In cool mode fan will work on all 3 positions. On auto and inter. But in heat mode fan will only work in the on position.will not kick on in auto or inter. Position. I've been advised its probably a bad relay. Is this 'probably' true. And if so how do I change it?
Model(-)BHA-17 H 11 S U B A this is the unit upstairs rheem acheiver unit outside

On most units there is a fan control for the heat. This control allows the unit to heat up before turning on the fan.

The model number you posted does not register or cannot be found. So try to find a different number. Is the inside unit also a RHEEM?

Inside the blower door of the inside unit will be a wiring diagram. This diagram will help you locate the part.

Yes inside unit is a rheem. I'm going up to try and locate part you spoke of. I take it that you think this part is faulty and therefore my problem. Thanks for your time and knowledge

The limit control/fan control has a sensor that sticks into the duct.

It's what allows the furnace to control the fan in heat mode. It waits until the plenum is hot enough to turn on the fan and once a call for heat ceases, it waits until there is no useul heat available and then turns off the fan.