Thermodynamics for Idiots

thermodynamicsDon’t worry: You won’t have to endure a lecture on thermodynamics to understand how to cool your home more efficiently. In fact, there are only two basic scientific principles that you need to know, and you’re sure to be familiar with at least one of them. The first is: heat follows cold. The second is: hot air rises. Thinking about these two rules of nature will help you see how air moves in and around your house and how that relates to cooling (and heating) efficiency.

Heat Follows Cold

If it’s 98 degrees outside but (thanks to central air conditioning) a comfy 72 degrees inside your house -- and you open a window, the heat from outside will jump right in through the window and keep jumping in until it’s just as hot inside as it is outside. Like water, heat constantly seeks equilibrium; heat moves to cold until everything is the same temperature.

Since you probably spend much of your summertime reminding the kids not to leave the doors open, you already know that opening a window when the AC is on is a dumb thing to do (unless you have an evaporative cooler, discussed below). But open windows and doors are just the largest and most obvious avenues for mingling indoor and outdoor temperatures. The smaller avenues, like gaps around light fixtures in your ceiling, are much less obvious and usually ignored, yet these are often the ones that matter the most.

Hot Air Rises

This is the law of nature that makes the upper floors of a house generally warmer than the bottom floor or basement. It’s called convection, and it’s responsible for much of the heat and cold lost inside our homes. In the winter, you heat the inside of your house, and that heat is constantly trying to escape through convection and other means. It heads right upstairs and sneaks through all the little holes in your ceiling between the top floor and the attic and from there makes its way outside.

As the warm air is leaving the house, it’s also pulling outdoor air into the house through tiny gaps around doors and windows, cracks in the foundation, around pipes entering the house, etc. This cool “makeup” air just makes your furnace work harder to keep you comfortable. In the summer, the same effect leads to inefficiency with air conditioning, because convection is pulling hot outdoor air into the house, making your air conditioner work harder.

Now that we’re done with the science lesson, let’s look at what home energy experts say about cooling your house more efficiently.

Air Seal and Insulate

Air sealing and insulation are the two secrets that allow a thermos to keep hot things hot and cold things cold. The insulated walls of a thermos create a barrier between the contents inside and the air temperature outside, interrupting the natural equilibrium of heat. The top of the thermos has a rubber gasket and screws on to create a tight seal. This prevents air flow, and thus rapid heat exchange via air currents, to maintain the thermal barrier.

Your house works pretty much like a thermos. Insulation prevents heat loss (in the winter) and heat gain (in the summer) through the walls and ceilings, while air sealing prevents heat loss or gain through air movement. Most people think of insulation only to keep out the cold, but according to Nancy Kellogg, a certified energy auditor with Lightly Treading, Inc., improving insulation and air sealing is just as important for the cooling season.

Kellogg once got a call from a client who had recently tightened up her home with insulation and air sealing improvements. It was June and quite warm outside, but the client was uncomfortably cold in the house and was reluctant to turn the heat on in the summertime. “I told her to open the windows,” says Kellogg. Clearly, the house had done its job of isolating the indoors from the outdoors (with somewhat humorous results, in this case).

When it comes to both air sealing and insulation, Kellogg stresses to her clients to “start at the top, and seal up the lid (meaning the ceiling under the attic) really well. This is the most important place to create a barrier. Holes in there are always the strongest for airflow.” This, Kellogg explains, is due to the “stack effect,” the fact that tall structures such as houses act like chimneys to promote warm air rising from natural convection. “And the taller the house, the stronger the stack effect.”

Kellogg also points to garages as other common problem areas. “If you have an attached garage, make sure it’s thermally separated from the house.” She explained that this is a commonly neglected area by builders, leading to significant heat exchange between the house and garage.

Seal and Insulate Forced-Air Ductwork

For those of you with conventional central AC systems, the network of supply and return ducts is critical to cooling efficiently and effectively. Air leaks in the many fittings, seams and joints in the ductwork can lead to serious energy loss. According to Cary Weiner, the Clean Energy Specialist at Colorado State University Extension, “You can lose up to 30% of your cooled air in ductwork. So one of the most important pieces is sealing and insulating your ductwork inside the house.”

