I recently had a Buderus G115 boiler installed, with a Buderus ST-150 40 gallon indirect hot water tank. The aquastat for the hot water tank is a Goldline SP-30D, one of the controls recommended by Buderus. The zone valve is a Taco 1 inch. The goal was to get 120 degree DHW. The DHW was running hot compared to the upper limit set on the aquastat, so we lowered the high temp to 105 degrees, with a 5 degree differential. (With this setting, the stat would call for heat to come to the tank at 100 degrees, and SHOULD stop calling at 105 degrees.) With this setting, the temp goes as high as 140 degrees and never lower than 120 degrees. Many times it is 130 degrees (all temps measured at the tap in the kitchen).
Can anyone explain why this is happening? The aquastat appears to be wired correctly and all the jumpers are set correctly. It was installed by a pro, and he is somewhat stumped by this. Perhaps the sensor is bad, but it is pretty close, because when the temp in the tank is 130, the stat won't go on until you turn the dial past 130. Is the Taco zone valve not closing, thus allowing hot water to continue to circulate in the zone? Any insight would be greatly appreciated!