Older homes were designed to breath.[1] I live in San Francisco and unfortunately contractors often forget this (or don't care or never think to consider it). For example, painters will often use a non-porous latex on the outside with a common consequence that you'll quickly begin to see mold grow on the internal walls where in bygone years this wouldn't have been a problem. Or the paint may be much more prone to blistering as humidity tries to escape.
Most old homes in SF originally had (or at least were modified 100+ years ago to have) gravity fed heaters--no forced air, and no return registers in each room; just a single giant return register at a low point on a bottom floor. I've spoken with A/C contractors who say that there shouldn't be any serious problems rigging up a forced air system to the output registers, even without proper return registers. And plenty of homes do this. But I guess maybe the real problems come if you then being insulating the home--can the forced-air heating system circulate air quick enough without return registers to compensate for the fact the building no longer naturally ventilates? I imagine in most cases it works well enough, but you're still moderately more likely to see mold problems.
[1] To varying extents. My house was built in 1926, and it seems they used a relatively thin tar paper to wrap the house, or at least part of the house. (Unless that was somehow added much later, but I doubt it as the wall facing an adjacent house a few inches away is papered, and the siding is original on that wall.)
Most old homes in SF originally had (or at least were modified 100+ years ago to have) gravity fed heaters--no forced air, and no return registers in each room; just a single giant return register at a low point on a bottom floor. I've spoken with A/C contractors who say that there shouldn't be any serious problems rigging up a forced air system to the output registers, even without proper return registers. And plenty of homes do this. But I guess maybe the real problems come if you then being insulating the home--can the forced-air heating system circulate air quick enough without return registers to compensate for the fact the building no longer naturally ventilates? I imagine in most cases it works well enough, but you're still moderately more likely to see mold problems.
[1] To varying extents. My house was built in 1926, and it seems they used a relatively thin tar paper to wrap the house, or at least part of the house. (Unless that was somehow added much later, but I doubt it as the wall facing an adjacent house a few inches away is papered, and the siding is original on that wall.)