I agree with Lane. There are maintenance companies that can services your entire Hvac system at a low cost so you keep the life of the units.

I really noticed a difference in my electric bill when I put in a programable thermostat for my HVAC. I tend to keep the air down low and would forget to turn it up when I went to work. The new thermostat does that for me. 

I think the plans are a good idea especially for older homes. Making sure you change your filters regularly is by far more important.

The energy efficiency of a home relies on several factors. 

The most dominant of these factors is the insulation used, both within the perimeter walls and attic.  This would include the houses ability to breath as well as maintain a good barrier.  It's not so much the amount of insulation as the type and installation methods.

Another factor that effects the energy efficiency of your home is your windows and doors.  They come in all varieties of energy ratings, and are also reliant on the installation methods used.

The third thing that I would look at is the ventilation of your attic.  If your attic has the necessary ventilation needed it can reduce energy cost as well as prolong the life of your roof system.

Great insulation plays a key factor in energy savings. There are many different options available to increase the energy efficiency of your home.  James Hardie Siding is specifically designed for the climate in which it is installed. Whether you live in a climate with freezing temperatures, snow & ice or a climate with high humidity, heat, and rain; James Hardie has a siding product built for your homes specific needs. Technology has changed tremendously over the years so homes built 15- 20 years ago may have been installed with what was considered cutting edge at the time. Window replacement and the new technology that windows have now can save a great deal of energy costs. A good product that we can recommend also would be the Mezzo Vinyl Window or Integrity by Marvin Windows. These are high performance windows that are engineered to provide outstanding energy efficiency. Major appliances such as heating and A/C units can also offer great energy savings. 

While there are pros to the Nest, there are other options out there that offer more features and better home comfort control.  The Prestige IAQ from Honeywell is a fantastic option. 

The Prestige is a feature rich, internet capable thermostat.  It has a large, full color touchscreen display.  Homeowners have the option of downloading the Comfort Control App from Honeywell at no charge; so it can be controlled from any phone, computer, or tablet.

The Prestige can also eliminate the need for a seperate humidistat to control the humidifier.  It has wireless outdoor sensors, so the temperature and humidity is monitored indoors and out. 

The Prestige has built-in Delta T Sensors.  These sensors monitor the temperature rise and drop of the HVAC system and will alert the homeowner with an email if there is a need for a service call.  Service reminders can also be programmed in to email the homeowner when it is time for annual maintenance, filter changes, and humidifier pad changes.

The Prestige IAQ is the choice for our company when a homeowner wants to upgrade to an internet capable, smart thermostat.

Oftentimes the attic is the place to start.  Make sure you have plenty of ventilation, both intake and exhaust.  Check the soffit cavity to make sure you it isn't clogged with insulation.  There are formulas for Square foot of attic in relation to ventilation.  If you have a vapor barrier in the ceilling a ration of 1:300 sf is ok, otherwise about 1:150 is the rule. Second, check the insulation.  All air escaping into the attic should be sealed using spray foam or other sealants. Next, depending on your geographic location, make sure you have plenty of insulation in the attic.  Finally, something as simple as installing blinds over windows can be an economical solution to saving on energy costs.  As recommended in some of the other answers, an energy audit or getting advice from any number of qualified professionals that are willing to visit your home is the best place to start.

John Ford, There are an abundance of energy efficiency upgrades you can do to your 2-story home. A question to your question - do you have a small or a larger budget? Small budget - 2 options: Attic Insulation or A Foil Energy Barrier, one example is Green Energy Barrier. Blown-In Attic Insulation is inexpensive and can help with an additional R-value of 18-60 in your attic. A Foil Energy Barrier - can be placed along the rafters or floor boards and is a foil used by NASA that greatly assists with energy efficiency. Larger budget - 3 solid options: Solar panels, Replacement Windows with Krypton gas, or Insulated Siding, an insulation with a system R-value of a 4.2 that contours to your siding panel. Best, Ken 

With anything like this, I hate to be the first especially with this manufacturer. They brought out the Nest smoke detector and it get recalled.  I would rather wait  and watch the press and blogs and when the dust clears and all is good then starting specifiing them. 

Phililp Anderson

Berkeley CA

I am assuming your home is ducted and has a forced air system.  If you you only have 1 furnace and 1 condenser, it my be more energy effecient to expend to two units.  I recommend Lennox.  You can also look at installing spilt units like these made by Fujitsu:

http://www.fujitsugeneral.com/products.htm

These are designed so that you can cool a room/rooms individually.  I suggest calling a few licensed HVAC contractors and get 3 quotes.