I live in Southern California, and am writing this at my desk in the large family room, which is about 45 sq.m. (500 sq.ft.). This room was added on to the house, and our natural gas furnace doesn't heat it very well (there is only one vent, which doesn't get great flow). For this reason I often run an electric space heater next to my desk when the weather is cold. (There is also a glass sun room next to the family room, which we basically don't use because it's too hot or too cold for most of the year.) Running the space heater continuously seems kind of insane -- its cost probably adds 50% to the total cost of heating our house (20 cents to run it for an hour, compared to maybe 40 cents to run the furnace on a 50% duty cycle over the same time period).
Our furnace and air conditioner are inefficient and reaching the ends of their lives, and my wife and I plan to live here for the rest of our lives, so now would be the time to make intelligent changes that would save us money in the long term. We are also interested in being environmentally benign, even if it costs a little more. In our area, the source of electricity is about 44% natural gas, with the rest being solar plus other stuff that has no carbon footprint. This area is one of the best on earth for solar power, so as time goes on, the share of zero-carbon sources for electricity will probably continue to go up.
Keeping in mind both cost and environmentalism, what is likely to be the best way to make this room more habitable and environmental? Options I can think of include: --
Install a single-room heat pump.
Just upgrade the central heat and AC to more efficient units, but widen or improve the ducts to get more air to the room.
Improve the circulation of air through the house by some other method, without messing with the HVAC vents themselves.
I'd sort of dread number 2 because of the lifestyle disruption and because there is a bunch of nice woodwork and cabinetry that would probably have to be torn out. Or is it possible to run ducts out over the roof?