When a furnace is too big, it can short cycle – start, heat up air very quickly that reaches the thermostat, and then shut down. Compare this to driving a car. A properly sized furnace is like driving along steadily at 60 MPH. Ductwork: If your ductwork and furnace size don’t match up, it could cause problems. For instance, if your ductwork isn’t big enough for your furnace, this will shorten the furnace’s lifespan because the air the furnace produces won’t have enough space to travel through.
Outdoor unit won't come on, Outdoor unit not running: This is a very common problem, whether it's an air conditioner or a heat pump. And there are many things that can prevent the outdoor unit from starting, some of which can be quite serious.