Question about high efficient furnace's

Here’s a call that might be interesting, but i have questions for some of the more experienced inspectors.

I’m a full time fireman and a part time home inspector. Last night we had a call for the CO detector that was going off. The weather was just above freezing with rain/sleet, very humid, the high efficency furnace was only 2 years old. The intake/exhaust was located in the rear corner of the house with a 5 ft wood fence on one side and the AC compressor on the other, the intake was facing down about 12 inches from the ground and the exhaust facing out. We had a low reading with the meter about 10 ppm in the house, nothing to be that concerned about. I went out to the back yard to check the intake/exhaust when my meter jumped to 800 ppm at the back door, the air was heavy, there was a sour smell and my eyes started to burn, and depending on how the wind was blowing the meter reading would change.
The reading at the intake pipe was 500 ppm depending on the wind which meant that the furnace was drawing up contaminated air. I believe that the high humidity had a part in suspending the CO in the air. I came to realize that some of the exhaust in the house was coming from the back door.

The questions are:

  1. Is this more common than I realize? Has anybody else ever heard of this?
  2. Since the reading was soo low in the house can CO be reburned by the furnace since it was in the intake air?
  3. Does the exhaust normally have a smell like this?

The exhaust from high efficiency furnaces is quite caustic.

Not common at all I think you have a very unusual situation and would have to think the intake or exhaust should be separated more due to the pocket it is discharging into created by the fence and the A/C unit.

No the furnace cannot re-burn Co it requires oxygen 20% just like you and I breath. When you start deluding the combustion air with Co the burner flame will turn yellow. Co has no smell one of the reasons it is so dangerous as to what the the smell was I don’t have a clue.

Thank you Mr. Bottger for the answer. I see that you’re a Master HVAC mechanic and I was hoping to get a answer from someone with your experience.
Have you ever questioned the placement of the intake/exhaust on a inspection? It seems to me that the two are too close together under normal installations anyways.

The Name is Charley, Mr. Bottger passed in 1962

I personally have never had to call one out on an inspection but they are mostly vertical in my area very few go horizontal but yes I would make the call if necessary. Normally they are installed in a open area where prevailing winds can carry the Co away before it can mix with the intake. Just because the installing contractor has a HVAC license does not mean they always install correctly just depends on who is watching as in AHJ.