Good morning all. Hope I can explain this clearly. As I was reading info for another project they discussed heat traps and thermal siphoning and I am wondering if this could be a cause for a problem I've had with my boiler/heat for years. I have 4- B&G flow control valves off an 1 1/4" header going out to various rooms. there are also 4 B&G circs on the return side. 1 zone goes off the side of the 1 1/4 about 4' horizontal then up to the fin tube. 2 zones go up off the header in the bottom of FCV and out the side to the zones. The 1 I'm having a problem with (it gravity feeds) goes off the side of the 1 1/4 thru the FCV and then 45's down about 6" lower than the supply header then continues to the zone about 10' away. The zone is 30' with the return running back above the fin tube then dropping. The return is lower than the supply by 2 or 3". I am now wondering if this is causing thermal siphoning leading to the plumber saying the FCV is bad?(it has been changed twice over the years) If this is what's causing my problem, what would be the fix? Thanks for any input.