Key Points
- Choosing a furnace that is too small or too large leads to uncomfortable temperatures and excessive energy use that inflates utility costs.
- Square footage and climate zone are used to determine how many BTUs, or British Thermal Units, are required to adequately heat your home.
- Factors such as efficiency, temperature preferences, the number of occupants, and the house’s layout and construction influence proper furnace sizing.
When the cold season approaches, you want to choose a furnace that will keep your home warm without breaking the bank or increasing your energy consumption. You may assume that larger furnaces heat homes better, but the adage that “bigger is better” doesn’t always apply to home heating. While large heaters may be perfect for large areas, it's important to find a furnace that is the right size for your living space. This article will help you understand what size furnace you need to keep cozy through the winter.
Why Does Furnace Size Matter?
Furnace sizing matters because a furnace that’s proportionally sized for your house will warm it gradually. It will require fewer repairs, last longer, and be more energy-efficient than an oversized or undersized furnace. It will also heat your house evenly, so no space is hotter or colder than the rest of the house. If your furnace is too large or too small for your home, you could end up with uncomfortable temperatures, high energy bills, or both. If your furnace is disproportionate to the size of your home, you may experience the following problems:
What Happens if Your Furnace Is Too Large?
Let’s return to the assumption that a large furnace will keep your home warmer than a smaller one. In actuality, an oversized furnace will make certain parts of your house feel extremely cold while making other parts feel too hot. Oversized heaters work in quick intervals. This means that they will turn on when it's cold and overheat certain rooms, which will then cause the furnace to shut off and make other rooms cold. The frequency at which oversized heaters turn on and off makes them less energy-efficient than a correctly sized heater. They use significantly more energy, resulting in increased energy bills. In addition, an oversized heater requires frequent and expensive repairs, which translates to a shorter lifespan due to frequent cycling between on and off.
What Happens if Your Furnace Is Too Small?
A furnace that is too small will not be able to warm your house to a comfortable temperature on very cold days. It will heat your home unevenly. Some spaces will get very hot, but most spaces in your home will remain cold. On the coldest days of the season, an undersized heater will run nonstop, causing it to wear out quicker and significantly increase your energy bills.
Calculating What Size BTU Furnace You Need
Sizing a furnace for a house takes only a few calculations. To find the right furnace size for the square footage of your house, you need to determine how many BTUs it needs. To do this, multiply your home's square footage by the heating factor of the climate zone that you live in.
What Is BTU?
BTU is the acronym for British Thermal Unit, the designated unit used to measure heat energy in energy sources or fuels. It is the amount of heat required to increase the temperature of one pound of water in liquid form by one degree Fahrenheit when the water is at approximately 39 degrees Fahrenheit, when it has its highest density. Energy is defined as the ability to do work, and heat energy comes from molecules and atoms moving within a substance. We use BTU to convert fuel from a physical unit of measurement, such as volume or weight, to its heat content or energy. The higher the BTU, the more heating power a unit has.
How to Determine Your Square Footage
To calculate the number of BTUs your furnace needs, determine the square footage of your house. Measure the square footage of each room by multiplying the length and width of each room together. Add each room's square footage to calculate the square footage of your entire house. This calculation will work for rectangular rooms, but rooms in other shapes will require slightly different calculations.
How to Find the Square Footage of a Triangular Room
To calculate the square footage of a triangular room, multiply the length and width and divide that number by two.
How to Find the Square Footage of a Circular Room
If you have a circular room, you will need to measure the radius, which is the distance from the center of the room to the edge. Multiply this measurement by 3.14 to calculate the square footage. If you have an abnormally shaped room, divide it into smaller sections, then measure and calculate each section separately.
How to Determine Your Climate Zone
Once you calculate your home's square footage, you need to identify which climate zone you live in. Climate is another factor that determines how many BTUs a furnace needs to heat your house. Your furnace typically needs more BTUs to heat your home the farther away you are from the equator. BTU requirements also vary based on geographic factors such as ocean currents and elevation. Figuring out your zone is easy because it is divided into five categories based on weather and average air temperatures. You can determine your climate zone based on the region that you live in.
Where Are The Climate Zones?
Southern regions in the United States, such as Houston, Miami, and New Orleans, make up zone one. Zone two includes states such as Tennessee, North Carolina, parts of Oklahoma, and coastal California. Zone three covers states such as Virginia, Kentucky, Kansas, and Delaware. New York, Chicago, and Boston are all in Zone four. Zone five covers the northernmost states in the country.
