After some discussions with Gary over at BuildItSolar I've decided that the hot water heater was coming on much more often than it should for its warming cycles when the desuperheater was turned off for the winter. Over the past month it has averaged coming on for 5-6 minutes about every 90 minutes. This results in about 90 minutes of "on" time per day just to keep the tank warm. The graph below shows the warming cycles of a typical day.
90 minutes a day @ 5500 watts = 8.25kwh per day = $.90/day. Since I keep the desuperheater off from Nov-March, that's about $140 per year in tank warming cycles. The question is why is it coming on so often. Gary suggested that I might have a thermosyphon loop occurring and after some investigation I believe he is correct. The downstairs heatpump sits right next to the water heater and the plumbing between the two units looks like this:
When the valve on the cold water line at the top of the water heater is open (as it has to be when the desuperheater is on), there is an instant flow of hot water out of the top of the water heater, down through the heat pump and back into the base of the water heater. Shutting the valve stops the flow and the lines stay cool. Even though all the piping is insulated, shutting the valve had an immediate affect on the number of tank warming cycles that occur during the day as shown below.
The tank now cycles on for 7 minutes every 6 hours. The "on" time for warming cycles has been reduced from 90 minutes to 30 minutes a day. For my 5500 watt water heater, that's 825 kwh per winter that I'm not using and 982 pounds of CO2 that's not being pumped into the air. In addition I'm also saving $.60 per day or $90 a year.