I am new to this forum. I have an Attack DP35 Profi, DHW FPHX and two hydronic floor zones, 100' each. My recirc loop uses a Danfoss ESBE VTC511 1" FNPT with a 140F thermostat. On startup, when there is a call for heat, my return temperature quickly drops to 120. As the zone heats, it of course comes back up, but then I activate the next zone, same thing. My understanding is the Danfoss begins to open at 140, so if your supply is 180, there is no reason the return should be less than 140, unless the valve is not working. Another odd thing is that I have an analog temp/press gage located in the supply about 5" from the boiler outlet. It reads 10F lower than the indicator on the boiler, even though it's only 5" from the boiler's sensor. I have verified this with an infrared thermometer and that's really what the temp's are. The boiler goes into idle with the supply at 180F+ and the return at 120F. Once the zones heat to normal operating temps, the return is of course fine and it seems to operate reasonably well. Because of condensation concerns, I don't want to continue running it until I figure out what's going on. Any thoughts out there?