There are basically two kinds of heaters, recirculating and fresh air. If your heater is round, it's recirculating. If your heater attached to the kick panel it's the more desirable fresh air type.
FRESH AIR HEATER Note that the valve you describe is hard to come by and runs around $200 if you can find one... same for the core. There is a rebuild kit for the valve but if your core is shot, you're going to pay to get a replacement.
