The old moncreif gravity furnaces have no moving parts, they'll probably last another 75 years.The funny thing is I bought for cash flow, not expecting any appreciation but they have apprreciated anyway (about 25% in the last 3 years).

Fans fans fans .. gravity will pull the water down fans will dry .. you may have stains as rust or dirt is pulled down through the drywall 

One final note, while everyone is familiar with gravity loading, very few are probably familiar with shear loading.

Heating systems  (FHW and gravity systems) need to be gone over carefully. 

I work at an oilfield company now known as Gravity Oilfield Services (formerly Globe Energy Services).

That forces water at a high rate through your drain rather than gravity fed.

If it has a pump it is more trouble but gravity systems can last a long time.

Hi All,I have an unfinished basement that's Kng great condition I would like to finish off.I would like to remove a gravity furnace and install a new furnace and ac, a bedroom with an egress window and split the basement into a bedroom and family room.

Where possible, when I buy,  I update electrical boards if they have fuses, new wiring for extra outlets when they need more power, plumbing to insure there is good water flow,  heating if the systems need replacing or are gravity based, and roofs when needed. 

-Lift Station-  pumps sewage from lower elevation to higher so gravity can then take over.