http://www.SwimmingPoolSteve.com - If you have a natural lgas or propane pool heater it can fail early (less than one year) if you do not protect it with a check valve. The check valve gets installed on or near the outlet from the heater and before the salt water cell / chlorine erosion feeder. This prevents the concentrated chlorine from these systems from travelling backwards through the plumbing lines and pooling inside of your heater where it will cause serious damage. Further to this if you do not have a check valve in between your heater and your chlorine injection system any warranty claims will be denied from the manufacturer due to improper installation of the heater.
If you want to learn more about pool heater check valves, how they work and where you should install one be sure to read this detailed article from Steve:
http://www.swimmingpoolsteve.com/pages/heater-reviews.html...
https://www.youtube.com/watch?v=wWOu-1J_56U