clamp on aquastat

My daughter has an hot water unit heater hanging in the lower lever of her home, it is plumbed in the with the living area thermostat upstairs, so when the living area calls for heat, the unit heater gets hot in the lower level, but i need to start the fan 110vac so what I need is an aquastat where i can clamp the bulb on the return pipe from the unit heater, this way when the water gets up to 130 or 140 deg, i want the fan to start and shut of at around 100 deg, i do not want to use a relay to do the switching, so i know i need an aquastat to make on rise ect, can anyone suggest what exactly i need or the model. there are so many choices i am not exactly what i need.
Chet
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