A drain-back solar heating system is designed to allow the water circulating in the solar collectors to drain back down into a small tank when the circulating pump is turned off. Since this tank is inside the building in a warm space, a drain-back system prevents the water in a solar hot water system from freezing if the temperature falls below -17.8 degrees C. The pump is switched on automatically in the morning when the solar collector panels are heated by the sun. If this control system is not working correctly, thermal shock may result when the cool water from the drain-back tank is pumped up to a very hot solar collector.
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Find the differential temperature controller (DTC) on your solar hot water system. This is a small unit with a digital display that will be installed close to the drain-back tank or the solar water tank. Watch how this controller works early in the morning when it switches on the circulating pump. There will be a temperature sensor on the solar water tank and another sensor on the solar collectors. The DTC compares these temperatures and will switch on the pump when the solar collector is hotter than the water in the tank. If the DTC is not set correctly, the pump may not be turned on until the solar collector is much hotter than the tank water. In this case, the water may boil when it enters the solar collectors, or it may boil when the water starts to run back down from the top of the collectors since this is when the pressure in the piping is at its lowest point. Either way, you will hear lots of noise in the pipes and the solar collectors as the steam tries to force its way up and out of the pipes.
Check the readings on the DTC in the morning when it switches on the pump. Write down the temperatures the DTC displays for the solar collectors and for the water in the tank. Note if you hear noise in the pipes that is evidence of thermal shock. The probable cause of the thermal shock is that the DTC is set so that it switches on the pump when the solar collectors are already very hot. The DTC needs to be adjusted so that the pump starts up earlier in the morning when the collectors are not so hot.
Consult the installation manual for the controller. It will explain how to adjust the unit so that it switches on the pump when the difference in temperature between the water tank and the solar collectors is less than it is now. Adjust the DTC and reduce the controlling temperature difference by two degrees. Check the DTC readings the next morning when the pump is switched on and note if you continue to hear noises from thermal shock. Monitor the system for a few days until you are sure that the problem has been resolved. If not, reduce the temperature difference by another two degrees and continue to watch how the system performs.
If the thermal shock noise persists, the problem may be that the solar hot water system is oversized and because of this the water in the main tank is too hot. The system may also lack a pressure relief valve and a vent. Call a qualified technician and ask for a detailed evaluation of the operation of the system.
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