Recently moved into a 4 year old home. Have a mid efficient furnace. Noted in the summer heat when we turned on the ventilation switch the upstairs bathroom fan came on and the furnace motor did not engage to blow air through the house, am I missing something or is there a part failure on the furnace

Can you give more detail. The bathroom exhaust fan would not be part of the heating and a/c system.

Agree with Harold. In no way should your furnace be tied into your bath exhaust fan. Please give us a better description of what is happening, or what you call a ventilation switch. We are here to help, but need more info to do so. Lee.