You should have all the exposed duct work inspected and sealed as needed. I would recommend a 95+ efficiency furnace. Spend the extra money on at least a 2-stage. For the AC I would also recommend 2-stage for your humid climate. A SEER range of 16-17 would be fine.

1916 that year likely has a number of things working against it. What is your utility rate as in how much does it cost per KWH of power?

How old is the AC? Furnace?

With lots of windows that maybe why the bills are so high depending on utility rate and age of equipment.

accessibility to get in to reinsulate an older home like this can be quite the challenge. I suspect the walls are made of lathe and plaster?

Appreciate the responses. KWH is .131110/kwh. Base gas is $.559700/therms plus distribution charge at $.150900/therms and facilities charge of $.33/day

Air conditioner was almost 30 years old, furnace is about 16 years

Walls are lathe and plaster on first floor and poorly insulated. Windows we put in remodeled areas on both floors are 30 years old and starting to fail. Extra attic insulation added about 4 years ago. Have done sealing and also sealed exposed ducts in basement but many of ducts are in finished basement ceiling.

Any recommendations on particular brand?

Ok, your electric rate is quite high. Given the age of your air conditioner it could be 6 SEER. Which is quite the relic. Minimum efficiency for the northern climates is 13 SEER which would more than double the likely efficiency of your current AC system. But realize if you go with 13 SEER in an Air Conditioner this is bottom rung and brand would mean nothing. (13 SEER is only allowable in the north, in my climate 14 SEER is lowest efficiency allowable by law today.)

SEER ratings for AC go as high as 24.5 SEER today. For your climate you may want to consider 16 SEER. However, the 2 speed variety is typically better made and 2 speed AC's start at 16 SEER efficiency --- so this would be considered mid level. All things equal installation is the most important and due to your climate you may want to focus more on heating than AC. Brand really doesn't mean much, in my opinion. I see regular failures of 3 years old or less equipment of every brand. No brand is immune to failure.

It is probably in your best interest to go to 95% or greater efficiency in a gas furnace. It would also be a good idea to find a insulation company that specializes in reinsulating the walls of older homes. If you plan to live in this home for any real length of time this would be money well spent.

Window replacement is probably not a bad idea either, but replacing windows without tackling the insulation issues would be pointless in my opinion.

Another thing about brands is that it's best to choose brands that have good support, great warranty coverage in the area or location of where you live. Every brand breaks, so you will at some point in time, depend on your installing contractor as well as the distributor of the brand you choose. If you don't have easy access to parts, it could spell long wait times to put your system back on line.

The major brands you should consider are Carrier/Bryant, Trane/American Standard, Rheem/Rudd. These pairs are sister brands. They offer most of the same modes with different names.

You have to do some research to see which brands are popular in your area. A good installation with the proper size is most important. Poorly installed equipment no matter how good it is will never operate properly and experience an early failure.

If you have the budget then replace the windows. They are the biggest source of leaks. Adding insulation in plaster lathe walls can be very expensive and has limited value. I am not sure if it is worth it unless you are willing to remodel down to the studs.