Energy Insight: How can you efficiently operate your heat and air-conditioning?

Clarksville is a beautiful place to live. The seasons certainly keep our weather interesting. They also make energy efficiency more challenging. There’s no one thermostat setting or schedule to operate your heating ventilation and air conditioning system (HVAC) efficiently all year long. It’s often best to make thermostat adjustments throughout the year.

Fact: Your HVAC system uses the least energy when it’s off.

Your thermostat controls when your HVAC system is on and off, and the trick is to keep your HVAC system off as much as possible, while also maintaining a comfortable environment in your home, but comfort is subjective. Since there’s no one schedule of thermostat settings that works for everyone, the schedules in Figure 1 are just suggestions.

Figure 1

If you’ve read our Energy Insight articles before, you know we’re not going to stop with a standard schedule of thermostat settings. We’re going to show you statistics, and dive much deeper than necessary, into how thermostat settings affect your energy use.

The schedules in figure 1 are effective for most homes. If you have a programmable or smart home thermostat you can set it to follow this schedule automatically, or you can manually adjust your thermostat. If you don’t have a smart thermostat click here and enter to win one.

If you have a heat pump and you’ve read Energy Insight: how do electric heat pumps work?, then you have a fundamental understanding of what’s happening when your thermostat switches on your HVAC system. Now let’s think about how your home and a heat pump work as a system to affect energy use.

In warm weather, your home is constantly gaining heat from the outside. No matter how well insulated your home is, heat is conducted through the exterior surfaces and convected through the interior areas. (In cold weather, it’s just the opposite; your home conducts heat from inside to out.)

The amount of heat conducted into your home in warm weather is determined by the difference between the outside ambient air temperature and the inside temperature of your home.

Heatflow through matter roughly equals the insulating value of the matter, multiplied by the area of the surface, multiplied by the difference in temperatures between one side of the matter and the other (Q=U·A·▲T). That little nugget is sure to make you a big hit at parties. You’re welcome.

The Air Conditioning Contractors of America (ACCA) HVAC system design parameters for Clarksville, TN assume that a thermostat will be set to 70°F in winter and 75° in summer. So if you have a 10’X10 13 R-value drywall and wood stud wall and the temperature outside is 95°F then heat transfer is assumed to be Q=.077·100·20, or Q=154Btu/hour.

To paraphrase Lucius Fox from Batman Begins, you don’t have to understand all that, we “just wanted you to know how hard it was.”

Your HVAC system is designed based on the size of your home, the climate of your region and the aggregated insulating properties of all the building materials that make up your home’s walls, floors and ceilings. It’s designed with heat gain and heat loss in mind because its primary function is to replace all of the heat you’re losing or to evacuate all of the heat you’re gaining, and these can be very energy intensive processes.

To explain exactly how energy intensive the process is we’ll need to do more math. Try not to be too excited about that.

If you live in a 1000 sqft home with R-value 13 walls, a R38 roof and a R14 door, then first of all you’re probably going to want to get some windows. Windows complicate the calculation though, so your hypothetical tiny house doesn’t have any. So the house looks like the one in Figure 2.

Figure 2

We never claimed to be architects, but this simplified house makes it easy to add up the R-values of the construction materials and plug them into our Q=U·A·▲T formula. If you did that – which we’re sure you did — then you would find that this house gains heat at a rate of about 2882.18Btu/hour, depending on how you calculate for the floor. That doesn’t include losses in the air conditioning ducts or infiltration of air around doors and windows. It’s just a number for our example.

If you’re still reading, we’re super surprised, and good for you. This is where it all comes back around to thermostat settings.

Every heat pump is rated with a Seasonal Energy Efficiency Ratio (SEER) and an Energy Efficiency Ratio (EER). For our purposes, we will focus on your system’s EER.

EER is basically the number of Watts of power an HVAC system requires to move one Btu of heat. So if our hypothetical tiny house has an EER of 12 then 2882.18 divided by 12 is 240.18. So it’s practical to assume that this structure requires 240 watts of power per hour to maintain a temperature of 70° at an outdoor ambient air temperature of 95°.

If you turn the thermostat up 4° to 74° and rerun the exact same calculation, changing only the inside temperature, then the tiny house only needs 201.77 Watts of power to maintain its temperature. That’s approximately 10 percent less energy.

There’s obviously much more to calculating heat transfer and energy use, but this example takes the fundamental principles of those calculations and shows that energy and money can be saved with thermostat adjustments.

Through similar processes, it is also possible to calculate the savings expected from new HVAC equipment, improved windows, insulation and weather sealing.

If you’d like to better understand the potential for savings at your home read our other articles.
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