Getting into buying a furnace can seem overwhelming. All the different acronyms and abbreviations can be confusing.
This article will cover the most common specifications for forced-air natural gas furnaces. When you are ready to hire an HVAC company (that’s Heating, Ventilation, and Air Conditioning), you won’t feel lost in the details.
First, A Little Bit About BTUs
Furnace heat output is listed in BTU (British Thermal Units) though you might also see kilowatts (kW) or megajoules (MJ). Any of these units refers to the heat energy output or input.
It is important to keep in mind that a higher BTU number is not better. he best furnace is the right-sized furnace.
One BTU is the amount of heat energy needed to raise the temperature of 1 pound of water by 1 degree Fahrenheit.
The higher the BTU capacity, the more powerful the furnace.
Measuring the Furnace's Efficiency
There are two values you will find, whether you are checking online or talking to an HVAC salesperson. Both are percent values, and in each case a higher number means a more efficient furnace. The percent value compares how much heat ends up in the home versus how much heat energy the furnace produced. These are the steady-state efficiency rating and the AFUE.
The steady-state efficiency rating is a measure of the furnace's maximum efficiency. It is measured when the unit is running at peak operating temperatures. This is the absolute best performance that the furnace can get. Much like you let your vehicle warm up before driving away in the winter, your furnace also runs best when warm.
But your furnace doesn't always run at peak operating temperature. So the more useful rating to compare will be the AFUE, or Annual Fuel Efficiency Rating. This rating takes into account inefficiencies that result in heat loss. It considers things like:
-heat lost out the chimney with the exhaust
-incomplete fuel ignition
-operating at below peak temperature (as during spring and fall)
-inefficient operation while cycling on and off
The various systems that make up your furnace are what affect the efficiency rating. Keep reading to learn about the big ones.
Sticker Labels in Canada
Furnaces in Canada can have 2 labels that refer to their energy efficiency: the Energy Star label, and the EnerGuide label.
EnergyStar is awarded by an independent third-party organization. They test the furnace to check if it meets strict high-efficiency standards. An EnergyStar label means that furnace model is very energy efficient, at least 6% better than standard models.
You might also see the EnerGuide label on furnaces. This is a voluntary label that shows the model's energy consumption. It will list the AFUE rating along with a visual scale that indicates how it compares to similar models.
Output Variance: How Hard is the Furnace Working
You will see a few different descriptors that all refer to the variation in levels of heat output:
-stages of heating
-levels of output
Whatever the name, there are 3 options for this category: single-stage, two-stage, or variable (also called modulating). This refers to how much gas flows into the furnace, which controls how much heat is produced.
A single-stage model has only on or off, and so only provides one temperature of air for the home. This is the least efficient output. It will also result in the biggest temperature, swinging above and below what is set on the thermostat.
A two-stage model has three settings: on, medium, and high. This is a more efficient system that reduces temperature swings and is much more efficient.
A variable furnace model has a sliding scale to control the output. It can adjust to many levels between nothing and full blast. Some furnaces can adjust between 1% of output. Precisely controlling how much heat flows out gives the most stable temperature. Any variation from the thermostat's set temperature will be unnoticeable. This is also the most efficient system, used on the highest-performing furnaces. As mentioned above, on-and-off cycling reduces the furnaces’ efficiency. Having the furnace continuously running is the most efficient and least wearing condition. A modulating furnace will keep running without overheating your home.
Heat Exchanger Technology Options: Managing the Exhaust
A very efficient, high-performing furnace will be a condensing furnace. This type of furnace has 2 heat exchangers in a series. The first heat exchanger captures heat from the initial burn. The second heat exchanger captures even more heat from the exhaust. This second heat exchanger pulls so much heat from the exhaust that the exhaust pipe can be PVC instead of metal. This drastic cooling will result in condensation as well--hence the name. There will also be a condensate drain that connects into your wastewater system. The efficiency values on condensing furnaces will range from 90%-98.7% AFUE.
