My system has been installed and working for just over a year. In that year, I've needed to use my electric hot water heater for less than 12 hours. I have not had much luck with sensors, though. I'm pretty sure the one I have in the top of my tank is not reading correctly, and it's not very old.
The heat exchanger/storage are configured as follows ...
The heat exchanger is a tank of potable water, inside an outer jacket that holds the mixture that flows through the collector loop. The tank has a fixture on the top where the temperature sensor is supposed to go. The trouble, from the beginning, was that the "dip stick" mounted on the fitting that screws into the top of the tank was too long for me to remove it from the tank without drilling a large hole in the ceiling, and the sensor wouldn't fit into the hole at the top.
My first solution was to shorten the "dipstick" by cutting it off, inserting the sensor inside the tube, and running just the wire through the small hole at the top. It was a great challenge getting that tube resealed so I could stick it back into the tank with the sensor inside. After a month, or so, the sensor quit working because a tiny bit of water was seeping into the tube and the sensor ended up in a few inches of water at the bottom of the tube, eventually.
So, I opted to screw a submersible sensor into the top of the tank, even though it would be reading the temperature at the hottest point in the tank. In order to monitor the bottom of the tank, I attached a sensor to the collector loop output pipe at the bottom of the tank. That isn't ideal, either, but I get a good idea of what the top and bottom temperatures are, between the two sensors.
Well, lately, the top sensor has been reading about the same or lower than the bottom sensor. Normally, the top sensor reading rises pretty fast when the sun is shining and the system is running. Now, the bottom temperature overtakes the top temperature, and the circulating pump runs a lot longer than it should. In the middle of the day, if I'm not using any hot water, the pump might run for an hour at a time. That leads me to believe that the top sensor is reading low.
I don't have any alternate way of testing the temperature at the top of the tank. The water is under pressure, so I can't just unscrew the sensor and stick a thermometer in there. Based on common sense, it has to be reading wrong. I checked for air at the top of the tank. There is an air bleeder that is working properly, and I unscrewed the sensor a little to let some water drip out before I retightened it.
So, are these sensors that unreliable? Could it be failing? Or is there another explanation I haven't yet considered?
