If a pressurized system is properly designed, there is no problem if the system reaches its high limit and the circulating pump turns off. The temperature in the collectors rises rapidly above the boiling point, creating steam. It takes about a thimbleful of water to make enough steam to completely fill a large flat-plate collector and even less to fill the manifold in an evacuated-tube collector, so very little water actually boils. This steam quickly expands to fill the collector and pushes the remaining heat-transfer fluid down to the expansion tank.
At this point, there is no fluid in the collectors, so the glycol is protected from thermal breakdown. When the collectors cool at the end of the day, the steam condenses, the pressure drops, and the heat-transfer fluid refills the collector. This “steamback” process is perfectly safe in a properly designed pressurized system. The key to success with this method is the size and location of the expansion tank within the solar loop (see “Steamback Specifics” sidebar).
Drainback and properly designed pressurized systems should not have overheating problems. However, there may be instances when systems reach their high limit and stop collecting heat while the solar resource is still available. This can occur during times when hot water is not regularly being drawn from the storage tank (for instance, if homeowners are away on vacation, especially during the summer) or with combination domestic water and space-heating systems, where there are lots of collectors for winter space heating that have no job to do during the summer. Wouldn’t it be great if we could use that heat while still making sure our collector fluid never suffers from thermal breakdown?
A great use for this excess heat is to divert it to seasonal swimming pools or outdoor hot tubs. Since excess solar heat production occurs mostly during summer, it offers perfect timing for providing heat to a seasonal pool. It is easy to divert heat to a pool or a hot tub by rerouting it through an alternate heat exchanger. Most swimming pools and many hot tubs use a chlorine or salt disinfectant in the water and both chemicals are extremely corrosive to most metals, so a special type of heat exchanger is required that will resist the chemicals’ corrosive effects. Titanium exchangers will last the longest, but marine-grade stainless steel or cupronickel (copper/nickel alloys) may work, too.
When the solar storage tank reaches its high-limit temperature, the solar fluid is diverted from the tank’s heat exchanger to the pool’s heat exchanger. A controller that includes this option is required, like IMC Instruments’ Eagle 2 dual-relay differential temperature controller with a built-in pump relay. When the solar storage tank temperature reaches the adjustable high limit, the solar circulating pump stays on and the second relay energizes a motorized ball valve or a circulation pump that diverts the solar fluid to the pool’s heat exchanger or diversion loop, and away from the solar storage heat exchanger.
Sometimes, a second circulating pump is required to overcome any additional pipe and exchanger friction; in some other cases, a second circulating pump is used instead of a motorized ball valve. The first priority of the controller is the domestic hot water, so if the solar storage tank cools because of hot water use in the home, the controller turns off the second relay and the solar heating goes back to the domestic water until the high-limit setting is reached again.
When heating a pool with a SHW system, the pool pump and filtering system must be on at the same time. This ensures that the pool/spa water is flowing through the solar heat exchanger so it can absorb the solar heat from the hot solar fluid. The easiest way to make sure the pool filter pump is on when the solar energy may be diverted to pool heating is to set the pool pump timer to operate during daylight hours. Pools normally need filtering six to eight hours each day—and the timer energizing the pool pump is set to coincide with the system’s solar day.
One of the solar heating systems at the Artha Sustainable Living Center in Amherst, Wisconsin, uses an outdoor hot tub as the excess heat diversion dump. That hot tub holds about 600 gallons of water, and it has no filtering system or filter pump. No chlorine or salt is used, so soft copper tubing is used as the heat exchanger. When the solar fluid is diverted to the tub, it simply travels through the copper tubing, transferring the heat to the tub water. If chemicals are used in a hot tub, PEX tubing can be used instead of copper, but the loop must be three times longer because the heat transfer capability of PEX is one-third that of copper.