A furnace that is too small for the house in which it is installed will put out too little heat to keep the home warm. So, the furnace will stay on for far longer than it is designed to do in an attempt to heat the home properly. This puts just as much stress on the system as if it were oversized, with the same result.
Too Small. Having a furnace that is too small for your home means the system won't be able to keep up with the dropping temperatures outside. They simply can't make up the difference to heat your home properly.
An undersized HVAC system can lead to various problems, including short cycling, inability to reach the desired temperature, high energy bills, and increased humidity levels.
The most common (and in many cases only) sign that your furnace is undersized is that the device simply doesn't maintain the temperature in your home properly. This means that when turned on to full and left for a few hours, your furnace doesn't heat your home to the thermostat setting.
it is best to have the right size furnace as oversize will cause too many on-off cycles. too small causes extended periods of running and the possibility that it won't handle extreme cold.
A basic rule of thumb for furnace size calculation is that it takes 30 BTUs for every square foot of house. So, if you have a 1,000 square foot house, you need a furnace that has a 30,000 BTU output.
An oversized furnace causes excess wear and tear, higher energy bills, and more problems (and expenses!) that make it tough to stay warm in the winter. This article goes over how to tell if the furnace installed in your home is too big.
A 1,500-square-foot home will require between 45,000 to 90,000 BTUs. A 1,800-square-foot home will require between 55,000 to 110,000 BTUs. A 2,100-square-foot home will require between 65,000 to 125,000 BTUs. A 2,400-square-foot home will require between 75,000 to 145,000 BTUs.
For example, in zone 3 to 4, which typically requires 40-45 BTU per square feet, you can determine that a 2,500 square foot home would need a 100,000 to 112,000 BTU furnace.
This can lead to a wide range of installation costs. The national average cost for installing a gas furnace in a 2,000 sq. ft. home is $4,000 to $8,000, with most people paying around $6,000 for a high-efficiency gas furnace installed with minor modifications to update the vent and drain.
An undersized furnace is incapable of keeping up with the heating demands of a big house on the coldest days. The result is that your furnace won't shut off automatically. It will instead run constantly, which means your heating bills will skyrocket and your furnace will break down often as well.
As stated by the National Fire Protection Association (NFPA), heating systems like furnaces are a major cause of home fires, contributing to almost 50,000 blazes, 500 civilian deaths and more than $1 billion in significant property damage every year.
Labor: Typical labor costs can range from $50 - $100 per hour for a licensed technician. Each additional team member will be an additional charge. Depending upon the complexity of the job, a new furnace installation can take 10-15 hours!
For example, the Lennox SLP98V Variable-Capacity Gas Furnace is at least 50 percent quieter than competitor's furnaces, making the SLP98V gas furnace the quietest furnace you can buy1. SilentComfort™ is achieved by utilizing variable-speed technology. In other words, this furnace has a high and low speed.
However, most gas furnaces typically last between 15 to 20 years before efficiency declines and repair costs increase, so it's wise to consider replacement around the 15-year mark to ensure optimal performance and safety.
When sizing your furnace, you need to count all the square footage of rooms you want to heat. If you want to heat your basement, count it when sizing your furnace.
Your home will be uncomfortable. When your furnace is too big, it will blast your home with too much conditioned air at one time. This can make your rooms feel too warm when your furnace is operating and lead to major temperature swings in your home.
A general rule of thumb is that you'll need about 30 to 60 BTUs per square foot of your home, depending on your climate zone. For example, if you have a 2,000-square-foot home, you might need a furnace rated between 60,000 and 120,000 BTUs. BTUs, or British Thermal Units, measure a furnace's heating capacity.
A furnace that is too small for the house in which it is installed will put out too little heat to keep the home warm. So, the furnace will stay on for far longer than it is designed to do in an attempt to heat the home properly. This puts just as much stress on the system as if it were oversized, with the same result.
Neither oversizing nor undersizing your AC unit is ideal. Both scenarios can lead to issues with efficiency, comfort, and longevity of the system.
Older furnaces, especially water boilers, used to take up extra room in the basement. As furnaces have become more efficient, they have also become smaller. Because it can heat water faster, the boiler tank does not need to have a large capacity.