Is there a problem of the fuel getting too hot when driving in warm weather while using a fuel line heater that uses the coolant water to heat the diesel fuel in your bus engine?

The fuel heater is built with a thermostat inside of it and it gets activated only when the fuel gets cold. It has been preset to a certain temperature when made to come on and go off automatically and can't be adjusted. It won't come on after about 40 degrees F.