I have a 9200sq ft house built in 1999. It has 4 total HVACs, 3 of them being AC/Gas furnace splits. The last one (basement one) is a Heat Pump with 14.4k backup heat strips. The entire attic has also been spray foamed (open cell, about 6") on the rafters, so its a pretty tight house as far as insulation goes, all the wood windoes in about 3-4weeks (the double hungs) will be repaced since the old ones are rotting, some dont open, many are sagging so its not sealing....
Now the basement unit is a 10 SEER 3.5ton Heat Pump, the basement footprint I would say is right around 3000sq ft. The unit is a ICP brand and was put in 2001 from the serial number I see. Recently (previous owners) must have put a Aprilaire 1750 whole house dehumidifier on that same 3.5ton Heat Pump because I think that 3.5ton is over sized. There are a few issues but dont want to dig too much into it, one being there is only about 900 CFM worth of return air but over 1200CFM of supply ducting. I know the unit is starving for air because when we pulled the front panel off while the unit was on, you could tell that all the sudden there was more air going out of all the supply vents. So additional return needs to be added, or correct the sizing.
Ive had about 4-5 quotes for replacement Heat Pump (since I dont think this is operating efficiently for me, we work out of the house, my wife and I, and our offices are in the basement,) so we ask that the unit try to maintain a 68F temp during winter and on colder days here in Atlanta like 30-40F outside, that unit has a cumulative runtime of almost 4-6hrs during just an 8hr period (we set it to say 68F at 8am and tell it to cut back to 62F by 4pm). Few have quoted the budget ones all the way upto the inverter based heat pumps, so the ranges are pretty wild.
Then looking at some open areas of the ceiling (some areas of the basement are unfinished like storage room) there is about a 1.5' gap between the actual basmement ceiling and the 1st floor joists. So I thought maybe we do a gas furnace, like a 95%. Getting a gas line isnt an issue since about 5ft from that unit is a gas line that Ts for my main 1st Gas furnace and a 40gal water heater. So I could T into that. Then looking at that gap in the ceiling, one HVAC guy said he can run the single 2-3" PVC about 40ft down the line and exit out the side of the house where all my AC condensers and pool equipment is. But that will require some cutting up of the drywall in a few places in the ceiling, probably every 8-10ft where he has to put join the PVC pipes or put an elbow in. I looked at the celing area a bit more and I dont think its going to be easily done, probably much more cutting involved, there is already a zoo of 8-10" insualted duct work all around that area so they will have to cut up alot of the area. And not really looking forward to that.
So getting to my main question, now Im thinking of going back to entertaining the heat pump idea. Lets say I go with these 18SEER, 2stage comrpessor (say a 3ton), HSPF of 9.5. How would you compare that to say like a 95% single or two stage 75k btu gas furnace? My electric rates here are between 9-15cents (right now 9 cause its winter, it goes to 15cents in summer during only like 3-7pm peak time). Gas rates are extremely low as well, right now around 50cents a therm. Or is this not as easy to compare these two since it is two different types of furnace. As far as the stages, Im not really concerned with all the hoopla around multi stage or variable speed and "comfort" you get. Just more concerned with heating this space during our working hours efficiently (unlike running for 6hrs during an 8hr period).