For a SHW system to rely on an expansion tank for overheating protection, the expansion tank must be large enough to hold the volume of heat-transfer fluid from all of the collectors, plus the expansion of all the fluid in the solar loop. Additionally, the expansion tank must be located on the collector feed leg of the solar pipe loop—not isolated from the collectors by the check valve (see illustration). A design problem with many pressurized systems has been undersized expansion tanks. Sizing expansion tanks is a complex undertaking that involves calculating the thermal expansion of the solar fluid, adding the collector volume, and determining how much expanded fluid an expansion tank can accept. It is best to use a larger tank than take a risk with one that is too small.
It is also important to make sure the collector being used can withstand dry stagnation (empty under full-sun conditions) and/or empty properly. These are the same requirements placed on collectors used in drainback systems. All collectors certified by the performance-rating nonprofit Solar Rating and Certification Corporation are subjected to a 30-day stagnation test prior to performance testing.