in line thermostat

hi to all

today I fitted a in line thermostat to my 75 diesel,started the engine and bleed the system,run the engine temp went up to 96 c and the cooling fan came on then I tried the heater and it was hot the switched the heater off and let the engine come up to temp until the fan came on,then I took the car for a spin down the road temp was between 76 c to 83 c tried the heater it was hot got back home parked the car ran the engine temp was 83 c turned the heater on to 28 c it was hot but the cooling fan came on and I don't know why can anyone tell me why this is

all the best david

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