Pool Heater iceing up.

Ran into this pool heater yesterday that has serious ice buildup on coils. Fan was running. What could cause this? See attached pic.

2015-02-05 02.37.59.jpg

Gary, check this out. http://www.aquacal.com/blog/post/143-Why-Is-My-Swimming-Pool-Heat-Pump-Covered-in-Ice–A-Troubleshooting-Guide

Thanks for the link?