An induced draft furnace is a step down in efficiency from a condensing furnace. This type of furnace has only 1 heat exchanger and uses a fan to push the exhaust out of the home. The exhaust vents to the roof through a traditional metal chimney. Because so much heat is lost to the outside, these furnaces are usually in the range of 80% efficiency.
A third furnace type is a natural draft furnace. This type also has only 1 heat exchanger, but no fan to push the exhaust out of the house. The exhaust rises out of the building because it is so warm. This results in a risk of back drafting and possible exhaust fumes in the house. The efficiency rating for this type of furnace is usually 77% or lower.
Ignition Method: Get the fuel Burning
Newer model furnaces also have newer technologies to ignite the natural gas inside. A natural draft system typically uses a pilot light: a constantly burning flame. Though small, the fuel the pilot light uses over the course of an entire year is significant. If your furnace uses a pilot light, we recommend turning off the pilot gas supply in the summer and re-lighting in the fall.
High-efficiency furnaces will use either a direct spark or hot surface ignition system. Direct spark ignition is like what a car uses. A high voltage charge arcs through a fuel-filled chamber to ignite the gas. Hot surface ignition uses a semi-conductive element, sometimes called a glow bar, that ignites the gas. Either of these is more efficient than a pilot light.
Any of these ignition types need electricity supplied to the furnace. The pilot light system (though it doesn't use electricity to ignite the gas) requires an electric-powered fan to send heat through the house. Direct spark or hot surface ignition systems need electricity to work, and also need an electric fan to send heat through the house.
The fan system can add a lot of electricity cost. Because it controls air distribution, fan efficiency will also impact the comfort level. High-efficiency furnace fans are so efficient that the added electric expense is usually minimal.
Just as fuel usage can be variable or simply on/off, the fan can also operate with variable speed or simply on/off. It's no surprise to you by now that a newer model furnace will also have a more efficient fan.
The fan, or blower motor, has a fan efficiency rating, or FER. It measures how many watts of electricity are needed to move a volume of air in a given amount of time. This could be either cubic feet per minute or liters per second: W/cfm or W/ L/s. The lower this number is, the more efficient the fan system.
The most recently manufactured furnaces might have a brushless permanent magnet motor fan. This type of motor is extremely efficient. This type of motor passes the required FER level without testing to determine the actual value.
Getting the Size Right
A furnace is definitely not one-size-fits-all. An important step in purchasing a new furnace is having your HVAC contractor measure your living space to determine what size of furnace you need. A furnace that is too large will blast a lot of heat quickly, then shut off. An undersized furnace will run constantly and still might not be able to keep up. Either way, you will notice hot and cold spots throughout your house.
The most efficient furnace is one that runs as long as possible. This avoids the loss from on/off cycling and reduces the wear on the furnace. Think of it like a diesel engine that runs best under a load and takes wear from shutting off and starting up again. Cycling on and off adds more mechanical and thermal stress to the system. If your furnace is properly sized, it will provide the right amount of heat over a longer time frame.
A forced-air natural gas furnace heats the air in your home. Yet air is a very poor way to distribute heat. The temperature of the surfaces in a space have a much greater impact on the comfort of that space.
Think about opening the door in the winter when it’s -30°C outside. That blast of cold air you feel is a large amount of hot air escaping, replaced with icy cold air. But, when the door is shut, your interior space is still a comfortable temperature. This is because the surfaces–furniture, floor, walls, etc–are still heated and are radiating heat back into the air. Air is such a poor heat reservoir that it returns to a comfortable temperature quickly.
Air sealing is especially important when air is the heating medium. When air can freely leave the house, it affects how much heat is transferred to the surfaces and occupants. The air might escape before you can enjoy its warmth.
If you are working towards a climate-resilient home, air sealing is an important step toward high-efficiency heating.
You Are All Set!
Want to know more about how all these parts work together? Learn how a natural gas, forced-air furnace works.
Searching for the perfect HVAC company? We have an article that will give you some guidance there, too.
Now that you understand the basics, you are ready to start looking for the right furnace model for your home. If you have any questions or want to speak to one of our Energy Advisors for advice, contact us.