If it's Cadets or baseboard heat, make sure they are not shoving a sofa in front of the heat to block airflow.

If the boiler and WH are connected to the same exhaust duct and the duct looked clean I would try an in-line exhaust blower wired to the heater/boiler to increase the airflow through the exhaust duct, since that's relatively cheap and easy. 

One reason is not only about air flow, it also adds aesthetic appeal to that portion of the house.

That coupled with the 2x4 stripping makes for some great air flow.

Air flow along with a dehumidification system would then be required to get the humidity level below 50%.

Great tenants, btw.Quote: $8300 w/out permits for 3 ton rooftop Tempstar, moved about 6 ft for more airflow and 75% new ducting.

This causes an extra evaporative cooling effect making the air conditioner more efficient, and, eliminating the drip hole all air conditioners used to have.It is possible when newer model air conditioners without condensate drain holes get dirty and airflow through the cooling fins becomes clogged, the condensate cannot evaporate properly and builds up, eventually freezing over on the inside and plugging up the unit.

If I did that would't I be stopping airflow to the entire duct, including the first floor?

Sometimes clogged up filters restrict airflow and leave the unit running harder and longer than usual.

They've complained about the upstairs bedrooms being hot and minimal air flow.