You should get a load calculation for the basement and main floor.

You have nat gas service to your home? What is your rate for nat gas/therm and electric/kWH?

Post back.

IMO

You seem to like a very cool house in summer and winter. Heating with a gas furnace in the winter may lower your heating bills depending on the utility rates. I don't think the savings would be significant given you set the thermostat at 60 and have mild winters.

The typical indoor design temperatures for cooling and heating are 75 and 72 degrees respectively. Your current equipment was designed to maintain these temperatures. If you do get a load calculation make sure you tell the contractor your desired temperatures. It will affect the size of the equipment. My initial thought was 7 tons of cooling was more than enough. I can understand why it can't keep both floors at 66.

How much insulation do you have in your attic? Why is it necessary to keep the bedroom window open?

Electric rate is .1063/kWh. Gas is .0196/cubic ft. Gas bill is a steady 40/month year round. Electric bill peaks at 500 for 3-4 months and is less than 100 during the winter.

We definitely like a cool house. My wife was AC deprived as a child and is making up for it now :)

In regards to the window, I was unclear in how I stated that. It is only open during the winter to make the bedroom even colder.

As to insulation, I would need to dig out the original paperwork. I know I spent extra beyond the builders reccommendation since I was concerned about cooling.

Thanks for taking the time to review my issue.

Bama

Do you mind sharing your zip code? I want to look up your summer and winter temp averages for your location.

Your nat gas rate looks high. I would appreciate if you recheck that number. What appliances do you currently use nat gas for in your home other than gas logs?

I assume those $500 electric bills are for summer AC cooling.

Post back.

IMO

Tiger, my zip code is 35905. below is how I calculated my NG rate based on the last 3 months bill.

______Usage Charges $/cubic ft
month 1 2100 41.03 0.019538095
month 2 2400 46.95 0.0195625
month 3 2100 42.27 0.020128571

We have a gas water heater that supp.lies the kitchen and laundry. In addition, we have a Thermador 48" cooktop that gets heavy use.

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.

Since you cook a lit, Induction hobs or a cooktop could take a lot of load off of the cooling system too!

I remain unconvinced and have little confidence in the nat gas rate that OP has provided.

The winter climate average for OP's location suggests an average temperature in the range of 35-40 degrees.

Based on this alone, I would recommend a nat gas two stg var speed furnace paired with a high eff heat pump like Trane's XL15i or XR15. Of course all correctly sized. I see nothing wrong with an 80% eff two stage furnace like the XV80. I just prefer nat gas heat for temps lower than freezing. In OP's location, this would not be too often.

IMO

Tiger - it is possible that since NG use is so light down there that they don't charge a flat monthly and just charge very high usage rates. Since no one uses 100 therms a month and probably most are below 50, they don't really balk at the high rates.

Also - the rates may be based on some higher historical cost and there is no float down based on today's super cheap NG. One could imagine a well financed private utility being able to lobby for this. They could hide their outrageous profit margin with lavish parties and executive perks.

If there is no furnace in place already, one could imagine the install costs for a chimney might be prohibitive. Suffice is to say that paying a lot for an NG furnace is not worth it given his climate and indoor temp and relatively new house.

I live in NC, much larger house, and replacing all my NG with electric resistance is a $300-$500 a year cost. For him it should be less .. much less.

For David

Something is missing about OP's nat gas rates.

I have a Trane DF system and prefer the nat gas at the lower temps although with last year's mild winter only used about 50-60 therms relying almost exclusively on heat pump heat. With that said, when I installed my DF system not so long ago, there was a clear operating cost advantage of over 30% difference between electric and nat gas. This has nearly disappeared thanks to DP. Of course, no one has a crystal ball. And I don't regret my choice then. But if I had to make the decision today, I doubt I would go DF, just high efficient gas furnace with high eff AC.

If the cost for the nat gas furnace versus air handler with strip heat is minor, I would go with the furnace. At least OP should investigate the option.

IMO

No disagreement - the cost should be investigated.