Solar Drainback Tank Sizing

Drainback Solar
Tank Quick Sizing

The expansion tank or the solar 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.

Accurately sizing a drainback tank is essential for a properly functioning solar thermal system. The expansion tank, or drainback tank, is a critical component of the system design and ensures that the water stored in the system remains pressurized while also minimizing thermal shock. It is essential that the expansion tank is sized precisely to provide the system with ample water volume and pressure to function correctly.

A properly sized drainback tank should be able to hold 5-6 gallons of system water, the required volume of water needed for each 10 sq. ft. of collector area. The actual size of the tank will depend on the total collector area of your system, as well as your local climate. If your system operates in a colder climate, you should consider sizing the tank larger in order to provide for more thermal mass.

The pressure rating of the drainback tank should also be calculated using the system’s maximum operating pressure. If the tank is undersized, the pressurized water may not be able to reach the required pressure levels when heating the system. If the tank is oversized, it can cause the system to take too long to reach the required pressure.

When installing a drainback tank, it is important to ensure that it is placed at the highest point in the system so that water can naturally drain back to the tank when the system is shut off. In addition, air vents must be installed in the tank to allow for the introduction of pressurized air and to allow for the ventilation of any gasses that may build up inside the tank.

By following these sizing guidelines, you can ensure that your solar thermal system will operate correctly and efficiently. Properly sizing the drainback tank will ensure that your system has the necessary water volume and pressure levels to operate correctly.

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 SizeType MType LType K

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.


Open drainback tank


Drainback tank with external HX

Design & Installation

Sizing Tools & Guides

Getting Started

Design & Installation

Solar Drainback Tank Sizing is a process that requires the correct sizing of a solar hot water tank based on the specific needs of a household. The goal of this sizing is to maximize the efficiency of the hot water system and ensure that the tank can provide enough hot water for the household at all times. While this sizing process has numerous advantages, it also comes with several drawbacks.

The primary benefit of Solar Drainback Tank Sizing is that it ensures that the tank is large enough to handle the needs of a household. Proper sizing ensures that a tank can store and release enough hot water to meet the demand of all the household’s hot water needs. Additionally, proper sizing ensures that the system will be as efficient as possible, maximizing the utilization of the solar energy being collected.

One of the significant drawbacks of Solar Drainback Tank Sizing is that it can be quite expensive. As different factors must be taken into account when sizing a tank, this process can require a large investment of resources. Additionally, there is always a risk of making a miscalculation, which could result in a tank that is either too big or too small for the households needs.

In conclusion, Solar Drainback Tank Sizing can be a beneficial process, as it helps maximize the efficiency and performance of a household’s solar water heating system. However, it also comes with a significant financial cost, as well as the risk of making a mistake in the sizing process.