Solar Drainback Storage Tanks


In indirect systems, expansion and drainback tanks must be appropriately sized for the system to operate properly. For expansion tanks, the acceptance volume must be sufficient to accommodate expansion of the glycol when the solar loop goes into stagnation. Drainback tanks must be of sufficient size that they can store the entire fluid volume of the array with enough reserve capacity to ensure proper operation of the pump during steady-state operation. Provided below are the basic sizing rules for determining tank sizing.

The first step in sizing solar drainback or expansion tanks is to calculate just how much fluid is in the indirect solar loop. Using pipe run lengths from the pump sizing section, fluid capacities can be calculated using the table below. In addition to the pipe run capacity you should add the capacity of the collectors as specified in the SunEarth spec sheets. This total will be the indirect loop volume.

Pipe Capacity (US Gal/100 ft)
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

Sizing of drainback tanks is quite simple and a standard rule is to select a tank with a capacity of twice the loop volume above the tank. Therefore, if the indirect loop volume were 40 gallons, an 80 gallon drainback tank would be suitable.

Sizing Example

Size drainback tank for system using 4 SunEarth EP-40 collectors; 1” Type L pipe, total pipe length (supply plus return) above the drainback tank is 60 ft.


Collector capacity = 6 * 1.2 = 7.2 gallons

Pipe capacity = 4.3 * 60/100 = 2.6 gallons

Total fluid volume above drainback tank = 9.8 gallons

Minimum required drainback tank capacity = 2 * 9.8 = 19.6 gallons

Select 20 gallon capacity drainback tank.

Standard electric water heaters may be used as drainback tanks and are commonly available in sizes from 30 to 120 gallons. SunEarth’s all copper CopperStor drainback tanks are available in 5, 7.5, and 10 gallon sizes and SunEarth Stainless Steel drainback tanks are available in 10, 15, and 20 gallon sizes. Morley Manufacturing http://www.morleytanks.com designs and manufactures custom drainback tanks with internal heat exchangers with capacities up to 2,000 gallons.

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