First before I start, there are wired CO detectors, battery CO detectors, and brand new honey well CO and smoke detectors that are linked to my monitored alarm system on every level of the house. There is also a rise of heat detector in the mechanical room. Please read entire post, I am desperate for help and direction.
The house is 3200sq feet colonial built in 2005, it has 2 zone hydro hot air with a dunkirk oil fired boiler. The basement is 1500sq feet finished but not heated. The dunkirk is direct vent with a fresh air intake to the outside via an air boot. We purchased the house in may of 2017, I had the dunkirk serviced immediately, my wife noticed an exhaust smell from the point when we moved in, the tech (owner of the service) said with oil you will always get some smell, he said he could come back and reseal the top, but likely would not do much. I switched companies after my wife complained more, I brought in a big HVAC contractor, they sent their oil tech 33 years of experience. Super nice guy spent 2 hours working on it. He cleaned the unit and sealed the top with cement or RTV. He told my wife that the way the flue is there would always be exhaust smell, especially with direct vent and no chimney. He sealed between the sections, as he said factory sealant broke down. He took an CO reading, it was fine, he said the boiler operates as it should. Also there is no soot on the side of the house. He recommended with switch to propane. Ever since the service I have a bad exhaust smell in the house and basement at random. I called the HVAC company they are sending an engineer out tomorrow, but I think he just wants to replace it. I need advice, I will try to attach pics. PLEASE HELP! I even built a temporary block wall to brake up the exhaust to the intake.