Is there carbonmonoxide entering the ductwork?

have you had the ductwork cleaned?

Ductwork Inspection: Inspect ductwork for leaks, insulation issues, and debris.

If you do this set the thermostat yourself and put a lock box over it so the tenants can't turn it up too high or down too low.I also like the idea of closing off the duct work to the upstairs apartment and adding electric baseboard heat, this might cost you a few hundred bucks.

.- This often results in city mechanical inspector being difficult about furnace and ductwork.

The single-family property doesn’t have any existing ductwork, so we’re planning to install a multi-zone mini split system (4 zones).We’ve received three quotes so far, ranging from $18K to $26K from HVAC companies.

The existing ductwork couldn’t accommodate this size hood, requiring new ductwork ran to the roof.  

You can easily burn up more labor reworking the ductwork than system install, which is really plug & play.

This may involve adding ductwork, installing a separate HVAC unit, or using portable heating and cooling units.5.

In the City of Philadelphia, you will also have to have an extra electric meter to serve common areas, such as lighting and fire alarms. http://www.biggerpockets.com/forums/84/topics/7504...Since you are doing a gut, now would be the time to get the ductwork for both supplies and returns into place.