Overheat-cooling with thermosyphon loop

By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.

A few months ago, I went on a service call to evaluate an old solar heating system at a house near Santa Fe. As we approached the house in our service truck, I was struck with a strong feeling of deja vu. As it turns out, I had designed this solar heating retrofit myself back in 1987 and had worked as one of the installers that put it together that summer.

If this heating system had been an automobile, it would have been designated a “classic car” and given special license plates. It is nearly 25 years old and still runs. In fact, the original owner still depends on it to control his heating bills. A new backup boiler had just been installed, along with some other new heating components, and the owner just wanted to upgrade the controls to handle the new equipment. The system is presently back in service.

Before the solar retrofit in 1987, we walked into a house where construction had recently been completed, including hydronic radiant heat downstairs with plastic tubing embedding in concrete mass floors. A conventional “hot water” electric boiler provided heat to the floors, using a single Grundfos circulator (UP26-96) and some zone valves. The warm mass floors added up to just under 2,000 square feet.

The retrofit included six 4 5 8 collectors mounted on the ground next to the house. The south-facing slope of the terrain allowed us to screen the collectors from view with existing trees. See Figure 33-1 for a view of the collectors as they look today. They appear in the trees to the left of the wooden “coyote” fence just below the house.

Single-minded design goal

This solar heating retrofit was not intended to be a combisystem. The original intention for these solar collectors was simple: to provide baseline winter heat for the mass floors. The owner wanted enough solar heat to temper the warm floors without using the electric boiler all the time and also wanted to divert unused solar heat to a spa when the heating season was over. The owner also wanted the retrofit to be as simple as possible, using the minimum equipment and simple controls.

We accomplished this by attaching the boiler, heat tubing and solar collectors together in a “solar direct” configuration, where a glycol mixture is circulated through all the heating equipment. The glycol is contained in a closed system under normal hydronic boiler pressure (typically greater than12 psi cold and less than 25 psi hot) and circulated directly from the collectors to the boiler and into the tubing in the floors. There are no heat exchangers in the system other than the plastic tubing in the concrete. The plastic is protected from solar overheating by using a thermal mixing valve.

Simple controls

A single hydronic circulator pump provides flow through the collectors and into the floors only whenever solar heat is available and the heat is needed in the house. The original Grundfos circulator was kept in service and used for both the solar and the boiler. The solar heat is controlled by a single differential thermostat that allows flow through the solar collectors only when they are hot, by activating a motorized valve. In this simple design, when solar heat is not needed in the house, the flow to the collectors simply stops, even when they are exposed to full sun.

When we designed this system, we knew that we had to think of a way to dissipate the unwanted solar heat from these collectors safely and reliably to prevent them from generating steam and causing high temperature damage to the glycol. Our solution was borrowed from the world of passive solar design and based on cooling by natural convection. A thermosyphon cooling loop was added to the collectors so that they could cool themselves by natural convection when the pump-flow stops.

Thermosyphon cooling with fin tubes

Thermosyphoning can be defined as the movement of fluid around a plumbing loop, driven only by a temperature difference across the loop (fluid “pumped” only by heat). Even today there are solar water heaters for sale that collect solar heat without electricity and without a pump, simply by thermosyphoning. The same principle can be used to dissipate solar heat. Hot fluid is less dense than cool fluid, so when it is contained in a loop, the cool fluid tends to “fall downhill” and the hot fluid tends to “float uphill.”

See Figure 33-2 for photos of the cooling loop installed on this job. (Remember that this equipment has been out in the weather for almost 25 years, so it is not showroom new.) When the pumped circulation in the collector stops, the hot fluid in the collector tends to rise to the top header. A tee connection at the outlet pipe (Fig.33-2, upper left) connects to cooling fins mounted on the back of the collector (Fig.33-2, center) that contain cool antifreeze, which is heavier than the hot antifreeze and tends to “fall” downhill. A tee connection at the collector inlet allows the cold fluid to slide downhill into the bottom of the collector as hot fluid rises up and out the tee at the top.

This thermosyphoning continues as long as the sun provides heat, or until the circulation pump turns back on. When the pump is on, the cooling loop snaps shut, using a passive check valve (located near the bottom inlet tee) that closes in response to the relatively high flow and pressure provided by the pump.

The vertical ball-check valve shown in the photo (Fig.33-2, lower right) is the original thermosyphon check valve installed in 1987 and still in operation today. It is the legendary “boogie” valve, manufactured by Zomeworks Corporation, but it is no longer widely available. As an alternative in this plumbing configuration today, an ordinary 1” brass swing check valve (with a vertical flap) could be substituted if mounted horizontally and tilted slightly downward toward the bottom tee.

20/20 hindsight

It is always valuable to learn from the past, unless you enjoy reinventing the wheel every few years. In this installation, we found some things that worked great and some things that could have been better. Generally speaking, it seems that much of our success here can be attributed to controlling and limiting high temperatures.

Good Stuff:

The thermal mixing worked to keep the plastic tubing safe from solar overheating.

The use of all-copper tubing near the collectors was a good idea. Keep plastic away from solar collectors.

The thermosyphon cooling loop kept the glycol in service for a very long time.

Direct floor heating with solar glycol works well even without heat exchangers and even without water storage tanks.

The large volume of glycol required to fill the floor tubing seems to contribute to the longevity of the glycol.

The original Grundfos circulator was still there and still running.

Do Over:

The thermosyphon cooling would probably work better with larger diameter tubing. Upsize the ¾ inch supplies and tees to 1".

The thermosyphon cooling would work even better with twice as many ¾" fin tubes placed in parallel for low resistance to flow. Larger diameter fin tubes are also a good alternative and available commercially.
The owner of this system willingly tended to some minor seasonal adjustments. We no longer design systems that require manual intervention such as collector covers or seasonal valves and switches. The success of a heating system should not be personality dependent.

Only high temperature pipe insulation withstood the test of time at this job.

The only solar pipe insulation that survived for the duration out in the weather was the stuff covered by metal foil.

These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement.

Bristol Stickney, partner and technical director at Cedar Mountain Solar Systems in Santa Fe, N.M., has been designing, manufacturing, engineering, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the Chief Technical Officer for SolarLogic LLC and is involved in training programs for solar heating professionals (visit www.cedarmountainsolar.com or www.solarlogicllc.com for more information.)