Most solar water heaters produce about twice as much heat in the middle of summer as they do in mid-winter. Long summer days, higher sun path, higher ambient temperatures and more intense sunlight make great conditions for solar-heating water. While an abundance of solar-heated water is welcome, overheating is not, and can be a big problem in antifreeze systems.
The same glycol solution that protects a system from freezing will boil between 220°F and 275°F, depending on the system pressure—and that’s a problem. If the solution gets too hot, the buffers in the glycol that prevent acidity can break down, leading to possible damaged pipes, and tripping pressure-relief valves—resulting in having to refill and repressurize the system.
Too much collector for the amount of storage available or too big a system. During summer, the conditions are ideal for solar hot water. With too much collector area compared to storage area there’s nowhere for that excess heat to go. In most of the United States, systems should not have more than 1 square foot of collector surface area per 1.5 gallons of water storage. In the sun-drenched Southwest, storage can be increased to 2 gallons per square foot of collector. Increasing storage can prevent overheating, but costs more, takes up more space, and takes longer for the collectors to heat the tank to the desired temperature.
Wrong system type or tilt too shallow. A collector gathers the most energy when its face is perpendicular to the sun—the lower the tilt, the more prone the system is to summertime overheating. Many people prefer a lower tilt for aesthetics, but a “laid-back” antifreeze system (one that’s too flat) is more prone to overheating. In snowy climates, laid-back collectors will take longer to shed the snow from the collector glass, leading to decreased winter production. Installing a drainback system instead can prevent this kind of overheating—when the tank reaches the desired temperature, the solution drains out of the collectors.
Lifestyle and load changes. Even correctly designed and installed systems can still encounter overheating problems, typically because of usage changes. When the home is vacant or has fewer occupants than planned for—and, therefore, lower hot water usage compared to what the system provides— the system can overheat. Families also change in size. Then there’s individual usage habits for owners living with a previously installed system—the SHW system may produce too much hot water for them.
Drainback systems. The warmer the climate, the better drainback systems perform—they don’t suffer damage from overheating. Although the collector can overheat, it won’t have fluid in it because it drains whenever the collector loop pump shuts off—so problems are avoided.
Drainback is the freeze protection for the collectors, but the pump can also be shut off to prevent overheating if the differential control has a high-limit function, actuated by the storage or tank sensor reaching the set point. Most controls have field-adjustable high limits; recommended settings are about 170°F.
Some designers and installers don’t like drainback systems—the farther north you go in the United States, the more likely this is true. In the 1980s, these systems got a bad reputation due to freezing because of substandard controls, poor installations, sagging pipe, and a few other reasons. In most locations, drainback systems are the first choice and antifreeze systems are the second choice, although drainback system installation is unforgiving, as the collectors and piping must be sloped correctly. For ground-mounted applications, the choice is almost always antifreeze systems.
Covering the collectors can be an easy solution to thwarting overheating—especially if they are ground-mounted. Covering collectors on steeply pitched roofs can be difficult—climbing can be dangerous. But it’s an inexpensive, effective solution to preventing overheating. Covering and uncovering collectors usually happens in the spring and fall.
Check-valve bypass. Installing a pipe bypassing the check valve can help prevent collector overheating. A manual valve is placed in the bypass pipe and is normally closed. In the summer—or whenever the system is prone to overheating—the valve is opened. When open, the system will thermosiphon at night from the storage tank to the collectors and lose heat through radiation to the night sky.
Controller vacation mode. Some controllers have a “vacation mode” setting which monitors the storage tank temperature as it does for a high-limit function. But instead of shutting off the collector-loop pump, the vacation mode turns on the pump at night to dump excess heat through the collector. Most controls that offer the vacation mode, as with the high limit, have limits that can be field-set. As with all the night-radiation strategies, this method only works well with flat-plate collectors, which lose heat more readily than superinsulated evacuated tubes. Evacuated tubes using heat pipes cannot reradiate heat because heat pipe cycles aren’t reversible.
Tilt. Most references recommend mounting a collector at an angle equal to latitude, which make its surface perpendicular to the sun on the equinoxes. However, an unseasonably warm spring or fall can cause overheating in many antifreeze systems. Setting the collector tilt to latitude plus 15° shifts the time at which the angle is optimal to sometime in November and February, limiting or eliminating overheating. The higher angle also enhances winter hot water production by a few percent.
Even more radical tilt angles are sometimes used with antifreeze-type solar space- and water-heating systems. These combination systems use the same set of collectors for both home heating and domestic hot water. Tilting up to 90° from horizontal (like a wall mount) can eliminate summer overheating while only slightly reducing winter heat production.
Automatic solution. Butler Sun Solutions’ fluid management system (FMS) takes over in a no-flow situation to avoid overheating. High-temperature fluid moves into a finned tube radiator, where the heat is dissipated to the outside air. Should the system pressure rise to 16 PSI, a radiator cap actuates and allows fluid to flow into a reservoir—temporarily, just as it does in an automobile coolant reservoir. The fluid that enters the reservoir is reintroduced to the system upon cooling. Should you use this system, pay close attention to the manufacturer’s instructions. The system should be installed with a system pressure no more than a couple of PSI for the FMS to operate correctly.
Heat dumps. A finned air-to-liquid heat exchanger or an extra underground radiant heating zone can be used to prevent overheating. Radiant floor tubing (PEX) can be installed underground and a control set up to pump water through that zone when the solar storage tank reaches a set temperature. The outside zone can be laid as a winter snow-melt system that gives it an additional use.
Another good heat dump is a pool or hot tub. If you are looking for a tipping point to get a hot tub, this might be your “good reason.” Many people use excess heat from their small SHW system to help heat hot tubs; larger solar home heating systems can help heat pools.
Large systems usually use a fan to blow air past a radiator that the storage water is circulated through when the tank is too hot. This design requires significant electricity to energize the blower, in addition to the heat-dump pump.
Apricus, a manufacturer of evacuated tube collectors, uses a radiator, but the operation is passive, using no additional energy. When the return antifreeze temperature exceeds about 170°F, a valve diverts the solution through a radiator, then sends it back to the return pipe after cooling.
Proprietary collector designs. At least three collectors have “built-in” excess heat protection. Flat-plate collector designs by EnerWorks and Pacific West Solar have passive vents that open when the collector temperature hits about 200°F. The vents allow outside air to enter the collector box, limiting the heat available to the collector fluid.
Thermomax’s evacuated-tube collector has a memory metal spring that actuates at 266°F, stopping the heat transfer of the individual heat pipe until the temperature falls. However, the manufacturer states that this temperature-limiting feature is meant to protect against component failure, not for protecting fluid from seasonal overheating.
The buffer breakdown temperature of generic propylene occurs at about 285°F. A few years ago, The Dow Chemical Co. introduced DowFrost HD, which has a maximum temperature of 325°F. This extra measure of protection has made the DowFrost a favorite.
Early on in the industry, chronic overheating problems were rare. But the recent popularity of evacuated-tube systems and a tendency for solar hot water installers to emulate PV installers, installing collectors at low tilt angles, has exacerbated the problem.
The best solution is prevention through design, but if that’s not your case, you may find a solution from the these strategies. Maybe that hot tub you’ve been thinking about isn’t such a bad idea after all.
Contributing editor Chuck Marken is a New Mexico licensed plumber, electrician, and HVAC contractor. He has been installing and servicing solar thermal systems since 1979. Chuck is a part-time instructor for Solar Energy International and the North Carolina Solar Center and works under contract with Sandia National Laboratories supporting the DOE-sponsored Solar Instructor Training Network.