Drainback Solar Hot Water Collection Innovations

By Peter Biondo

Drainback methods for solar hot water heating are not new to the solar industry. However, drainback technology has changed over the years to include innovative features from design installers all over the United States. In this article, I will discuss these innovations which are widely and successfully used by licensed design installers who prefer the service-friendly qualities that drainback systems tend to offer. In my own experience, designing for drainback is an application-driven decision. In a drainback system, the collectors are in failsafe mode — always completely empty except for cycles of solar collection. Ordinarily, the collectors and the collector piping slope to drain into a reservoir within the building envelope — most likely in the mechanical room. The other form of solar collection is the solar closed loop, in which there is a pressurized antifreeze loop between the collectors and the heat exchanger. These are very different collection systems.

The use of drainback is found to be preferable when hot water loads (demand) have wide variations. This is often determined by the occupancy of the building (for domestic hot water) or seasonal variations (for space heating). The all-important specification to consider for drainback is that the collectors and piping must slope to drain back to the drainback reservoir (there are ways around that — I will point them out later). Roof-mounted solar collectors offer this advantage to drainback. The advantage of the closed loop system is that the collectors may be mounted anywhere in relation to the solar storage tank. There is a great deal of debate over which system is better than the other, and I have listened to both sides. I personally recommend understanding the benefits and disadvantages of both systems and then choosing the one that most appropriately matches the building and the application. This article will focus on drainback solutions.

The most significant advantage of drainback design is built-in freeze protection with the added benefit of collector overheating protection. An empty collector won’t freeze at night or during cloudy days with freezing temperatures. When the solar tank reaches its high limit temperature during a collection cycle, collection ends, circulation ceases, and every bit of working fluid drains out of the collectors. Most solar thermal collectors, without working fluid, cannot be damaged by sitting in the sun in the heat of the day. By design, drainback requires that solar thermal collectors be emptied by gravity.

Most domestic flat plate manufacturers internally pipe collectors in what is known as a harp design, with high and low manifolds connected with vertical risers. These collectors will typically drain empty, as long as the manifolds are sloped to drain. Many European collector models have what is referred to as a serpentine pipe configuration. Serpentine collectors may drain by gravity, while others that cannot are designed exclusively for closed loop systems (see Figure 1). Consult the manufacturer about a drainback option for serpentine models. Few heat pipe evacuated tubes can be integrated for drainback. The manifold must be drainable, and the tubes able to reach stagnation without compromising the vacuum seal. Consult the evacuated tube manufacturer for the drainback option.

For buildings such as schools, seasonal lodges, second homes and offices that are unoccupied for days, weeks, or even months at a time, drainback should always be given serious consideration. This is also the case for space heating systems where the collectors would not be providing for a summertime load. Drainback systems do not require a heat dump, which would be necessary for some antifreeze closed loop systems. In a closed loop system, excess solar energy must be dumped when chronic and excessive temperatures are anticipated in the glycol loop. That would be a requirement for the applications listed above and especially for space heating. However, there are multitudes of domestic hot water and space heating/pool heating/reheat combinations where the solar tank temperatures seldom reach high limit due to the size of the load relative to the collector area. Closed loop systems lend themselves well into these load profiles due to the simplicity of the design and installation.

There is one important detail to pay attention to in either system, however — sizing the solar storage tank to the collector energy capacity. The goal to meet for a solar hot water system is to raise the temperature in the solar tank so that it first preheats cold water and climbs at or above design temperature (i.e. 120°F for DHW) while having enough storage volume to absorb the remaining daytime collection cycle. The general rule of thumb for solar storage sizing is that for every one square foot of collector aperture, you will need to store anywhere between 1 to 2½ gallons of water, depending on your location and the application. Large collector banks with small storage vessels can be problematic and should be avoided. The collector investment is wasted by too many idle hours during a daytime cycle due to excessive solar storage temperatures. The exceptions to that rule would be buildings, such as hospitals or process heat factories, with consistent daytime hot water loads.

