[Richard]

The big tank inside the stainless cover on the Aquahot is full of coolant (Aquahot calls it boiler fluid). The sensor for the temperature probe is threaded into that tank so that it reads the temperature of the coolant. If you simply removed the sensor, you ARE going to get a lot of fluid pouring out, so the level of the tank must be drained below the sensor level. Easiest way for me is to insert a tube into the radiator cap on the AH tank and siphon it out.

Don’t worry about that plug on the overflow tank.