We have a 60 year old house (uninsulated) and we've been having difficulties keeping it adequately heated over the past 10 years or so (short of cranking up the thermostat). We've replaced windows and reinsulated the attic yet during the winter the upstairs is about 2 degrees cooler and in the summer about 3-4 degrees warmer than the downstairs.
What surprises me is the variety of "advice" from HVAC "pros" that we've gotten that don't seem to work.
What did not work:
1. In the winter, turn on the ceiling fans (but reverse the direction)
2. In the winter, leave the furnace fan on (this sort of works in the winter but unless we leave the thermostat at 70 and above it blows cold air around) - we have yet to try this in the summer.
3. Close up some registers.
4. Conflicting advice:
(a) The air return is too low or corrollary the air return is too high.
(b) The are too many air returns in the house - you don't need one in every room, or the corrollary, there isn't enough air return, you need one in every room.
(c) The thermostat is too low - by about 6 inches (like this would make a difference).
The real problem is that the houses in this era really were badly built for heating and cooling. (Some romanticize that houses built today are just not as durable as the ones built in the 40s and 50s but heck, these houses are just really energy inefficient).
1. The ducts are too small. (For instance, the register in the master bedroom is approx 6"x6".)
2. There just ain't enough insulation.
I would have thought that by now we would have HVAC down to a science but apparently not.
My prescription: There needs to be 2 systems - one for heating and one for cooling. The air return for the summer months should be up to draw away the warm air and down in the winter to draw away the cool air. The registers should be up but angled down and not directly across the air return otherwise it would just get sucked out of the room in the winter and in the summer the registers should be down but angled up. Then again, it probably can't beat a nicely insulated modern house.