# Is It Cheaper To Leave Your Thermostat At One Temperature?

by | Feb 6, 2020 | Industry News

Is it really better to keep your thermostat at a constant temperature? The physics and math say no. So, are you willing to adapt to save money?

You’ve probably heard somewhere that keeping your Texas home at a steady temperature will save you than if you turned down the thermostat. It sounds like it should make sense, right? After all, if you turn down the heat in your home, the temperature drops  and then you’ll have to pay those high Texas electricity rates to heat the place back up. Right?

WRONG!

While that notion sounds plausible, it’s really just myth. And it’s all because of Newton’s Law of Cooling.

## What Does Turning Down The Thermostat Do?

Newton’s Law of Cooling shows that the rate at which the temperature of a thing changes is proportional to the difference between its temperature and the temperature of what’s around it.

Let’s say you’ve got two bricks, one heated up to 70°F and one heated to 40°F. You set them outside where it’s 32°F. According to the Law of Cooling, the 70°F brick will initially loose heat faster in the cold than the 40°F brick. However, the rate of heat loss slows as the bricks’ temperatures gets closer to 32°F. Why? Because the temperature change slows down as the bricks’ temperature gets closer to the surrounding air temperature.

### How does Newton’s Law of Cooling work with my home?

Let’s assume the inside of your Texas home is 72°F and the outside is 32°F. When the heat is turned off your home will begin to cool. Of course, exactly how fast that happens depends on the home’s heat loss (how well insulated and air sealed the building is). But because the rate at which the building cools is proportionalto the outside temperature, the house loses heat more quickly at 72°F than it does at 62°F. That happens because heat loss slows down as the inside and outside temperatures get closer to being equal.

So instead of your home’s inside temperature dropping to 32°F faster than a jalapeño dropped into a jug of liquid nitrogen, the temperature actually glides down a slope that gets more and more gradual.

### Constant Temperature and Heat Loss

Starting with some very broad assumptions, let’s compare the heat needed in British Thermal Units (BTUs) to heat a moderately insulated 2,000 sq. ft home. Let’s also assume that it’s 32°F outside.

• At 72°F, the amount of heat energy needed to keep the home heated is 32,000 BTUs.
• At 62°F, the amount of heat energy needed drops to 25,600 BTUs.

Now, let’s say you leave your thermostat set to 72°F for 8 hours while you’re gone at work during the day. Given Newton’s Law of Cooling, your furnace will be running longer and more often during that entire 8 hour period in order to maintain the temperature at 72°F than it would if you set the thermostat to 62°F.

Consequently, setting your thermostat back to 62°F will use less energy to maintain temperature and also save you money on your Texas electricity bills.

### Won’t my furnace use more energy to heat the house back up?

That’s a good question. But the problem is that it assumes your home’s heat loss is at the same rate no matter what the temperature is. As we see with the Law of Cooling, heat loss just don’t work that way. Now it is true that when you turn down the thermostat, the home does lose heat. However, over 8-10 hours, the house doesn’t lose all of its heat because the heat loss slows down. Plus, it takes less energy to heat something from 62°F to 72°F than it does heating something from 32°F to 72°F.

Raising the temperature in our example house would require roughly using 12,800 BTU of heat. That means during that heating “recovery” hour, the furnace would use 25,600 BTUs plus an additional 12,800 BTUs (burned by running more often and longer during the recovery hour) for a total of 38,400 BTUs. However, this heating “recovery time” would only occur when the thermostat is set to raise the heat. For most working day schedules, that’s about twice a day.

Just exactly how long the furnace needs to run during the heat recovery time to raise the temperature depends entirely on how well the home is insulated and air sealed, the efficiency of the furnace system, and how cold it is outside.

### Does Turning Down The Thermostat Save Money?

Before wading into the math, we need to outline how we’ll figure cost. For that, we’ll calculate energy in BTUs but to help calculate the cost of the energy we’ll convert those BTUs to kWh.

• First, 1 BTU = 0.000293071 kWh of electricity.
• Second, let’s say we’re paying 10 cents per kWh.
• Thirdly, an average sized furnace for this size house is rated around 55,000 BTUs. That’s more than adequate for our example house.

