I would like to thank everyone for there thoughtful comments. Today another Senior Tech stopped by (the sales rep also a former tech arranged this for us) He walked in and immediately I envisioned Sherlock Holmes just by his demeanor. We went into the basement and unlike the previous 4 or 5 techs who spent hours on this rather then opening up the oil furnace He simply looked at the large pipe that extends from the furnace to the chimney. Then like Sherlock he rubbed his ring finger over it near the furnace. He smiled & said I know what the problem is. (I thought he was joking) After showing me a little dust. He pointed out a little discoloration on the pipe . He then said occasionally the heat is going in the wrong direction.. To prove this he connected a Draft Meter. When the system is on (flame on) its negative but when the fan goes on it is positive (looking for air) We went up stairs and once again he smiled. We have chairs and stuff in front of the outgoing vents and he moved them a few inches & then said problem solved. He said the newer powerful systems simply need to circulate more air. both in and out. We went back to the basement with his Draft Regulator and it tested perfect. IT only took this gentleman less then a minute to resolve a problem that we had since November or December -A problem that most of the other techs downplayed etc The chairs and stuff were there with the old less powerful furnace without any issues. . again thanks for your input.

Hi - a lot of these posts seem to be slightly different but thought I would share my experience in my home that may prove helpful or be similar. My wife and I moved to our home newly built 5 years ago tomorrow! I can't say I've noticed the smell in the first year but maybe it's poor memory. After the first year, we'd notice this propane/rotten/dead animal like smell. At first I thought the builder put something in the vents - no signs of any rodents ever and ruled this out. Next I checked the line/drain from the AC/furnace and even had a tech check and clean the pump - no help. plumber says there is no issue with drainage or sewer or any back up venting - ok. what HAS helped intermittently is calling the gas company to check the gas lines inside. I've had them 7 times in the past 2 years, each time they find a small leak - nothing major, no reading except with the sniffer right at the source of the leak. {I also have multiple CO and gas detectors on the floor (heavy gas). They have never gone off}. After the gas company repairs the pipe the smell is gone for a few weeks maybe months depending on warm or cold season. It comes back, gas company is back out and repeat. I have replaced two ovens in the kitchen due to them being the source of the leak. I find that the air return on the main level of my house sucks in the smell and returns it to the furnace. When the furnace kicks on we smell it out of the vents. Occasionally I can smell it in the room where the furnace is also. Gas company has done line checks, pressure checks, fixed leaks - I've paid them thousands to come out and fix this over and over then the smell returns - I've returned two "new" ovens due to leaks after minimal use. I have a 2 yr old son and new baby due in a month and NEED this resolved. Please let me know if you have any suggestions or have been through something similar.

Any "loose air" in the house is going to be attracted to the return ... even if it's coming from a burned piece of turkey in the toaster oven. Is there more than the one return, which smells? I don't think the heat cycle should be that short. I'd look into the fan, which is on the other end of the "return".

Chances are your furnace fresh air intake is drawing in the odor from the vent pipe. The reason you only smell it at the end of the heating cycle is because when the burner is running the air coming through the intake is being used for combustion. But once the burner shuts down, and the blower continues to run for 2-3 minutes, then that air is being sucked in and not burnt off, so the smell continues out through your registers. Solutions include making sure your drain traps are refreshed, filling and priming your furnace trap, and reducing the amount of time the blower is on after combustion ends.