Yes! Well, mostly yes. Well, it depends. About 99 percent of the time, setting back your thermostat at night will save you a substantial amount of money. The other one percent of the time is a much more complicated discussion, we'll explore further below as it relates to the sub-zero temperatures we’ve been experiencing recently.
This post dives into the nuts and bolts to not only help you make informed decisions but also give you a solid foundation of how forced air furnaces work.
So what do you need to know?
This article is for forced-air, natural gas or propane gas burning furnaces only. Some of this won’t directly apply to your home if you have electric radiant heat, a heat pump, geothermal, oil, or any other type of heating system in your home. Fortunately, most heating systems in Kalamazoo are forced-air gas systems. Not sure what type of heating system you have? Google your unit's model number.
Note: This post focuses on concepts and generalizations so that the average homeowner will understand their heating system. If you are an expert, please do not pick apart things like latent versus sensible heat, how energy converts, density ratios, specific heat, thermal inertia, altitude, U values, etc. Want to learn more? Request a future post on specific aspects of these by submitting a Contact form here, or posting to our Facebook page.
The Basics of Heat
Before answering the thermostat question, we must understand what heat acts like, how heating systems are designed, and how homes are heated.
The most important thing to remember is that heat moves from warm to cold, and the greater the difference in temperature between a warm area and a cold area, the faster the heat moves. The cold weather from outside does NOT come in to your home. The heat that is IN your home moves OUTSIDE. When heat leaves your home, the temperature drops inside. There is actually no such thing as “cold.” Cold is simply a concept to describe the absence of heat. Cold does not move – only heat moves.
Heat moves in three basic ways; convection, conduction, and radiation. We aren’t going to get into the differences between them right now although your furnace uses all three to move heat in your house, and heat leaves your house in all three ways.
Heat does not care which way is up. We often say that “heat rises.” Heat does not rise, but rather, warm air rises because warm air is lighter than cold air. This is an important concept to keep in mind because heat actually escapes your home into the cold outside from all sides, including through your basement. About a third of the heat lost in a typical old home is through the basement, while only about 10 percent is through the attic.
The way to keep heat from escaping from your house is with air sealing and insulation. One of the best known insulators is actually air itself. Different types of insulation – fiberglass, cellulose, spray foam, or foam board – do nothing more than trap air pockets inside it.
It’s the air that is doing the insulating, not the insulation itself. BUT, and this is a big but, air that is moving is also one of the best ways to move heat. This is why air sealing is so important. Air moving through any type of insulation will move the heat out of your house at a very fast rate and make your insulation worthless.
Heat is a type of energy, much like electricity, light, or motion. Heat energy is measured in British Thermal Units (BTUs). The amount of heat energy stored in one cubic foot of natural gas is 1000 BTUs. When you burn one cubic foot of natural gas, you release 1000 BTUs of heat energy, which heats up your furnace and is transferred into your home by the blower fan.
Furnaces are sized according to BTUs; they most commonly range anywhere from 40,000 BTU to 120,000 BTU. When you get your bill from the utility company, they charge per MCF of gas used. One MCF is equal to 1000 cubic feet, or 1,000,000 BTUs worth of heat.
The Basics of Heat Loss and the Right Size Furnace
Now that we have a basic understanding of heat and how heat moves, let’s look at how we decide what size furnace to install in your home. If you install a furnace that is too small, it won’t be able to heat your home adequately in the winter. If you install one that is too big, it costs more money to run and your home becomes uncomfortable from drastic temperature swings (more on this further below).
To “right-size” a furnace, we calculate the amount of heat that will move through every part of a house that is connected to the outside. This includes exterior walls, ceilings, doors, windows, basement walls, basement floors, crawlspaces, and any other place that has the outside on one side and the inside on the other. In addition, we calculate the amount of “air infiltration” that moves in and out of your home between the inside and outside.
This is done using the amount of surface area of a house component and the “R value” of the material to figure out how much heat moves through it. We figure out air infiltration either by running diagnostic tests with a blower door, using a computer simulation model, or making an estimate from a chart based on how the home was constructed. We put all of this information into a very large spreadsheet and calculate the total heat loss.
Now you are probably thinking “But wait! What if I build the exact same house in Florida as the one I have in Kalamazoo? You can’t say that the house in Florida needs the same size furnace!”
You would be absolutely correct. If you remember from the beginning of the article, the larger the difference in temperature between the inside of your home and the outside world, the faster heat will move to the outside. To account for this, we use what is called a “design condition.” Design condition is what mechanical contractors are required to use by the State of Michigan to properly size a heating system.
Pro tip: if you ever replace your heating system in your home, be sure to ask the contractor for the “Manual J” calculations before hiring them. Manual J is the standard they are required to use to size your furnace. If they don’t have one or tell you that one isn’t needed, don’t hire them. Your existing furnace is NOT a good indicator of what size your new furnace should be. Rule of thumb, experience, or any other method they may mention is almost always wrong as well.
Design condition simply means keeping your home at 70 degrees when it is about as cold as it will get outside in your location. This is based on the 99th percentile of cold temperatures where your house is located. In Kalamazoo, this temperature is five degrees outside. Why did we pick five degrees for Kalamazoo when it’s below zero right now? This number is calculated based on the actual winter temperatures over the last 10 years. Ninety nine percent of the time, the winter temperature was above five degrees, and one percent of the time, the temperature was below five degrees.
We add all of this information to the big Manual J spreadsheet and calculate the total heat loss of the home when it is 70 degrees inside and 5 degrees outside. This is given in the number of BTUs per hour the house will lose during that time. We then match that total potential heat loss of the home to the correct size furnace to have a “right-sized” heating system. This means when it is five degrees outside, the furnace will run nonstop to keep the home at 70 degrees inside.
