less than the volume of HTF in the system, damage to building materials and contents of the building can occur, and bursting can damage the absorber plate in collectors, requiring potentially difficult and expensive repairs.

A common “fix” for a poorly plumbed or aging drainback system is to add propylene glycol to the water, ranging from 30% to 50%. In cold climates, the system may even be designed to use an antifreeze mix to increase the reliability of the freeze protection. Because the glycol will leave a film inside the collector when it drains, it is very common to see the HTF mix become brown and acidic over time. As with a pressurized glycol system, periodic testing of HTF acidity should be followed with its replacement as needed.

Scale buildup inside the heat exchanger can also be an issue, since minerals precipitate out of water as it’s heated. They accumulate on the pipe walls, reducing the heat exchanger’s flow and efficiency. Eventually, the heat exchanger may become completely clogged, rendering the system ineffective and causing the motor in the end-use water’s circulator pump to overheat. Depending on local water conditions, occasionally flushing the heat exchanger with a chemical descaler or a compound such as muriatic acid may be necessary. Scale buildup can also occur in the collectors, which is why distilled water should be used as the HTF. This is especially critical in systems that lose HTF over time due to evaporation, since this results in mineral deposits being concentrated in the collectors, which is especially problematic for the narrow riser tubes.

Design & Sizing

The drainback reservoir is unique to this type of system. Its capacity should be at least twice the volume of the HTF loop, including the volume of the collectors. It must be able to hold enough fluid so that, when pumping, the pipes to the collectors and the collectors themselves are full. Also, enough fluid must remain in the reservoir to ensure that the HTF level stays above the pump’s inlet so that it does not suck air. If air bubbles get stuck in the pump—a condition called cavitation—the pump must work harder to move the fluid. This can cause the pump to overheat, increasing the possibility of premature failure, and cavitation can even completely stop the flow of the HTF. Furthermore, if the reservoir is undersized, the HTF can operate at too high of a temperature, reducing the efficiency of the collectors. If the reservoir is too large, the HTF may operate at too low of temperature, resulting in a poor heat exchange efficiency between the HTF and the end-use water.

If the heat exchanger is located inside the drainback tank, the HTF level must be high enough so that the coil is submerged while the system is pumping. Consult the manufacturers’ spec sheets for collector volume, as well as for the recommended system capacity for a specific drainback reservoir. The pipe length and diameter will differ on each installation and can be calculated as shown in the pipe sizing table.

Many prepackaged drainback systems locate the drainback tank on top of the storage tank, which can reduce the head—and thus the pump size—for the system. In some cases, the drainback reservoir is placed in a different location than the storage tank—for example, the storage tank may be in a basement, while the drainback reservoir is on the ground level or second floor. This can dramatically reduce head, but the reservoir must be located in conditioned space to avoid freezing (i.e., not installed in an uninsulated attic).

Access to the fill and drain ports in the drainback tank should be considered when designing the system, so that the HTF can be checked and topped off if needed. A sight-glass can be installed on the HTF loop plumbing, level with the top of the reservoir. This makes it easy to check the HTF level when the system has drained back into the reservoir. Using a transparent flow meter as the site tube allows the flow rate to be verified and fine-tuned if necessary.

Heat Exchanger Configurations

Integrated Exchanger in the Reservoir. Many domestic hot water systems use a separate, special drainback reservoir that contains a heat exchanger. The reservoir tank, typically ranging in size from 7 to 15 gallons, has collector supply and return ports for the HTF loop. A second set of ports connects to the internal heat exchanger and is used to route water from the storage tank to the heat exchanger in the reservoir. This system requires two pumps, which are operated by the same controller. One circulates the HTF, which bathes the heat exchanger through which domestic water is pumped by the second pump. In domestic hot water systems, the two pumps will typically be running at the same time. Larger heating systems with big reservoirs are usually installed so that the pumps can run independently because there will be usable heat in the reservoir after the HTF loop shuts off. Likewise, the HTF loop may have to circulate for a while at the start of the day before there is enough heat in the reservoir for the space-heating loop to circulate.

External Exchanger. Another option, which also requires two pumps, is to use an external heat exchanger and a 10- to 20-gallon electric hot water tank as the drainback reservoir. The electric element is not hooked up—the tank is used to simply hold the required volume to fill the HTF loop when pumping and provide an air space for the system to drain. When the system is operating, the HTF passes through one side of the heat exchanger. The domestic water is pumped through the other side of the exchanger in the opposite, counterflow direction. 

Storage Tanks with Integrated Exchangers. Using storage tanks with integrated heat exchangers is another popular drainback option. A separate reservoir is still required for the HTF, but only a single pump is needed to circulate the HTF through the collectors, the heat exchanger immersed in the storage tank, and back to the reservoir, completing the circuit.

