I received a call the other day from a customer that was testing the stagnation response of a bank of solar collectors. He filled the system completely with water and then just left the pump off while the sun baked the collectors. He was doing a form of pressure testing of the system for leaks. Since the system is ultimately going to be a closed loop glycol system he has a generously sized expansion tank connected to the system. He is using an immersed thermal well with the temperature sensor for measuring the temperature of the solar collector array. What he noticed was under stagnation conditions the temperature sensor went up to about 252 degrees Fahrenheit and then would drop down in temperature about 10 degrees for a period of time before it would build up to 252 degrees again. The system was repeating this cycle over and over again and he wondered why.
What the installer was experiencing was the result of the fluid boiling, turning to steam, and then cooling as a result. He confirmed that his solar system was operating around 30 psi when it was doing this. You can see from the charts below that the boiling point of water at 30 psi is roughly 250 degrees. This matters because any closed loop glycol system can exhibit the same behavior.
When the fluid in your system is undergoing this phase change it will experience significant amount of pulsing as the pressure spikes from the creation of steam. During this pulsing you don’t want your pump running. If the pump is running you end up rattling the pipes by continuously pumping cold fluid into the collector to experience boiling. You also can damage the pump by having it operate when the shock waves of the fluid phase change are propagating through the system. The issue comes in when you use the standard turn off point of a solar controller. The different manufacturers of solar differential controls have different safety turn off points. Steca uses 265 degrees as the point after which the pump will shut down. Other manufacturers use 249 degrees. All of these settings are adjustable. To prevent customers complaining about the rattling of their pipes and to prevent damage to your installed equipment make sure that the turn off point for your differential control is set to keep the system off during these times. I would recommend you set your turn off point for the system at 249 to be safe.