Drainback designs with pressurized solar storage vessels

There are a multitude of drainback designs which could be covered in this article. Since the reader may be accustomed to using pressurized vessels, I have included a diagram for a basic drainback system that is comprised of a drainback reservoir, an external heat exchanger, and a pressurized solar storage tank (see Figure 2). I have also included a “how-to” design for drainback so the system will operate efficiently and trouble free. There are variations to the basic design that I will also cover.

The most obvious criteria to be met for a drainback system is that the top of the drainback tank must be below the bottom of the collector level. The collectors must drain into the drainback reservoir during non-collection periods. There are specifications about the drainback reservoir itself that are important to meet. One is that the volume of this reservoir must be larger than volume of collectors and piping above it. Another is that a piped vacuum break should be included at the top of the drainback vessel so that during drainback, immediately following the ceasing of circulation, air can be released from the top of the drainback reservoir to escape out of the tank and back into the collector array. A site glass should be included to view the water level in the drainback reservoir. A flow meter may be substituted for a site glass if it is located at the level of fill.

Variations of the drainback tank may include its location. It is not necessary to install the drainback tank in the mechanical room with the heat exchanger and the solar storage tank. In tall buildings where the mechanicals are in the basement, it is better to locate the drainback reservoir high in the building. This choice is primarily made to size a pump for the collector loop lift that would not have to overcome the entire height of the building, saving on both pump size and operating cost.

The drainback pump is also something important to pay attention to. The pump selection and installation are very different than those chosen for a closed loop system. Drainback pumps for drainback reservoirs are stainless steel or bronze construction. As fresh oxygen is usually present in a drainback tank, cast iron circulators would rust and fail. Always mount the pump below the static fill line as far as possible. In order to do this, the drainback vessel will sometimes need to be installed below the drainback tank. For this reason, it is not unusual to see a drainback tank installed above ground level, and the pump located under the bottom of the drainback reservoir. This is not always the case, but for small drainback tanks under 20 gallons it usually is. The circulator must be capable of delivering the reservoir fluid at 2 feet per second through the piping. The speed of the water is necessary to push steam through the collector, which will occur during start up in the heat of the day. As an added precaution, balancing valves can be installed on the collector return side to keep the fluid velocity into and through each collector array high during the initial fill.

Another consideration is mounting a drainback pump in a vertical position rather than horizontal. I have seen horizontal pump installations be the cause of a troublesome and hard-to-pinpoint problem causing pump cavitation. Another thing which will help with fluid pumping dynamics is to seal the drainback loop and pressurize it to 15 to 25 psi. There would be no airvent installed anywhere in a drainback loop; the piping and thread joints will be airtight. Adding an air charge to a drainback system would verify any leaks that may be present and help the pump do a better job lowering the potential for bothersome issues of pump cavitation.

The last thing I would like to point out is that in a drainback system, the collectors and the collector piping can be installed dead level. The collectors and piping will drain even if mounted level. As a safeguard, the drainback fluid would include an antifreeze mix. Some state programs require an antifreeze mix in the drainback reservoir as a precaution for freeze protection anyway. However, there may be codes and rebate programs which would require that the slope drain at ¼ inch per foot for collectors and piping. I will point out that collectors and piping installed dead level for drainback are being successfully implemented in extremely cold climates and for large commercial systems of 100+ collectors. The practice of doing away with slope in order to drain collectors and piping may give drainback a whole new appeal to the solar industry, as well as a whole new set of options for how to (or how not to) install for drainback.

Peter Biondo is the technical sales coordinator for Oventrop Corp. He has been involved in hydronic heating and solar thermal technologies for more than 25 years. His primary work is assisting mechanical engineers and contractors with balanced distribution hydronic systems and solar hot water and space heating. His solar thermal and hydronic heating workshops are featured in trades shows throughout the country.