Just Say No to Glycol

“Dr. Ben, I am in the process of setting up a solar water heating system for a client and would like to get some assistance. Basically the system will be closed loop using the following components:

• 1 no. 119 gallon capacity Solar indirect tank by Lochnivar
•  3 no. Solar flat plate collectors as per Lochnivar
•  1 no. Solar recirculator pump
•  1 no. 15 gallon expansion tank as per WATTS DETA 30
•  1 no. Air Separator as per WATTS AS-B
•  Miscellaneous controls, valves and sensors etc.

I have also included a sketch showing schematic flow diagram of the system. Please note that the calculated volume of glycol in the closed loop system including collectors & will be will be approx 20 gallons, and this is based on the length of 1″ dia piping which is 150 feet. My question is how to determine the correct pressure for the pre-charged expansion tank, owing to the fact that the system will not be subjected to the mains cold water pressure which will be approx 80 PSI.” – Paul

Paul,
Unfortunately, I can’t be much help. I focus on non-pressurized drainback systems. These systems don’t need expansion tanks, pressure relief valves, air vents, check valves, or air separators.  In fact they don’t even need a heat exchanger in the collector loop. While I know the principles of glycol pressurized systems, I don’t know the installation details that you’re asking for and don’t want to give you the wrong information.

In general, the collector loop in a glycol system has a pressure much less than the CW mains.  They are not connected, so they don’t related to each other.  The last thing I heard was the charging pressure came from a table of values related to the temperature of the system at the time of charging.  Since most of your system components are Lochinvar, I suggest you ask them what they recommend.

I have also heard that the expansion tank should be over-sized to take into account stagnation temperatures and “steam back” conditions when the power fails.  Generally, this means making sure the expansion tank volume will accommodate emptying the collector volume into the line.  I didn’t see a pressure relief valve on your collector loop.  I think that is standard.  You also need to pipe the PRV drain away from shingles, since the hot glycol can dissolve shingles and plastic tile flooring.

Good luck on your system!
Dr. Ben