How to measure water heater temperature?
I need a similar setup for recirculating the hot water in the boiler through an adjacent solar water heater tank. This would increase the storage capacity of the system as it moves heat to the boiler when the temperature of the solar water tank exceeds the temperature of the boiler.  In my case I'm more interested in the temperature at the bottom of the tank.
A possible solution would be to replace the drain valve with a "T" such that one "path" goes to the tank straight and the other one forms a 90 degree angle. On the straight through path insert an immersion well for the thermocouple on the other path the drain valve.  All standard parts and no holes to drill.  The immersion well will constrict the flow to the drain valve a bit which is not really an issue on the drain port. For the hot water port it might still work with a 1/2 inch immersion well on a 3/4 pipe.