Our heater blows warm air out as it is supposed to, but also blows cold air out of the cold air return vents. Any idea on how to fix this short of replacing the system?

How old is the system, and how long has it been doing it? What other work may have been done on the house since the last time it worked right? Did it work OK in the cooling season? Where do the ducts run? Most systems have ducts gathering the return air in the center of the house, and ducting it to the furnace. At the furnace, it passes through the filter, the blower itself, through the heat exchanger, then to the A coil, and then into the supply ducts. If you have a heat pump, it is much the same except no heat exchanger. Are the filters are clean? My guess is that somehow you have damage to the ductwork, or even a cover left off the furnace. You need to start at the furnace and follow the ductwork in both directions looking for problems. If you find a problem, it may or may not be a fairly simple fix. If you can't find a problem, better call a pro. First give us a chance to help you find something easy.

If you answer my questions, either some of the others or I may have a better idea what the problem is.