Conditioned air leaking from ducts often never makes it to your living spaces; it ends up in the basement, inside walls, under floors and other places where it’s essentially wasted. When ducts run through a hot attic or exterior wall, you need duct insulation to keep the air cool as it travels through.

Shade Thy Home

“Another big factor is heat avoidance,” stresses Weiner, “using shade trees, managing ventilation, adding window coverings (on the inside) and awnings (on the outside) to keep heat from reaching the house in the first place.”

Shading of any kind is helpful, but it’s always best to block the sun before it gets to the house. Alex Wilson, Executive Editor of Environmental Building News, writes in Green Building Advisor: “Shading windows is the easiest way to keep your house cool or keep your air conditioning bills down. Pulling down interior window blinds will help (the more reflective the outer surface of the blinds the better), but shading is even more effective if you can block the sunlight on the outside of your windows... Climbing vines on a trellis, nearby trees, large potted plants that can be rolled in front of doors or windows and awnings can also help a lot. If there’s no way to block that sunlight on the outside of windows and patio doors, install and use interior blinds.”

Customize Your Thermostat

Whether you still have an old mercury-vial thermostat or a newer, basic digital model, consider upgrading to a programmable thermostat. These allow you to program automatic temperature settings to suit the household’s activity on a daily or weekly basis. For example, you can have your thermostat turn off the AC just before you leave the house for work, then turn it on again just before your return at the end of the day. This is one of the simple measures that Weiner recommends to homeowners looking to save energy on cooling. “It just makes it easier to manage household comfort without wasting energy.”

Tip to Lower Summertime Indoor Temperatures 

Nancy Kellogg has a clever trick to capture a little free cooling and balance the temperatures inside her home. Her thermostat has a “fan only” option that simply runs the blower motor on her forced-air furnace. During hot days, when the upper-level rooms are heated up and the basement (where the furnace is) stays much cooler, she runs the fan for a while. The system draws in some of the cool basement air and circulates it throughout the house. In her case, this can lower the temperature in her living spaces by 6 to 7 degrees.

Clean and Maintain Your AC Equipment

According to Weiner, “keeping your AC and furnace filters clean can improve the system’s efficiency by 5 to 15%. Many systems have a filter in the outdoor condenser unit that helps keep the coils clean, maximizing efficiency. For indoor air quality and proper flow, it’s also important to replace furnace filters as needed throughout the cooling season.

Kellogg offers similar advice and adds, “I tell people to forget the standard routine of checking filters every three months and do it once a month. An easy way for them to remember is to check the filters every time they get a utility bill.” Kellogg also recommends shading your outdoor unit and making sure it is level and has at least 1 foot of free air space on all sides.

Consider an Evaporative Cooler

As an energy expert based in Colorado, Weiner is a big fan of evaporative coolers (AKA swamp coolers). These are relatively simple systems that pass air through a water-soaked pad to cool it before sending it into your house’s ductwork. Because they add moisture to the indoor air, they are appropriate only for dry climates. The best thing is that they save a lot of energy.

“An evaporative cooling system can be 75% more efficient than conventional AC,” says Weiner. “An additional benefit of evaporative systems is that some natural ventilation is not only ok, it’s actually required. You manage your comfort level by how much you open your windows.” With conventional AC, you never open the windows because it lets out the cool air and lets in humidity, which your system is working hard to eliminate.

Use Ceiling Fans With or Without AC

Ceiling fans help cool you when the AC is off, and when it’s on, fans can make it more comfortable to keep the thermostat at a higher temperature. According to Alex Wilson, “All other things being equal, a breeze will keep you a lot cooler. If you’re normally comfortable at, say, 72 degrees, using a ceiling fan or oscillating desk fan may enable you to be just about as comfortable with an air temperature of 75 to 80 degrees -- because the moving air evaporates moisture from your skin. Note that fans don’t actually cool the air (in fact, they increase air temperature slightly from the motor’s waste heat), so turn them off when you leave the room.”

Weiner says that ceiling fans, which use much less energy than AC systems, can make you feel up to 4 degrees cooler.

Check for Rebates before Making Improvements

Kellogg is eager to tell clients where to find information about government and utility incentives and programs for home energy improvements. Ask a local energy auditor or your utility company about resources for finding available incentives and programs. Some of these benefits can be surprisingly good—while they last.

For energy-saving improvements to your home heating and cooling system, contact a reliable HVAC professional.

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