Getting Hot, But Not Too Hot: Page 3 of 3

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The author’s pressurized system is 30 years old and still has the original propylene glycol in it—a testament to good design.
Large, wintertime-weighted—or space-heating combisystems—often produce excess heat in the summer, but a properly designed system will include measures for mitigating overheating. The author’s pressurized system is 30 years old and still has the original propylene glycol in it—a testament to good design.
An outdoor hot tub as an excess solar heat diversion dump.
The Artha Sustainable Living Center in Amherst, Wisconsin, uses an outdoor hot tub as an excess solar heat diversion dump.
The Artha Sustainable Living Center in Amherst, Wisconsin
The Artha Sustainable Living Center in Amherst, Wisconsin
Caleffi StarMax V collector
Caleffi StarMax V collectors are built with sloped internal headers, negating the need for mounting collectors at an angle.
Drainback systems require sloping the collectors and all piping
Drainback systems require sloping the collectors and all piping, which allows them to drain by gravity when the pump is shut off.
A pressurized glycol system can use “steamback” as a way to empty the collectors and prevent overheating.
With the correct positioning of an expansion tank, a pressurized glycol system can use “steamback” as a way to empty the collectors and prevent overheating.
The Apricus HD25 water-to-air radiator is a purpose-built dissipator for excess heat.
The Apricus HD25 water-to-air radiator is a purpose-built dissipator for excess heat.
IMC Instruments’ Solar Eagle 2 is a dual-relay differential controller
IMC Instruments’ Solar Eagle 2 is a dual-relay differential controller capable of operating valves or pumps on diversion-load loops.
The author’s pressurized system is 30 years old and still has the original propylene glycol in it—a testament to good design.
An outdoor hot tub as an excess solar heat diversion dump.
The Artha Sustainable Living Center in Amherst, Wisconsin
Caleffi StarMax V collector
Drainback systems require sloping the collectors and all piping
A pressurized glycol system can use “steamback” as a way to empty the collectors and prevent overheating.
The Apricus HD25 water-to-air radiator is a purpose-built dissipator for excess heat.
IMC Instruments’ Solar Eagle 2 is a dual-relay differential controller

Fin-Tube Heat Sink

There can be times when there is no way to use the excess heat produced, like in a pool or hot tub, but it still needs to be dissipated with an automatically controlled heat dump. These systems usually use copper fin-tube to shed heat to the atmosphere. The method is simple, economical, and effective. The recommended fin-tube is the core of baseboard heaters—a readily available product from hydronic-heating suppliers. There are commercial water-to-air heat dumps available as well.

The fin-tube is installed behind the solar collectors and the solar fluid is diverted through the fin-tube before it enters the collectors. One way to divert hot solar fluid through the fin-tube is to use the same control method mentioned above with a dual-relay controller. When the storage tank reaches a preset temperature, the main circulator stays on and the second relay also turns on. In this case, the relay energizes a motorized valve to divert the solar fluid to the fin-tube before it flows back to the collector array.

Another way to control the fluid flow is to use an additional set-point thermostat or an aquastat to operate a motorized valve that diverts the flow of the solar fluid. If the solar fluid reaches a preset temperature as it travels to the collectors, the flow is diverted through the fin-tube, where it cools. This fluid does not need to be cooled to ambient temperature; it only needs to be kept below 200°F.

There are several strategies you can use to keep your SHW fluid from overheating, and the best solution will depend on the climate and system specifics. A pool or hot tub can be an excellent choice and add to the enjoyment of living with solar. If properly designed and installed, SHW systems can provide decades of service—and hot-tubbing or pool heating.

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Bob Ramlow has been active in the solar thermal industry for more than 40 years as an installer, consultant, manufacturer, and teacher. He is an ISPQ-certified solar thermal independent master trainer and a NABCEP-certified solar thermal installer. He is the co-author of Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems. He teaches about solar heating and is a consultant to the industry (see arthaonline.com).

Resources

IMC Instruments • solar.imcinstruments.com • Solar thermal controllers

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