Now, obviously, because this is an over-simplified example the cost amounts we’ll see will be exaggerated. Considering real world heat loss conditions in most homes, the financial costs would actually be much, much less. So for convenience, let’s assume that the heat our furnace produces is limited to meeting the thermostat setting during one hour of operation. Therefore, if our thermostat is set to 62°F, it will produce 25,600 BTUs during one hour to maintain the temperature at 62°F. If our thermostat is set to 72°F then it will produce 32,000 BTUs during one hour to maintain the temperature at 72°F. And finally, the furnace will produce 38,400 BTUs during each heating recovery hour.

### What Should I Set My Thermostat To When I’m Not Home?

Using our example home, if it takes 32,000 BTUs per hour to keep the heat in our home at 72°F, that means it uses about 9.3782742222 kWh. Therefore, at 10 cents per kWh, we’ll pay about 94 cents per hour.

But if we use only 25,600 BTUs to heat to 62°F, then we use only 7.5026193778 kWh and we’ll pay just 75 cents per hour.

Now, let’s look at heating the home for a full 24 hour period.

If we’re keeping the thermostat set to 72°F for an entire 24 hour day the costs come out to be:  \$0.94 x 24 = \$22.56

But if we use setbacks, we can do better. Let’s suppose this is your week day schedule:

• Wake at 7 am, leave by 8 am.
• Return home between 4 and 5 pm.
• Bedtime at 10 pm.

If we setback the thermostat following a schedule that only warms your house when you’re at home and active, then we see the savings making a difference.

Morning (6 am – 7:59 am @ 72°F) \$0.94 x 2 hours = \$1.88

At Work

Away period setting (8 am – 2:59 pm @ 62 °F) \$0.75 x 7 hours = \$5.25

Recovery Hour (3 pm to 4 pm) to raise temperature to 72°F. Total = 38,400 BTUs (1.125 kWh) = \$1.125

Total At Work Amount: (7 hours) \$5.25  +  \$1.125 (1 Recovery Hour) = \$6.375 for total of 8 hours

Evening ( 4 pm – 9:59 pm @ 72°F) \$0.94 x 6 hours =\$5.64

Night (10 pm – 5:59 am @ 62°F) (7 hours) \$5.25 + \$1.125 (1 Recovery Hour before Morning) = \$6.375 for 8 hours

TOTAL COST: \$20.27  That’s \$2.29 less or 11% in savings.

Over the course of 22 work days per month, that savings could grow to over \$50. For three months of winter, that’s potentially \$150 in savings.

### It Is Better To Turn Down Your Thermostat!

As mentioned since we’ve used an over-simplified example, the energy costs here are exaggerated. However, depending on how well insulated and air sealed your home is, the percentage of savings from thermostat setbacks could actually be much more. The Department of Energy estimates homeowners using programmable thermostats this way can save as much as 15% a year on heating and cooling by simply setting back the temperature by 7°-10°F for 8 hours a day from its normal setting.

### What Should You Set Your Thermostat At In The Winter?

What’s a good winter setting for YOUR thermostat? That depends entirely on what you and your family are most comfortable with. The problem is that a “comfortable temperature” has never been an absolute fact. In the 1800s, the indoor comfort range ran from 55°F to 62°F mostly because everyone was wearing woolen clothing. In the 20th Century, clothing became lighter and the accepted comfortable temperature rose to 72°F. Even regionally, there’s a lot variation for winter comfort which is mainly dependent on what people are wearing. In some states, 63°F is comfortable, others think 70°F. However, in one experiment by the Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) showed that as long the experiment participants didn’t know the room’s temperature, they indicated they were as comfortable at 68°F as they were at 72°F.

In short, people are very adaptable when comes to temperature; more so when they don’t know what it is!And the lower you can set your home’s thermostat, the more money you will reduce what you pay on your Texas electricity plan. So, see how low you family can go during the day and especially at night when they’re asleep. The results may surprise both you and your pocketbook.

For more tips on how to save money on your energy usage check out https://www.texaselectricityratings.com.

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