Alternative-energy heat sources are not always well-behaved. Unlike conventional hydronic boilers, solar heat collectors will not produce heat on-demand, and will not always produce a temperature that is entirely predictable. The plumbing details and control functions must be designed to accommodate spontaneous heat availability and widely fluctuating delivery temperatures. 

In this column, I have been describing how I accomplish this in my own projects using a piping and control strategy that I call the New Standard solar hydronic configuration. I have been using this standard method on virtually every solar combisystem I have designed dating back over 14 years and well over 100 installations.

One compelling reason that I use this standardized design configuration is because alternative energy heating systems can have occasional episodes of extreme overheating. When a perfect storm of adverse conditions occur, solar collectors can produce temperatures high enough to melt plastic plumbing components. And, even if this only happens once in the life of the heating system, you want the system to survive and continue to function normally afterwards. So, many of the best practices that I use in every standard heating system are included mainly to ensure high-temperature survival over the long term. 

The anatomy of an extreme overheat episode

Figure 76-1 shows sample plumbing details for a typical New Standard solar hydronic combisystem (similar to dozens of systems installed in recent years).When an extreme episode of solar overheating occurs, the source of the high temperature is the solar collectors seen in red. In normal operation (e.g. on a sunny afternoon), these collectors are cooled by the control system that powers the solar glycol pump simultaneously with the primary pump and at least one other pump. These pumps carry the solar heat away from the solar collectors, through the heat exchanger, into the primary loop and then into one or more heat loads. 

In this example, the heat loads include Masonry radiant floors, baseboard and fan-coil fin tubes, a domestic hot water (DHW) tank, and a heat storage tank. These heat loads are assembled around a primary loop in the correct temperature order so that they can each receive solar heat (or not), one by one or all at the same time. The control system distributes the solar heat in a controlled way to all the loads so that none of the heat sources and none of the heat loads ever get too hot.

As a former solar repair man, I can tell you from experience that as long as the control system is designed properly and is working correctly, overheating can always be prevented by proper control. But, there is one thing that can incapacitate a solar/glycol heating system no matter how well the control system is done, and that is when an extended electrical power failure occurs on a sunny day. The odds of this happening at any location over the lifetime of a heating system are very high. And when it happens, here is the sequence of events that occur even in a well-designed system:

1. It is a normal sunny afternoon and the solar collectors are hot (e.g.120°F – 160°F) and this heat is being pumped around the primary loop to any heat load that needs it.
2. The electric power to the building is interrupted and all the controls and pumps go dead.
3. The glycol coolant stagnates in the hot solar collectors and in less than half an hour the solar collectors begin to generate steam in excess of 240°F at 30-40 psi.
4. If thermosyphon self-cooling fins or photovoltaic (PV solar-electric) coolant pumps are provided the collectors will stagnate below 240°F and will not generate steam. (These options are not shown in Figure 76-1.)
5. If the power failure continues for an hour or more without self cooling fins, the collectors may fill with steam and generate internal temperatures that exceed 300°F.
6. If steam-back expansion tanks are properly installed, the liquid in the collectors is forced out by the steam and is contained in the steam-back tanks.
7. If the electric power comes back on while it is still sunny, the control system will “wake up” and resume normal operation by turning on the circulator pumps.
8. For the first few minutes after restarting on a sunny afternoon, circulator pumps may move super-hot glycol (e.g.240F) out of the solar collectors and around the solar loop (seen in red). This may cause the primary loop to also get much hotter than normal (seen in orange).
9. Once the normal control system functions have returned, a well-designed heating system will return to normal operational conditions on its own without any special attention or service calls.

Metal, plastic and tempering valves

The heating system will only survive and recover normal operation as outlined in the nine steps above if it is properly installed with a careful attention to detail. For example, if cooling fins are not installed as in Step 4 and a steam-back expansion tank system is not properly installed as in Step 6, a pressure relief valve will pop open and hot glycol will be lost. If enough glycol is lost, the system will not restart and recover in Steps 7 – 9.

There are many ways that this scenario may play out if the wrong materials are installed as well. If plastic parts are exposed to the extreme high temperatures mentioned in Steps 3 – 5, they may melt, deform, leak or fall apart. If PEX tubing is used anywhere in or near the solar collector loop (seen in red) or the primary loop (seen in orange), it will experience high temperature failure during Step 8. That is why only metal tubing should be used in the Red and Orange piping zones shown in Figure 76-1.

The other components in the red and orange pipe loops must also resist high temperatures. That includes pipe insulation, air vents, spring check valves and gasket materials. This means choosing components that have a “Solar” rating, avoiding plastic component parts and looking at the temperature ratings and choosing the highest ones available. Don’t forget that any sensors, wires and wire connectors that are in contact with the red or orange plumbing must also be high temperature compatible.

Black plumbing in Figure 76-1 indicates that temperatures are controlled in such a way that common hydronic equipment is typically used. The same kind of components that are used in conventional hydronic boiler systems are typically used in the sections seen in black, and metal piping is recommended here as well.

The blue pipe seen in Figure 76-1 is PEX tubing, (cross-linked polyethylene) commonly used in hydronic Combisystems for heat distribution in radiant floors. PEX is certainly not banned from solar heating systems, it just needs some reliable temperature protection to keep it below 180°F. This is provided by the tempering valves seen in green on our diagram. There are two on this example drawing, one to help prevent scalding from the solar heated DHW tank, and the other one prevents extreme high temperatures from reaching the PEX tubing in the floors. They both work on the same principle, where cool fluid is allowed to mix with the hot fluid to provide a safe output temperature. A thermal element inside the valve moves in response to the hot source temperature to provide enough cool fluid into the mix for a reasonably constant output temperature. The example seen in the diagram is adjusted using a manual knob, but electronic mixing controls and mixing pump blocks can be installed here to do the same job. 

Figure 76-2 shows a photo of another option: a pump module made by Caleffi that provides the manual mixing valve, the circulator pump and some other plumbing hardware all in a single package.

Summary

Hydronic combisystems that include alternative-energy heat sources are likely to experience overheating episodes that might cause damage to PEX tubing and other common plastic plumbing components. This can happen with solar heat as described above, but also with other closed-loop hydronic intermittent heat sources such as wood-fired hot water coils. These heating systems can be designed to survive high temperature events by following the recommendations discussed above. 

Tempering valves may be used on any secondary loop where PEX tubing or other plastic components require temperature protection, not just for radiant floors as seen in the diagram. The examples and recommendations seen here are based on successful installations that have been duplicated may times as part of a reliable standard design method.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than ten thousand 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 has been designing, manufacturing, 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 in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.