That is a crazy high gas rate. Do you have a flat fee worked into that? We pay $10 a month flat so if you look at what 1 therm costs, it is $11. But truly the therm costs $1. A NG furnace will always save a few in a dual fuel situation but not enough to make it worth it for you. Where is your MBR? Don't you really just want the MBR at 66 at night? What you probably want is more tonnage which isn't going to work with your existing ductwork most likely. So it is pretty cost prohibitive compared to other options. The other options are to reduce your heat load. Air sealing is cheap enough to do some things DIY - do you have attic stairs? Do you have cans into the attic? Sealing these things may lower your temp enough. Controlling solar gain is really where the real savings is. The cheapest is solar screens but they do effect the transmission of light. Plant trees. If you don't have low SHGC windows this can make a huge difference. Switch to LED lights - anything used over an hour a day is a reasonable payback and it will lower the a/c load. LED is far better than CFLs and we find it acceptable everywhere except the bathroom. We keep our master at 65 at night (and high 50s in the winter) with a ceiling fan so I understand where you are coming from. Since your usage is so high, it would definitely pay to get some high efficiency equipment. But overall, the best answer to a cooler house in the summer is not to change the a/c size but to control the loads. I do feel like your usage is pretty high even given your locale and requirements. I peaked at $150 (probably $90 in a/c) in July this year with a 65 degree bedroom suite for 5000 sqft in NC. Obviously you are warmer but not double.

where does one find "degree days Centigrade scale" for their climate? I am in Sarnia, Ontario and this is an area where a heat pump supposedly feasible. (i would mate it with a high efficiency furnace), but all the techs i speak to have advised against it saying i am wasting my money, it won't work...etc. I have gotten a couple of quotes for 16 seer 2 stage heat pumps with HE furnace and they have been 11-12K. i can get the same setup with 16 seer ac for about 8K... doesn't that seem like a huge jump for a heat pump over an a/c condensor? i'm about to give in and just go with an a/c unit, but the math says it is about 30% cheaper to heat my house with at heat pump over a 95% efficency furnace...and that is using a COP of 2.5 at 30 deg f. i am thinking about importing my own equipment from the states where prices seem much more reasonable (96% amana furnace 16 seer 2 stage heatpump, coil, txv for $3800) and just having a local guy who is willing to install it. that blows some of the rebates that are available, but it still might be worth it. all i know is it would take more than a decade (and likely much more...) to recoup an extra $4k.

HVAC equipment is mostly designed to load specifications of the structure. So changing configurations within the home envelope can bring unintended consequences in that the equipment in question is 'over sized' for a specific area. 'Over sized' is a convoluted term because due to variations of the building load --- either hot or cold --- what is considered a properly sized system is actually over sized upwards of 90% of the time. Climate can make all the difference. If you're in a more moderate climate it *may* not make much difference. However, realize this: your systems and the duct work it is attached to is *sized* for the system it is attached to. There are likely differences in duct sizes and possibly number of ducts. If you attach a large system to a duct system that is not properly sized for the equipment --- it won't work for long, if at all. If you attach a small system to a larger duct system the low volume will be low and won't work properly either. The other issue is Humidity. An over abundance of humidity in a basement is *extremely* bad. Because basements can be a haven for mold growth. Over sized AC will not run long enough to pull the humidity out. So you get a renter that has a habit of long hot steamy showers and an over sized AC = recipe for disaster. If you're in a moderate climate the basement may not need AC, but you'll probably want to install a good sized and capable dehumidifier to be safe and even then you could still have trouble.

Propane has 91K btu's per gallon input, Electric has 3400 Btu's per kilowatt. Propane gives you 72,800 btu's output at 80% efficiency. Electric aprox. 3400 Btu's output per kilowatt, (100%) with only a small amount lost. 72,800 btu's divided by 3400 btu's = 21.4 kilowatts to equal 1 gallon of propane. The average cost of 1 kilowatt in Austin - $0.1142 cents, times 21.4 Kilowatts equals $2.42 for electricty and little more expensive then propane at $2+ per gallon. But this will change drastically if Biden does ban fracking as he stated. But then what happens to the price of electricity? It's a guess, and having the ability to switch between the two may be a valuable option in the near future. But the cheapest option with a stable price for ever would be to capture the heat in the sun with active solar collectors to heat water (not to be confused with solar panels to produce electricity), and store it in insulated underground tanks to use for the winter. After the initial investment it would cost you a few dollars a month in electricity to run circulation pumps, and you could even get the electricity from solar panels for that. They do it in Alaska for building bigger than yours, easy and simple, only problem is getting insulated storage tanks in the 5K to 20K gallon range. I'd do it in a heart beat if I had some really big tanks, I have the land, backhoe and the solar collectors, 9 of them 5' x 9', salvaged from a $35,000 government subsidized program project in the late 70's. (The government loves wasting other peoples money.) Johnson controls started a line of HVAC equipment called champion about 6 years ago. Parts?