After two HVAC companies serviced the two furnaces in the duplex I own in the course of the last 12 months, the third one discovered that both furnaces have multiple cracks in the exhaust systems. 

I'd like to exhaust other options before that, as lucrative as it may be.If we had thought of it prior to getting our primary residence, it's the first thing I would have pursued.

@Dan Hoehn never said if he had an exhaust fan. 

Just keep the gas engine away from the window so I dont die from  the exhaust 

I have not exhausted all my options in my search for the correct unit.

But my tenants are pipefitters (they can change a hot water tank), boilermakers (can change a hot water tank with their eyes closed), electricians (can replace a fuse box with a breaker box), electrical crews (can wire a 2 story house for cable tv), scaffold builders (can trim a tree with a pole saw), welders (can replace the factory exhaust with a custom stainless steel exhaust on a riding mower), construction foreman (can add a bathroom counter and vanity).

Asbestos positive materials I've run in to -* Caulk around windows and doors* HVAC duct work tape* HVAC exhaust pipes* Resilient flooring glue* Resilient flooring* Mortar in fireplaces* Lining on underside of sinks* Plaster* Drywall* Drywall mud* Siding* InsulationGoogle "Asbestos in construction materials" and the results are staggering.

Insufficient fresh air to support combustion can lead to "back drafting" (air for combustion is sucked in via the chimney and exhaust vents, thus pulling exhaust gases back in as well).

One of the reasons for going high efficiency is that you can direct vent the exhaust through PVC piping through the wall rather than having to vent through a chimney and over the roof.Chimneys need liners, they can leak exhaust (carbon monoxide) into the living space, they typically are brick structures rising above the roof line (oh and they usually seem ready to drop bricks due to lack of maintenance) ...

Exhaust that list.