Are Air Conditioners More Efficient Than Heaters?
by Mason PUD 3 on
May 18, 2017
Why are winter heating bills so much larger than summer cooling bills?
READING TIME: 5 Minutes
This article was previously published in the Shelton-Mason County Journal in April 2017.
I’ve heard the question, “Why can I run my A/C at full blast all summer and not have an electricity bill as high as one month of winter heat?”
The answer: PHYSICS!
WHAT IS AN AIR CONDITIONER?
Think of an air conditioner as a “put the heat somewhere else” machine. Did you know that you can’t “add cooling” to a room? Heat is energy; cooler temperatures are the lack of heat energy. If you cool your home, you’re gathering the heat energy inside, and moving it out. An air conditioner uses electricity to remove heat from your home. That lowers the temperature, and makes it more comfortable in the summer.
THE REFRIGERATION CYCLE SIMPLIFIED
The magic of an air conditioner is found in the refrigeration cycle.
Here’s the quick version: An air conditioner uses electricity to change refrigerant between a liquid and a gas, which captures heat indoors and dumps it outside. The result is a cooler home. A refrigerator or freezer works the same way, on a much smaller scale.
For a little more detail: Picture a closed-loop system, in the shape of a circle. On one side of the circle is the inside of your house; the other side of the circle is the outdoors. The goal is to gather heat energy in your house and dump it outside. An air conditioner uses electricity to compress a refrigerant which increases pressure, and therefore its ability to hold heat. In the next step, a condenser causes the heat energy to be released from the refrigerant to the outside air, which lowers its temperature. This causes the refrigerant to condense and become a liquid. This refrigerant is then transported via copper tubing into your home. Inside your warm home, air is blown over the refrigerant lines, raising the temperature of the refrigerant. The gathering of heat evaporates the refrigerant and changes its state from a liquid to a gas. Completing the cycle, that gas is then compressed using electricity and the heat energy released outdoors.
FUEL SOURCE OPTIONS
Air conditioning can only be accomplished by using electricity.
On the other hand, there are many ways to heat your home. You can burn wood or pellets, which release heat energy; you can use natural gas or propane, which is a fuel that is burned in a furnace to release heat; you can use electricity in baseboards, wall heaters, or electric furnaces, or you can use electricity in a heat pump, which is essentially an air conditioner that can be operated in reverse (moving heat inside instead of outside).
When using natural gas to heat your home, depending on the efficiency of your heating system, you get between 60% - 98% of the heat energy out of the fuel. Propane and pellet stoves are about 70% efficient. For wood, it’s approximately 60%. When using electricity in resistance heat baseboards, wall heaters, or an electric forced air furnace, you’re operating at a 100% efficiency ratio, which means for every kilowatt-hour you put in, you get an equal number of BTUs out. BTU stands for “British Thermal Units,” a common measurement of heat energy.
That sounds pretty good, until you consider that an air conditioner or a heat pump is up to 250% efficient! This is because you get to take advantage of the phase change of the refrigerant (gas to liquid and back again), which gives it the ability to gather and release energy more efficiently.
Additionally, each air conditioner or heat pump system has a different level of overall efficiency. The efficiency rating of a system is called the coefficient of performance (CoP). The higher the CoP, the more efficient a system, and the lower it costs to operate. When looking at a cooling system, you also may see a Seasonal Energy Efficiency Ratio (SEER) rating. A heating system has a Heating Seasonal Performance Factor (HSPF). In all cases, the higher the number, the more efficient the system. PUD 3 would consider a system “energy efficient” if it has an HSPF rating of 9.0 or higher and a SEER: 14 or higher. In Mason County, a higher HSPF rating is much more important than a higher SEER rating.
System efficiency estimates don’t take into consideration the “delivery” method of the heat energy. For baseboards, wall heaters, ductless heat pumps, fireplaces, or pellet stoves, you could say you get a 100% efficiency rating on heat delivery because they generally are operated in the same room as you. If your home uses duct work, depending on the condition and design of your ducts, you could see a 70% or less efficiency penalty.
When comparing an air conditioner (or heat pump) to an electric forced air furnace or other electric resistance heat, it’s obvious that an air conditioner is much more efficient at moving heat energy.
DIFFERENCE IN TEMPERATURE
Another large contributor to the low cost of cooling compared to heating is the difference in temperature. The largest factor in determining how many BTUs (heat energy) to put in (or remove) from a space when heating or cooling is the difference in temperature between the outside air and the inside air, set by the thermostat.
Most building science experiments use 65°F as a suitable baseline for human comfort. I understand that may be cold, or hot, to depending on your preferences. For our example, let’s say your thermostat is set to 65°.
In August, when the average high temperature in Shelton is 78°, your cooling system has to remove heat at a rate to reduce temperature by 13°. In December and January, when the average low temperature is 33°, your heating system has to add heat at a rate to raise temperature by 32°. As the temperature difference increases, heat transfer (loss or gain) speeds up. In the summer, heat enters your home at a much slower rate than in the winter, and therefore your air conditioner doesn’t have to work as hard to move it outside. Having a well-insulated home can help to slow heat loss/gain, but the largest impact is always the difference in temperature.
With our recent historically cold winter, your heating system had to work harder than it has since 1984!
Many utilities around the country see their largest loads during the summer, when air conditioners use electricity to keep their customers cool. In the Pacific Northwest, we experience relatively mild summers, so we don’t see a significant amount of cooling load on the system.
Our low energy rates due to hydropower, and overall mild climate, have made electricity a preferable fuel source for heating. Much of the rest of the country uses fossil fuels such as gas, propane, and oil to heat their homes. That’s one reason customers who relocate here are often surprised by large winter heating bills. These customers may sometimes forget that they aren’t paying a separate, large bill for a different heating fuel during the winter!
About 20% of Mason County homes have cooling available through their heat pump, and that number grows each month. If you’re considering upgrading your home’s heating system, please call our conservation department to learn about the availability of rebates.
With a heat pump, you can harness the magic of the refrigeration cycle to efficiently heat and cool your home, all year long!