When it comes to saving energy, misconceptions and mistaken assumptions often mislead people, causing more problems or costing them more money. Some were part of standard building codes until research proved them wrong.
The vexing thing about these misconceptions is that, at first glance, they seem to make sense. Only after closer examination do you find out they’re not based on a complete understanding of how energy and structures (like your home) behave in the physical world.
In our new Challenging Common Energy Misconceptions series, we will look at several energy-related myths to learn how they affect energy efficiency. To kick it off, let’s start with the most commonly held ones.
1) Heat Rises.
Put your hand up near the ceiling. Does it feel warmer? Why? If you said, “Yes, because heat rises”, then ask yourself, “If you heat up a brick with a blowtorch, does the brick rise up and float about?”
Heat is energy. Heat always moves towards cold, until the two locations reach thermal equilibrium. A hot brick transfers heat energy to the air around it. Heated air rises because it is less dense. So when you held you hand up near the ceiling, what you felt was heated air. The heated air transfers the heat energy to the sheetrock and wood framing in the ceiling.
Once the heat leaves the air, it sinks to the floor. This type of heat transfer and movement of heated and cooled currents of gasses (like air) and liquids is called “convection”. Heat transfer and heat loss play crucial roles in energy efficiency. Insulation is rated in terms of R-values (heat resistance). Energy efficient windows are rated in terms of U-factor that describe how fast heat moves through the materials making up the entire window.
So next time, remember: it’s heated air that rises, and heat moves toward cold.
2) Fluorescent Lights — It takes more energy to turn them on than just leaving them on.
Yes —BUT — this was mostly true 40 years ago, especially for the older style T12 and T8 style fluorescent tubes that utilize preheating. Many of these are found in warehouses, garages, older office buildings, schools, auditoriums, and other institutions (T12 and T8 tubes are being discontinued).
Preheating at start-up heats up the gas in the tube and also causes the cathode to emit electrons which soon excites the mercury vapor in the tube and eventually causes the glow. The voltage in-rush on these lamps can be up to a thousand volts, and if the lamp does not light right away, the preheat cycle can repeat several times.
So, when should you turn off the lights? Energy.Gov recommends this rule of thumb:
- If you will be out of a room for 15 minutes or less, leave it on.
- If you will be out of a room for more than 15 minutes, turn it off.
3) Keeping your home at a steady temperature uses less energy. Making your furnace work so hard to bring the house up to a comfortable temperature will wear it out much quicker than letting it maintain the heat in your house.
This assumption makes sense only if you believe that heat loss acts the same way as a pig charging off a cliff — a sharp arc that plunges straight down. However, physics shows us that this assumption to be hogwash.
Newton’s Law of Cooling states that the rate of change of the temperature of an object is proportional to the difference between its own temperature and the ambient (or surrounding) temperature. The cooler the interior of your home gets (relative to outside temperature), the slower the heat loss. In other words, your home loses heat energy more quickly at 72°F than it does at 62°F because heat transfer slows down as the difference in temperatures gets closer to thermal equilibrium. This includes structures in the home like fireplaces and interior walls of house that act as thermal masses that store infrared heat and release it overtime.
So, if it’s 32°F outside, your furnace will run more often over an eight hour period maintaining a 72°F temperature inside than it would if it were set back to 62°F for eight hours. During the recovery phase when the furnace is heating the home back to a comfortable level, the furnace doesn’t work any harder than it would maintaining a 72°F temperature for eight hours. How long the furnace will need to run during that recovery period depends entirely on how well the home is insulated and the efficiency of the furnace system. A clean, well-maintained system will run efficiently; a dirty, neglected one will struggle and be costly.
So, heat loss is more like a pig skiing down a slope. How steep the pig’s initial decent is depends on how well your home is insulated because that has a direct effect on how fast your home cools down during an eight hour period. Well-insulated homes may only cool part way and never fire their furnaces, while poorly-insulated homes may lose that ten degrees of heat within an hour. In that instance, the furnace would more than likely run almost continuously trying to maintain a 72°F temperature.
By not heating your home as much when you are not there or not being active through using a programmable or smart thermostat, you can save about $180 every year in energy costs. And that’s nothing to squeal about.
Want to learn more? Stay tuned for our next installment!