As far as I can figure there are two parts of a system controller that need a tank temperature reading: the part where we decide if the tank is hot enough to provide heat to the house (I'll call this the THH sensor for Tank Heat House), and the part where we decide if the tank is cool enough that we can keep heating it (i.e. the differential control that keeps coming up in various threads, I'll call this the TAH sensor for Tank Accept Heat). These two temperatures don't need to be from the same sensor and it may make sense to make them separate sensors. In my system I use the sensor at the top of the tank for the THH and the middle for the TAH. In another thread, someone mentioned that they are using a sensor on the tank output. I think this sensor location could be used for both the TAH and THH. However, I can't see how you would avoid a delay problem. For example, let's say we are charging the tank and the TAH sensor tells the controller to stop charging the tank because the output from the tank is higher than the input. The tank circ shuts off and now the TAH sensor starts to cool down. Since it cools down it will now cause the controller to start heating the tank again and the the TAH will heat up and shut down the circ again, etc, etc. It seems liek we'd get some nuisance cycling. How do people with their sensor on the output avoid this?