A little background, my system has been running since January 2008. It has been trouble free and the only one happier then me with the system is my wife . I have a Eko 25 with 500 gal. of pressurized storage. I am running Primary / Secondary system. My backup boiler is a direct vent propane boiler. I have made no changes to the system this year. This past weekend we had to head south for the weekend to do Christmas with the in-laws. When I left storage was at 180*, with a boiler 3/4 full of wood. We get home Sunday night the system had switched to Propane as designed. I get the Eko going to heat storage, when I get up in the AM storage is at 180, there is a call for heat, to my surprise the temp on the propane boiler is at 136*, boiler isn't running. Seems that when the CIRC on the Primary loop is running water is flowing into the Propane loop and running through the boiler back to the Primary loop. In six years this has NEVER happened. I thought maybe because the Propane boiler had been running that the water in the loop was hot so thermal siphon started up. So I closed the ball valve's to isolate the loop and let the water cool. This AM I opened the valves and when I got home form work it thermal siphon is going again. Any ideas? System is running normal between 15-20 PSI.