A Guide to Choosing the Right Size of Furnace

When having a new furnace installed, it is vital that the unit you choose is the right size for your home. Many people wrongly believe that the bigger a furnace is, the better it will work. Unfortunately, this simply isn’t the case as numerous issues can result if the furnace is either oversized or undersized. It is always best to have a certified HVAC technician calculate what size of furnace your home needs, but here is an overview of how you can at least get a rough estimate of what size of unit you need on your own.

Calculating Your Home’s Square Footage

The first thing you will need to do is calculate exactly how many square feet your home is, not including your garage or anywhere else that isn’t connected to your heating system. If you’re unsure of how many square feet your home is, you will need to calculate the square footage of all of your rooms and then add these together to determine the total square footage. To calculate square footage, measure the length and width of each room and then multiply the two to find square feet.

Why Climate Zone Is Important

Calculating your home’s square footage is important as your furnace will need to produce so many BTUs (British thermal units) of heat for every square foot, and exactly how many BTUs per square foot are needed is determined by what part of the country you live in. The U.S. Department of Energy separates the country into seven different climate zones, and each zone has its own BTU requirements. These requirements are based on an estimate of how cold it can get in the winter. The colder the winter temperatures are, the more BTUs of heating you need per square foot.

Plainfield and the entire Chicago area are in Zone 5. In this zone, the general recommendation is that you need somewhere between 45 and 55 BTUs of heating for every square foot. If you have a 1,500-square-foot home, your furnace would usually need to produce somewhere between 67,500 and 82,500 BTUs to effectively and efficiently heat your house.

How HVAC Technicians Size a Furnace

HVAC technicians will always start by calculating square footage and BTU requirements based on your climate zone. However, these two factors alone aren’t enough to ensure that the furnace is the appropriate size as there are numerous other factors that will also affect how much heating you need.

One of the most important factors is how well-insulated your home is. Especially important is the level of insulation in your attic and basement or crawl space as these are the areas that will let the most heat out and the most cold air in. If your home is poorly insulated or has lots of air leaks, you will need a much larger furnace than you would in a home that was well-insulated and sealed up tight. If the level of insulation isn’t accounted for, then you may not end up with a big enough furnace, and your home will always stay colder.

Another important consideration is the number, type, size, and location of all doors and windows in the home. The more windows you have and the larger they are, the bigger your furnace will need to be as windows always let in cold air. You will also need a larger furnace if your home has old single-pane windows than you would if all of the windows were double- or triple-pane.

Window location is also important since this plays a role in how much heat gain you get during the day through your windows. South-facing windows will let in the most heat while you get almost no heat gain from north- and east-facing windows. West-facing windows also contribute to more heat gain since they receive lots of light in the afternoon when the sun is hottest. Homes with lots of south- and west-facing windows will often need a slightly smaller furnace than the same size home with more east- and north-facing windows due to the amount of heat that they let in.

The amount of direct sunlight or shade that your home gets throughout the day is also important. If you have lots of tall trees surrounding your home, you will often need a larger furnace since your house will get less heat gain from the sun.

When determining what size of furnace or other HVAC unit is needed, HVAC technicians always perform something known as a Manual J calculation. This calculation figures in not only square footage and climate zone but also windows, insulation, age and condition of the ductwork, how many people live in or occupy the building, and any other factors that can affect how well your heating system works and how much heat is needed.

Issues Caused By Oversized and Undersized Furnaces

If your furnace is too small, it won’t produce enough heat at a time to effectively raise the temperature of your home. This means that your heating system will need to run for much longer times and your energy bills will be much higher. A properly sized furnace will usually perform two heating cycles an hour. Depending on how cold it is outside, each cycle will usually last for anywhere between 10 and 20 minutes.

If your furnace is undersized, each heating cycle could take an hour or more, and your heating will often only ever shut off for 10 or so minutes before the next cycle starts. This means your home will often feel cold, your heating costs will be higher, and your heating system will experience much more wear and tear. The fact that undersized furnaces will get far more use means they also typically have a much shorter life expectancy.

Oversized furnaces can also create numerous issues. For starters, you will be paying far more for the unit than you really need to since it is bigger than what you need, and your energy bills will be higher since the furnace will burn more gas at a time.

Another issue with oversized furnaces is that they frequently do something known as short cycling. This is when the furnace shuts off before it completes its heating cycle, i.e. before it has raised the indoor temperature to what your thermostat is set to.

Short cycling usually occurs because the furnace is overheating. If a furnace is oversized, it will usually produce more heat at a faster rate than your blower fan can handle. If the blower fan isn’t circulating enough air, lots of the heat will remain inside the furnace and can quickly cause it to overheat. Overheating will lead to the furnace automatically shutting down before it has had time to fully heat the building.

Oversized furnaces will typically break down much more frequently and suffer from more serious issues that require expensive repairs. If a furnace is oversized, its life expectancy is often not even half of what it would be otherwise. A properly sized furnace should last for 15 years or more, whereas an oversized furnace may need to be replaced after just five to seven years.

If you’re not sure what size or type of furnace you need, TR Miller Heating, Cooling & Plumbing is here to help. We are experts in furnace installation and will perform all of the necessary calculations to ensure you get the right size of unit for your home. We also install air conditioners, boilers, mini-splits, UV air purifiers, and air handlers, and we can assist with any heating or cooling repairs and maintenance as well. In addition, we offer a range of residential plumbing services for residents in Plainfield and the Chicago area. To schedule a furnace installation consultation or any other service, give us a call today.

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