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Old 06-23-15, 06:02 AM   #17
Mikesolar
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Sorry guys but lots of odd and old info in this thread.

First, I have put in lots of all types of solar thermal systems and really the only drawbacks to a drainback system is pumping power and a bigger pipe size/ need for slope on the pipe.

You only need one pump. With your "flat plate" HX at the bottom of the storage tank, and the one pump pumping up to the panels, you can have a good system. The heat will thermosyphon to the top of the tank and draw from the bottom.

You need a 20-30L tank (placed as high as possible) as a water reservoir for 6m2 of flat panels (do not use tubes), DO NOT use a vacuum breaker or auto air vent on the solar side (this is 70s thought) and they always fail after some time. A good pump will push the air/steam/water back through the system until the hot water reaches the HX then it will flow well.

Use a RESOL BS2 controller (good German brand) but also put a secondary sensor (it can just be any old switch that only turns on above 20C) on the panel to prevent the controller from switching on at night. They seem to have this issue often enough and at -20C it is a big problem. It is a problem with glycol systems too. I have had 4 failures in the last 2 winters because of this.

And (sorry Jeff), recirc systems are not less efficient than drain back and if you look at the SRCC docs, the top systems are glycol based. A properly designed glycol system will "steam out" when the panels reach stratification. In other words the heat will drive the glycol out of the panel and into the expansion tank and all that is left in the panel is water steam, which does not degrade. Of course an improperly designed system will allow glycol to be captured in the panels and will degrade.

All systems, everywhere, need a pressure relief and this is usually set at 6-7bar (some are 10bar) and are there to protect piping etc. If you look at PAW pump stations for example the pressure relief, check valves and exp tank are designed to allow the glycol to flow back to the exp tank. This is actually German law.
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