Improper solvent-welding can cause leaks in joints allowing both water and exhaust gases to enter your house.

These images are the result of not having your furnace serviced annually and doing home renovations while the furnace blower motor is running.  A partially plugged secondary heat exchanger can cause numerous issues including no heat situations caused by over heating, lack of air flow, air conditioner freeze up, drop in efficiency for both heating and air conditioning.

Improper installation of exhaust vent on a furnace caused water to build up and freeze causing a No-Heat situation.