> My experience (sample of one) is that they don't.
There is something very very wrong with your equipment (or you're measuring something else not having anything to do with the heating equipment - more likely). A high efficiency furnace has a sealed combustion chamber, the entire thing runs in a circuit vented to the outside - combustion air comes from the outside and it is vented to the outside. If it is not airtight sealed from the indoors, it is broken. Even a mid-efficiency furnace with a non-sealed system will vent 100% of the flue gas outside.
> Natural gas combustion is at best _mostly_ exhausted.
Bullshit for any modern equipment (ie installed in the last 50 years).
There is something very very wrong with your equipment (or you're measuring something else not having anything to do with the heating equipment - more likely). A high efficiency furnace has a sealed combustion chamber, the entire thing runs in a circuit vented to the outside - combustion air comes from the outside and it is vented to the outside. If it is not airtight sealed from the indoors, it is broken. Even a mid-efficiency furnace with a non-sealed system will vent 100% of the flue gas outside.
> Natural gas combustion is at best _mostly_ exhausted.
Bullshit for any modern equipment (ie installed in the last 50 years).