If you are getting a faint smell of gas at the end of the heating cycle, there is a possibility that the pressure regulator could be the problem. All regulators have a vent hole on them to allow air into the case so the diaphragm can move. Whenever the gas valve shuts off, there is a momentary pressure buildup within the line and regulator. If there is a small hole in the diaghragm, the gas could be released to the atmosphere, thus the smell of gas for only a short period of time. (You will sometimes notice the smell of gas around the main regulator outside a building, as this is a normal occurance because regulators that size have an internal relief valve.) However, most domestic units have the regulator built into the gas valve. I have not worked on these systems for many years, so I am not sure how they are configured, but it is worth checking into.