Larger space-heating systems often use custom drainback tanks, with options for multiple heat exchangers. A large coil of stainless or copper piping submerged in the reservoir tank is used to transfer heat to the storage water; additional exchangers are used to provide heat to a specific location. A backup heat source, such as a boiler, can provide additional heat to the end-use water, activating automatically if a sufficient temperature is not being attained by the solar thermal part of the system.

Choosing a Tank & System

The selection of drainback tanks and systems is growing. While there is the option of using a standard hot water tank as the reservoir, or to use a storage tank with integrated heat exchanger (with the HTF filling the tank), specialized tanks and even complete, packaged systems are available.

Alternate Energy Technologies, Energy Labs, Heat Transfer Products, and Radco are just a few manufacturers that make drainback tanks for residential applications. Some are complete, plumbed systems with the drainback tank mounted on top of the storage tank, and the pumps and controller included. Most make reservoirs with or without integrated heat exchangers. This allows great system flexibility: the heat exchanger model can be used with a standard, four-port storage tank; the tank-only model can be used with an external heat exchanger/four-port storage tank; or a storage tank with an integrated heat exchanger can be used. Models with heat exchangers typically range in capacity from 7 to 20 gallons; models without exchangers are available in larger sizes. Some feature a built-in sight glass in the reservoir. Smaller reservoirs are capable of being mounted on top of the storage tank, saving floor space as long as enough height is available. 

SunEarth’s Copperstor drainback tank takes a slightly different approach. Designed to be used with an external heat exchanger or a storage tank with an integrated exchanger, this drainback “tank” is merely a bulge in the plumbing. Comprised of several large-diameter (4”) copper tubes plumbed in parallel, the “tank” provides a simple and easy-to-install reservoir available in 5- or 7.5-gallon capacities. The Copperstor tank is heavy when full and, unlike a reservoir tank that sits on the floor or on top of the storage tank, will require more support than that provided by the piping that runs to and from it.

Trendsetter Solar Products and Morely Manufacturing make large, unpressurized tanks for use in space heating applications or for large-capacity domestic water systems. Various sizes and numbers of heat exchanger loops are available for these large systems.

Choosing a Circulator Pump

In domestic drainback systems, the Taco 009 and Grundfos UP26-64 and UP26-96 are popular HTF pump choices because of their proven durability and ability to lift the HTF 30 feet or more in an unpressurized loop. These (relatively) high-head, cast-iron circulator pumps use more energy than those in pressurized systems. Whatever pump is used, it must allow the reverse flow of the HTF back into the reservoir when the pump turns off; the majority of small circulator pumps do this, but verify that there is not a built-in check valve in the pump (an option in some models), as this will prevent the loop from draining.

If the pump is not adequately sized, it may be able to pump HTF into the collector, but will have difficulty completing the circuit and exiting the collector. During hotter months, this can result in a collector full of HTF that quickly boils and releases steam, reducing the loop volume. As HTF is lost and the loop volume is reduced, it becomes less likely that the pump will be able to complete the circuit, resulting in more steam being produced and more HTF loss. Eventually, the pump will overheat. During colder months, this scenario can lead to burst pipes in the HTF loop or the collector, as the water enters the collector, does not circulate all the way through, and, unmoving, eventually freezes. 

When a two-pump system is used, the domestic/storage water circulator must be bronze or stainless steel, because the impeller housing of an iron pump will clog within a few months due to the corrosion caused by the dissolved oxygen in the water. Common choices are the Taco 006 and the Grundfos 15-18 SU, which use much less energy than the high-head HTF pumps, and are easy to service or replace if necessary.


Brian Mehalic is a NABCEP-certified PV installer, with experience designing, installing, and servicing PV, solar thermal, wind, and water-pumping systems. He develops PV curricula and is an instructor for Solar Energy International.


“SDHW Installation Basics Part 3: Drainback System,” by Chuck Marken & Ken Oldham, HP97

“Single-Tank Solar Water Systems,” by John Patterson & Suzanne Olsen, HP124

“Solar Water Heating Systems Buyer’s Guide,” by Chuck Marken & Doug Puffer, HP125

“Solar Hot Water Storage: Residential Tanks with Integrated Heat Exchangers,” by Brian Mehalic, HP131

“Pick the Right Pump: Choosing a Circulator for Solar Hot Water Systems,” by Chuck Marken, HP121

Comments (1)

Dustin's picture

Dear Mr. Mehalic,
Thank you for making this article, I learned a lot from it.
I was wondering can you just explain one thing: When you say:
"Most locations in North America are subject to at least one freeze per year"

What does the "one freeze" relate to? Is this a one freeze-thaw cycle of the soil? Or is "one freeze", simply an hour during a year when air temperature is below 0 Celsius (32F)?
Can "one freeze" be an air temperature of -1C (30.2F)?

Thank you for the reply.

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