When people think green, the first thing that comes to mind is probably switching to those twisty fluorescent bulbs, or cutting back tailpipe emissions from a big SUV. In fact, the place where the most energy is used is actually to maintain a comfortable temperature in our homes. According to the US Green Building Association, heating and cooling costs account for:

• 36 percent of total energy use
• 65 percent of electricity consumption
• 30 percent of greenhouse gas emissions

That means there is a real opportunity for cost savings through greener, more efficient heating and cooling products and systems. Even modern houses are full of places where energy can escape. Proper insulation and ventilation can reduce heating needs to one-third or even one tenth of what an average house would need.

That's great if you're building a new home, but what about a drafty old house or apartment? Not everyone can afford a big investment to overhaul their insulation and install new windows. But there are plenty of opportunities for smart people to make small steps that can create a greener environment and make a real difference in energy bills.

Seal Leaks

As you walk through your house, it's easy to feel where heat is being lost: those drafty doors, windows, crawlspaces and basements are usually the culprits. While a house needs a certain amount of ventilation to properly "breathe," older homes have a long way to go before they are considered too tight. The following are some tips on how to seal the most common air leaks in your home:

• Windows: Older, single-pane windows are a major source of lost heat in a home. Even more efficient modern windows result in wasted energy.  The energy lost in the U.S. each year through our windows is equal to the amount of energy currently carried through the Alaska oil pipeline. The easiest and quickest fix is to install storm windows. Unfortunately, storm windows are usually sold along with the windows when they are installed. If you live in an old home or apartment, the original storm windows may be lost and hard to replace. However, another effective solution is affordable and easy to install window films. Applied to the inside frame of windows, these create an insulating barrier of air, similar to the principle behind double-paned insulated windows.
• Doors: Similar to windows, doors allow a great deal of heat to escape from a house. If it's cold where you are now, put your hand near the bottom of a door. Chances are there is a healthy draft, especially in doors that lead directly to the outside or to a basement or attic. An easy, immediate way to avoid this cold draft is by installing a simple draft catcher.  However, the best way to prevent heat loss is through weather stripping.
• Attics: The attic is often the last place people think of to curb heat loss.  However, a great deal of heat is lost through the roof. A simple, effective solution is radiant barriers. These block much of the radiant heat from the attic, reducing the greenhouse effect inside that space.
• Heating and Cooling Ducts: Each home has dozens of feet of ductwork snaking through it. Heat can be lost through bad seams between sections. In fact, studies show that this loss can be as much as 20 to 40 percent of energy costs. The good news is that you can seal off these "energy bandits" with an afternoon's worth of work. Start with ducts that run through the attic, crawlspace, unheated basement, or garage. Use duct sealant (mastic) or metal-backed (foil) tape to seal  seams. After sealing the ducts in those spaces, wrap the ducts in insulation to keep them from getting hot in the summer or cold in the winter. Next, seal ducts that you can access in the heated or cooled part of the house. Homeowners are often concerned about sealing their house too tightly; however, this is very unlikely in most older homes. But to be safe, have a heating and cooling technician check to make sure that your combustion appliances (gas- or oil-fired furnace, water heater, and dryer) are venting properly. For more information, check out Residential Energy: Cost Savings and Comfort for Existing Buildings.

Change your air filter regularly

All of the processed air in your house runs through a filter attached to the furnace. With the volume of air moving through most household heating and cooling systems, there are many opportunities for impurities to clog the filter. Check your filter every month, especially during heavy use months (winter and summer). In most systems, the filter is located adjacent to the furnace, within the first few feet of ducting. Most people change their filter at best once a year -- but truly, it should be changed four times a year. A dirty filter slows down air flow and makes the system work harder to keep you warm or cool – wasting energy. A clean filter will also prevent dust and dirt from building up in the system – leading to costly maintenance and repairs.

Install a programmable thermostat

Every house has a thermostat: that little box or knob on the wall that you are always being reminded to keep at a low temperature to save energy. But did you know that new thermostats actually can be set to allow the temperature to creep up or down a few degrees while you are at work to save energy? Today's modern programmable thermostats can be had for a reasonable price, and when installed in place of a standard non-programmable model, can double your investment in just one year.  What's more, over the lifetime of the device, the units can save money in the thousands of dollars. The EPA's ENERGY STAR program has detailed information about how to squeeze the most savings out of these thermostats. Visit their page on programmable thermostats here.  These new thermostats can be installed to replace an older model with only a few twists of a screwdriver.

Invest in ENERGY STAR qualified heating and cooling equipment

While replacing your HVAC equipment can require the largest investment, it also holds the most promise for savings. If you have older equipment in your house, or if you notice your bills are high, and your house is still not comfortable, consider replacing it with a unit that has earned the ENERGY STAR. Depending on where you live, replacing your old heating and cooling equipment with ENERGY STAR qualified equipment can cut your annual energy bill by nearly $200. A licensed contractor can inspect your system and make sure that it is running properly.

Finally, it's important to have your HVAC system inspected by a licensed professional annually to make sure it is running at peak efficiency. These inspections can be obtained inexpensively through your oil or natural gas provider.