Learn tips on conquering your air-conditioning costs
By Bill Bullock
This is the third in a series of articles on energy use and practical information to help a resident of Memphis make wise decisions regarding ways to lower energy consumption and overall utility costs.
In midtown, most of our homes were built before modern air conditioning was available. Many homes incorporated design features such as tall ceilings, large double-hung windows, opening transoms, sleeping porches, ceiling fans, and whole house fans. Many also had shading features such as porches and awnings, especially on the east and west sides of the house.
With the advent of air conditioning, many of those features are no longer utilized. However, home designers in the first half of the last century knew a lot about trying to keep cool. They knew that hot air rises, that moving air feels a lot cooler than still air, and that heat gain through windows is a significant contributor to the inside temperature. We can benefit from that knowledge as we attempt to reduce the amount of energy we use to stay comfortable.
Most of us tend to associate comfort solely with temperature. But in the summer, another key factor to comfort is humidity. Our air conditioners not only remove heat, but they also remove moisture from the air, as well. They are cooling and dehumidifying.
Have you ever awakened at 3 am with your thermostat still at the temperature that was comfortable when you went to bed, but now you are warm and clammy? If you’ve ruled out menopause, this is a result of the humidity rising through the night. The air conditioner got the temperature down, but the humidity crept up during the night. The temperature is the same, but you are not comfortable. This phenomenon is captured in psychrometrics—the study of the thermodynamic properties of moist air.
As an example, people are more comfortable at 78 degrees with 45% humidity than they are at 76 degrees with 55% humidity. From a geek’s standpoint, this is because there is more energy (enthalpy) in the more humid air. From a real person’s standpoint, this is because we are cooled when perspiration evaporates from our skin. And this is a key reason to have a properly sized air-conditioning unit. Larger units use more energy when on, but run less often. Smaller units use less energy when on, but run longer to get the temperature down to the thermostat setting. As long as the unit is running, air is being dehumidified. A properly sized air conditioner dehumidifies more than an oversized unit, and, therefore, you can be comfortable at a higher temperature.
You are also just as comfortable with moving air at several degrees warmer than if the air is not moving. This, again, has to do with the ability of moving air to allow more perspiration to evaporate from our skin.
The geeks will tell you that heat gets into your home in the summer through conduction, convection, and radiation. You already know this because you know that heat can come in through cracks around doors, through the ceiling from a hot attic, and through a west-facing window.
The last scientific fact we’ll cover is that the conduction of heat through a wall or ceiling is directly proportional to the difference in temperature between the two areas. So, if you can lower the temperature of your attic or raise your thermostat, you will save energy.
So, armed with a thorough understanding of psychrometrics, thermodynamics, and heat transfer, here are some specific tips you can use to stay cool using less energy.
Move the air – When you are home, use fans. They will allow you to be comfortable at a higher temperature. Ceiling fans don’t cool at all, so turn them off when you are out of the room for any length of time. In shoulder months, consider opening windows and/or using a whole-house fan when temperatures are cooler at night.
Attic – Make sure all attic vents are open in summer. If your house is not surrounded by shade trees, consider adding a radiant barrier to the underside of your roof rafters. Have as much insulation in the floor of your attic as possible. If you have AC ducts in the attic, seal all joints and seams with mastic, and make sure the ducts are wrapped well with insulation. If you have a whole-house fan, make sure it is closed and sealed from the interior when not being used.
Windows – Consider trees, awnings, and solar screens to block sunlight from entering east- and west-facing windows. If the weather stripping is in poor condition around windows and doors, have it replaced, and you will reap even more benefit in the winter.
“Off” – The geeks can calculate that it uses less energy to cool a hot house than to keep that same house cool all day. Use a programmable thermostat and utilize the automatic setback features. Remember, the most cost-effective thermostat setting is “off.” So get close to that as often as you can.
Summer heaters – When cooking, use the vent hood to remove excess heat and humidity. Replace incandescent bulbs with compact fluorescents (CFLs). These have gotten better each year, and many brands offer instant-on, good-color rendering, and a long life while cutting energy use (and heat generation) by about two-thirds. Keep big electronics in off mode instead of standby—many use a good bit of energy (and generate heat) when in standby mode.
Maintenance – When the air-conditioning filter gets dirty, replace it. Check the outdoor condenser coil routinely, and clean it when obviously dirty and at least annually. Many can be cleaned with a hose from the outside. Some require an experienced technician to clean from the inside out. If in doubt, watch your technician. Avoid damaging the cooling fins.
Replacing your unit? – Choose a system with a very high efficiency. Minimum code is now a Seasonal Energy Efficiency Rating (SEER) of 13. Go with SEER 16 or better. Require your contractor to perform a heat loss/heat gain survey and get a properly sized unit. Avoid the tendency to oversize. Consider purchasing a dual-fuel heat pump that operates as an air conditioner in reverse in the winter, with natural gas as the auxiliary fuel. Demand that your ducts be sealed with mastic and be fully insulated.
Log on to mlgw.com and click on “In Home Evaluation Program” to learn of MLGW/TVA incentives for making energy improvements and investments. Look at “Energy Tax Incentives” so see how some of these improvements qualify for Federal Tax Credits. Use “My Account” to track your energy use, get energy conservation tips, view and pay your bill, or sign up for paperless billing.
Bill Bullock has a degree in Mechanical Engineering, has been working in the energy field for over 25 years, and is a long-time resident of Midtown Memphis. If you have questions regarding this information or energy use in general, contact him at firstname.lastname@example.org.