[Destroyer]

Originally Posted by RWilson2526

For cold air to be coming in through that return duct you need to be losing hot air out somewhere.

Totally agree. In your mind think of heat as a liquid. It will always flow towards cold. That heated air, flowing out of your house, needs to be replaced with makeup air, which will flow into your home from outside. The heat flows because the body and the surroundings are attempting to reach the same temperature, at which point they are in thermal equilibrium. The spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature, as described by the second law of thermodynamics. Now, some heat is always lost through walls, windows, doors, etc., But to have a situation such as you described you need to have a hole someplace. It's probably costing you large money also, as wasted energy, so it is a good idea to locate it and seal it. I'm with RWilson on this one...look for a fresh air intake first and seal it, then look for any holes or large leaks between the heated interior of your home and the unheated exterior (including the unheated attic if you have one)