I attached a HOBO data logger to the system so I could monitor the performance of the system.  I made up my own sensor probes to monitor the temperature coming into the Wand and out, the temperature at the top of the tank, and the PV voltage (to track the solar exposure).  The HOBOware software allows me to produce a very clear graph of my first full day.  The outdoor temperatures peaked in the mid '70's and there were no clouds.  We used hot water in a normal way, and the graph clearly shows a significant solar contribution (green line).   I will add more graphs as I tweak the system.

Aside from all these detailed statistics, we are noticing a distinct change in the pattern of the water heater's operating time.  Our heater uses a forced air vent driven by a loud blower and typically we used to hear it kick on very soon after using hot water.  In the last day or so we have barely noticed it running at all!  This is a blessing for my wife who uses the daylight basement for her business as it is much quieter now.

One thing I learned was that the sensor that reads tank temperature for the Differential Temperature Controller should not be at the bottom of the tank.  That is where it was in the morning on the day of this graph.  The other sensor is right at the top of the collectors. The pump turned on before there was enough sun to run the pump, since the PV was not getting direct sun at that point.  The bottom of the tank has 60F water coming from our well that keeps the sensor too cool, and when the collectors got above 60F or so the DTC turned on the pump before the water was hotter than the average tank temperature.  When I moved the sensor to near the top of the tank later in the day, the pump turned off at a more appropriate time.

 Average tank temperature 87.329F

 

We just returned from a vacation and I had left the data logger running to monitor the system, taking readings every 60 seconds -- the interval for all these plots.

The plot showed a surprising loss of heat each evening, I would have expected to see a continuous gain in stored temperature in the tank, and a much higher temperature overall.  I assume that the collectors are reverse thermosiphoning (cold water in the collectors flowing down) in the cool night air which is dropping to the low 60's and upper 50's.

I emailed the plot below to Barry Butler, and he suggested that I shut off the Solar Wand flow at the valve right above it overnight, and turn it back on in the am.  This would prevent any reverse thermo-siphon from the collectors. I will also turn off the pump using the ON-AUTO-OFF switch on my Differential Temperature Controller.  Barry said that they can send me a one-way check valve if this test prevents heat loss.

Average tank temperature 102.097F

Today's plot (below) shows a cloudy day that shows only moderate solar gain that does not fully offset the water usage for the day.  (Each drop in the blue line is a cloud blocking the sun).  Keep in mind that the water entering this pre-heat tank comes from my well which is about 60F.  Any temperature exiting this tank that is significantly above 60 is a net solar gain that reduces the need for propane to heat the regular water tank up to 100F.

Note that the pump now turns on exactly when the collector becomes hotter than the tank top.  I moved the sensor that runs to the Differential Temperature Controller so that it can respond appropriately to the tank temperature rather than turning on (as above) based on the temperature of the lower part of the tank.

Average tank temperature 89.365F

 

Below is a plot that shows my test as suggested by Barry (see above) to see if a reverse thermo-siphon effect in the collectors at night was causing heat loss in the tank. 

I shut off the Wand circulation using the valve right above the Solar Wand at 7:00pm and opened it at midnight.  What happened at midnight clearly shows a thermo-siphon, but the flow is in the normal direction which is peculiar.  I would expect to see the red line bump up from a reverse thermo-siphon that would result from the larger cold mass in the collectors falling down through the plumbing.  Barry Butler is just as perplexed, since a check valve will not prevent this effect.

Also notice the drop in tank temperature from the cooler water that is now circulating through the Wand from the collectors.

I had not gotten around to filling the system with glycol mixture, so you can see a dip in temperature at 7:45 as I filled in fresh 50% glycol mix.

Average tank temperature 85.462F

The last few days have been progressively cloudier, with yesterday being fully overcast and rainy all day.  The plot below clearly shows that you can't get solar heat without direct sunlight.  Even on days when clouds pass over the heat gain is compromised significantly.  The tank temperature is returning to the default 60F from the well.

Below is an 8 day plot that compares propane consumption to solar gain.  You can see s slight decrease in the frequency of the propane heater's run time when the solar storage tank is hottest on 8/31.  It is interesting to see the sudden drop in the temperature of the bottom of the tank (red line) when water is used and cold water is drawn in.  The biggest drops are from showers which consume 10-20 gallons, and the propane kicks in right away for 5 to 10 minutes.  The hotter the solar heated water is the less time the propane runs .  Smaller drops are from the clothes washer and dishwasher etc.