Why Not Just Put in a Super-Sized Furnace?
By now, you might be thinking that we could skip all of this stuff and just put in a great big furnace that will keep the house warm even when we have arctic temperatures outside. This is how it used to be done years ago and we find many very old furnaces in homes that are two or three times the size they should be. Oversized furnaces not only cost more to keep your house warm regardless of the outside temperature, they make you uncomfortable as well.
First: the comfort issue.
Imagine standing in front of a bonfire on a very cold night. When you face the fire, your front gets very warm and it feels nice but your back is still freezing. You find yourself turning around over and over again in order to keep both sides of your body warm but you are never truly comfortable because you are too hot on one side and too cold on the other.
Oversized furnaces work the same way. When they turn on, they send out a lot of heat all at once and immediately make the air warm in most areas of your home. They turn off very quickly and wait for the house to cool down before turning on again.
The sudden inrush of heat from the big furnace heats the air but not the furniture, walls, and other stuff in your home. Thermostats detect air temperature. This means that the furnace will turn off but the “stuff” in your house will still be cold. The heat from the air will move into the cold stuff in your home to warm it up causing the air to cool down quickly. This means the thermostat will then turn the furnace back on to heat up the air in your house again. This results in the furnace “cycling” on and off many times per hour, creating an up/down/up/down temperature swing and making the house feel drafty inside. The industry term for this is called “short cycling.”
Oversized furnaces cost more to run
The second reason not to oversize a furnace is that a furnace becomes more energy efficient the longer it runs. When a furnace first turns on, all of its parts are cold and the ductwork that delivers the air to your room is cold. The burning gas in the furnace first heats up all of this metal before it can move the heat to the air in your home. This usually takes about two to five minutes to happen. A furnace which is oversized might only run for five minutes before turning off again. This means that you are spending a larger amount of your gas bill just heating up the mechanical system in your basement and not in heating your living space.
So why did the industry decide to use a design temperature that only covers the heat load of a house 99 percent of the time? Remember that the furnace needs to be sized to handle really cold days, but not be too large for the warmer 30 degree days. The magic number to produce energy efficiency, indoor comfort for you, and make sure you are safe in case a polar vortex comes to town is 99, resulting in a furnace that is slightly smaller but more than up to the task. If it is below five degrees outside only one percent of the time, the outside temperature will not go too much below this for very long in most cases. Even with the bone-chilling cold of negative 10 degrees in Kalamazoo over the last few years, it generally only happens at night and the temperature rises back up during the day. This means that the furnace will be undersized for only a few hours at a time and the inside temperatures will stay warm enough because of thermal inertia.
Answer the Thermostat Question Already!
Now we have almost all of the information we need to answer the question about setting back a thermostat at night. The simple answer to the question is yes, 99 percent of the time you should set back your thermostat at night to save money. In Kalamazoo, if it is more than 5 degrees outside, your properly sized heating system will have no problem bringing your home back up to temperature in a reasonable amount of time.
The complicated answer to the question is the 1% of the time you should not set back your thermostat at night, even though it would still save you money.
If the temperature is 5 degrees outside, your properly sized furnace will run continuously to keep the inside at 70 degrees until the outside temperature rises again. If the temperature is 4 degrees outside, your furnace will run continuously and will keep the inside at 69 degrees. When it is 0 degrees outside, your furnace will run continuously and will keep the inside at 65 degrees.
Of course, we know that outside temperature doesn’t stay constant – the temperature typically drops at night and rises again during the day most of the time. If you go to bed at 10pm with your thermostat set at 70 degrees when it is 5 degrees outside, your furnace will continue to chug along to keep the temperature constant in your house. However, if the temperature drops to -5 by 2am, the house will not instantly become 60 degrees. It will most likely be around 69 degrees inside at 2am. This is because the temperature outside doesn’t drop instantly and the inside of the house is already at 70 to start with.
Because of thermal inertia, it takes time to lose the heat already in your house. As the night goes on, it may drop to negative 10 degrees for an hour right at sunrise and your house will most likely be about 67 degrees. Once the sun is up and the day warms, the temperature outside increases back to five degrees or more, and the furnace will be able to gain ground and bring the house back up to 70 degrees and turn off.
The thermal inertia in the house is why the temperature inside the home doesn’t drop as fast as the temperature outside of the home. This means that the temperature swings inside your house are substantially leveled out and most of the time you probably won’t even notice.
So what happens when I set the thermostat when it’s 0 degrees at night?
If you take the above scenario except that you set your thermostat to 60 degrees when you go to bed at 10pm, your furnace will turn off until the temperature inside drops to 60 degrees. The furnace is able to keep up with the 60 degree temperature all night, but you will have lost all of the thermal inertia in the home between the 60 and 70 degree marks when the furnace turned off at 10pm.
When you turn the temperature back up to 70 in the morning when it is -10 degrees outside, your furnace may not be able to get the temperature above 60 degrees until the outside temperature rises. Once the temperature outside starts to rise, the furnace will slowly gain ground to get the house back up to 70 degrees. This can take hours on a very cold day, and in some very cold stretches lasting several days, you may never get your house back up to 70 degrees until it is consistently above 5 degrees outside. Your furnace ends up fighting a ten degree temperature rise instead of a three degree temperature rise had you not set the thermostat back at all.
This assumes that the furnace in your home is correctly sized. If your furnace is oversized right now, then the 5 degree temperature may be much lower before it can’t keep up. If your furnace is undersized, then you may be in trouble when it is 10 degrees or warmer outside. The only way to know the exact temperature at which you should not set back your thermostat is to either run a heat load calculation on your house or to experiment every time it gets very cold and track your results.
Looking for an expert consultation about replacing your furnace? Contact us for a free assessment and quote.
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