Each zone has its own recommended number of BTUs, which includes the following:
- Zone 1: 30 to 35 BTUs
- Zone 2: 35 to 40 BTUs
- Zone 3: 40 to 45 BTUs
- Zone 4: 45 to 50 BTUs
- Zone 5: 50 to 60 BTUs
Each zone includes the recommended number of BTUs needed per square foot to heat your home. For example, if you live in Florida, your furnace should produce 30 to 35 BTUs per square foot. If you live in Maine, you need a furnace that produces 50 to 60 BTUs per square foot. The type of house you live in greatly affects whether you need the lower or higher number in your zone. If you live in an older or poorly insulated home, go with the higher number in your designated range. Use the lower number if you live in a newer or well-insulated house. If you're not sure about the quality of your home's insulation, it's typically safer to choose the higher number.
How to Calculate BTU
Once you calculate the square footage of your home and figure out which climate zone you live in, you can use these numbers to calculate the BTUs your furnace needs to heat your home efficiently. Multiply your home's square footage by the recommended heating factor for your climate zone. You can also use a furnace size calculator to determine what your home needs. For example, if you live in a well-insulated house that is 2,200 square feet in zone one, you would multiply 2,200 by 30, which calculates to 66,000. This means you need a furnace with a 66,000 BTU energy output. If you live in a poorly insulated house that is 1,600 square feet in zone four, multiply 1,600 by 50 for a result of 80,000, meaning you need a furnace with an 80,000 BTU output.
How to Calculate the BTU Output Rating
Some furnaces do not include a BTU output rating. To size a furnace that only includes a BTU input rating, you have to calculate the BTU output. To calculate the BTU output, multiply the furnace's efficiency by the BTU input. For example, if you look at a furnace with a BTU input of 80,000 and 95% efficiency, you multiply 80,000 by .95 for a result of 76,000, which is the BTU output the furnace can produce. Efficiency is important when searching for a furnace. Two different furnaces can have the same BTU input but produce different amounts of heat energy for your home. For example, a furnace with 80,000 BTU input and 78% efficiency will produce 62,400 BTU output, while a furnace with the same BTU input and 95% efficiency can produce 76,000 BTU output. This means you can potentially purchase a smaller, more efficient furnace, saving you money from the start.
Other Variables That Affect Furnace Sizing
Square footage, efficiency, insulation, and BTU output are important when choosing a correctly sized furnace. However, you should also consider other variables, including the following:
Family Size
The more people who dwell in a household, the warmer it will be, and the fewer BTUs you will need to keep your home warm. This is because people dissipate heat into their surroundings.
Windows and Number of Floors
Windows also affect furnace sizing, because having many windows allows more heat to escape your house, even when they are closed. This is especially true for older windows. If your house has a high number of windows, you may want to choose a higher number of BTUs. The number of floors in your house also affects heat because a second-story house will provide more insulation and keep heat within the house.
Sun Exposure and Roof Color
The amount of sun exposure your home gets also affects its temperature. If the rooms in your house get a lot of sunshine, you will need fewer BTUs than if your rooms get more shade. The color of your roof affects how much heat energy your house absorbs from the sun. Darker roofs absorb more heat energy from the sun, and light roofs absorb less heat energy, so a darker roof may help keep your home warm with a smaller furnace.
Ceiling Height and Ceiling Fans
If you have high ceilings, you may need more BTUs to heat your home. However, ceiling fans located throughout your home circulate air, resulting in fewer BTUs needed.
House Shape
In addition, the shape of your house affects how much BTU output your furnace needs. A long, narrow house has more exterior walls, causing it to lose more heat and requiring more BTUs to keep your home warm. A square house requires fewer BTUs.
Ductwork
You also need to ensure your ductwork is the right size for your furnace. If the ductwork in your house is too small, the air from your furnace won't have enough space to travel through, and your furnace could overheat. Consult a professional if you're unsure.
Preferred Temperature
Another factor to consider is your preferred temperature. Most people like to keep their thermostat between 70 and 80 degrees Fahrenheit, but you may prefer to keep it lower or higher. If this is the case, your BTU requirement will be slightly lower or higher than normal.
Understanding Furnace Efficiency
The efficiency rating percentage of a furnace shows how effective it is in converting air to heat. This is important because it affects your energy consumption and how much money you will spend on heating your home. A heater with a higher efficiency rating requires a lower BTU input number to heat your home. A smaller heater with a high enough BTU output can heat your home just as well as a larger heater. Efficient furnaces are also safer and more eco-friendly.
Get the Furnace You Need at IWAE
When searching for a furnace to heat your home, sizing is important. A furnace that is too large or too small can unevenly heat your home, increase your energy consumption, and cost you more money. However, a furnace that appropriately fits your home can keep you comfortable while saving you money and keeping your energy consumption low. IWAE has a gas or electric furnace that is the perfect size for your living space. We offer fast, free shipping and price-matching services as well as lifetime tech support over the phone any time you need help with installation or repairs. Find the right furnace for your living space with IWAE today.
