Okay, got the hx cleaned out. It took about a 1/2 hour of soaking in vinegar before I heard it gurgle and start to move. I then connected my pump to it and ran it for a good 1/2 hour. I was somewhat surprised that the vinegar came through it as clean as it did. Anyway, after that I flushed it good with water. I then reconnected everything and got the OWB flowing it. Here's the thing that concerns me. When only 2 zones were calling for heat, the Hx was able to keep water in oil burner at 160/170, BUT, if more than the 2 zones called for heat, the temp dropped in the oil burner. When all 4 zones called for heat, the temp dropped even further to 120/130. Once it hit that low number it never recovers, it just sits there. It's quite obvious to me that something is undersized, my thoughts are the underground piping from the OWB. I'm thinking of changing out the E-Classic 1400 this spring/summer for something a bit larger and at the same time, replace the 1" Thermopex with a larger diameter pipe. My first thoughts are to go with 1/ 1/4. I kind of hate to go that route due to the work involved but on the bright side, I'll have help this time instead of doing alone. Roger