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Design Resources: Sizing The Drainback Tank

How Do You Properly Size Your Drainback Tank?

The expansion tank or the drainback tank are critical components of the system design, and must be properly sized to ensure proper operation.

Expansion tanks must be sized to accommodate the expansion of the glycol when the system stagnates to protect the system.

Drainback tanks must be sized to accommodate the volume of water that drains back down from the solar collectors when the pump is off, as well as enough reserve capacity for correct operation during standard operation.

Use the following process to size your drainback tank properly:

Step 1 – determine how much fluid is in the solar loop

Use the table of piping volumes and the length of your pipe runs to determine the volume of your solar loop.

Pipe Capacity (US Gal/100ft)
Pipe Size Type M Type L Type K
3/4" 2.7 2.5 2.3
1" 4.5 4.3 4.0
1-1/4" 6.8 6.5 6.3
1-1/2" 9.5 9.2 8.9
2" 16.5 16.1 15.7
2-1/2" 25.4 24.8 24.2
3" 36.2 35.4 34.5

Step 2 – choose the correct size tank

A good rule of thumb that SunMaxx engineers recommend is that you use a drainback tank that is 2X the volume of solar loop above the drainback tank. For instance, if you have a solar loop with a 25 gallon capacity, a drainback tank of 50 gallons would be appropriate.

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