Everything you need to know about furnaces: Types, pros and cons

Whether you’re a homeowner seeking efficient heating solutions or a property investor looking to optimize a real estate investment, understanding furnaces is crucial to heating your home efficiently and saving money.

Furnaces remain one of the most common types of home heating systems in the U.S., offering a range of options to keep spaces warm and comfortable. In this article, we will talk you through the different types of furnaces that exist, the advantages furnaces offer, and potential drawbacks.

What is a furnace?

Furnaces work by blowing heated air throughout the house via air registers or grills. This type of heating system is known as a ducted warm-air, or forced warm-air distribution system. They are normally powered by electricity, natural gas, or fuel oil.

Within a gas- or oil-fired furnace, the fuel is being mixed with air and burned. This heats a metal heat exchanger where the heat creates hot air. This hot air is pushed through the heat exchanger by the “air handler’s” furnace fan and then forced through the ductwork downstream of the heat exchanger. At the furnace, any combustion products are vented out of the building via a flue pipe. Older “atmospheric” furnaces normally vent directly to the atmosphere and waste about 30% of the fuel energy just to keep the exhaust hot enough to safely rise through the chimney.

However, current minimum-efficiency furnaces reduce this waste substantially by using an “inducer” fan to pull the exhaust gasses through the heat exchanger and induce draft in the chimney. “Condensing” furnaces are designed to reclaim much of this escaping heat by cooling exhaust gasses well below 140°F, so the water vapor in the exhaust gets condensed into water. This is the primary feature of a high-efficiency furnace. These typically vent through a sidewall with a plastic pipe.

Modern heating systems have controls to regulate the various components within the system. The most important control for households is the thermostat, which turns the system — or at least the distribution system — on and off to keep you comfortable. A typical forced air system will have a single thermostat. There are many other internal controls in a heating system, such as “high limit” switches, etc. that make up an invisible yet critical set of safety controls. The efficiency of a fossil-fuel furnace is measured by the amount of useful heat generated per unit of input energy (fuel). Combustion efficiency is the simplest measure of furnace performance; as it tracks the system’s efficiency while it is running. Combustion efficiency is similar to miles per gallon measured by your car when it is cruising on the highway.

In the U.S., furnace efficiency is regulated by minimum AFUE (Annual Fuel Utilization Efficiency). AFUE is an average value, which considers seasonal efficiency, averaging peak and part-load situations. The higher the AFUE value, the more efficient the furnace.

Types of furnaces

Furnaces can be classified into four general categories, based on efficiency and design, natural draft, forced-air, forced draft, and condensing. Below we’ve described these categories and how they were used historically in more detail:

Natural Draft

These have been around for many years. These furnaces usually consisted of cast-iron or riveted-steel heat exchangers built within an outer shell of brick, masonry, or steel. Air circulation depended on large, upwardly pitched pipes, and these pipes would channel the warm air into floor or wall vents within the home. This method of heating worked because warm air rises.

The system was simple, had few controls, a single automatic gas valve, and no blower (so there was little that could go wrong). Also, these furnaces could be made to work with any fuel simply by adapting the burner area. They could have operated with wood, coke, coal, trash, paper, natural gas, fuel oil as well as whale oil for a brief period at the turn of the century. However, furnaces that used solid fuels would require daily maintenance to remove ash and “clinkers” that accumulated in the bottom of the burner area.

In recent years, natural draft furnaces have been adapted with electric blowers to aid air distribution and speed moving heat into the home. Gas and oil-fired systems were usually controlled by a thermostat inside the home, while most wood and coal-fired furnaces had no electrical connection, so were controlled by the amount of fuel in the burner and the position of a damper on the burner access door.

Forced Air

This category of furnace uses an atmospheric burner style with a cast-iron or sectional steel heat exchanger. Through the 1950s and 1960s, this style of furnace started to replace the big, natural draft systems and could be installed on the existing gravity duct work. The heated air was moved by blowers which were belt-driven and designed to run at a range of speeds.

These furnaces were still big and bulky compared to more modern furnaces. They had heavy, steel exteriors with bolt-on removable panels. Their energy efficiency would range anywhere from just over 50% to upward of 65% AFUE. This style furnace also used large, masonry or brick chimneys for flues, ultimately to accommodate air-conditioning systems.

Forced Draft

The next category of furnace is the forced draft, mid-efficiency furnace with steel heat exchanger and multi-speed blower. These furnaces became physically more compact than the previous styles. They were equipped with combustion air blowers that could pull the air through heat exchangers, and this greatly improved fuel efficiency, enabling the heat exchangers to become smaller. These furnaces would have variable, multi-speed blowers.

Condensing

The modern category of furnace is the high-efficiency condensing gas furnace. High-efficiency condensing gas furnaces typically achieve between 90% and 98% AFUE. A condensing gas furnace would include a sealed combustion area, a combustion draft inducer, and a secondary heat exchanger. The primary gain in efficiency for a condensing gas furnace is the capture of additional heat from the exhaust gasses, using a secondary heat exchanger. The secondary heat exchanger also removes most of the heat energy within the exhaust gas, actually condensing water vapor and other chemicals as it operates.

The vent pipes are often installed using PVC pipe instead of metal venting pipe to prevent corrosion, but this could vary based on the geographical location of the installation and local regulations. The draft inducer allows exhaust piping to be routed vertically or horizontally as it exits the structure. A typical high-efficiency furnace would include a fresh air supply pipe that brings fresh combustion air from outside the home, into the furnace combustion unit. Normally the fresh combustion air is routed alongside the exhaust PVC during installation and the pipes exit through a sidewall of the home in the same location.

High-efficiency furnaces typically deliver a 25% to 35% fuel savings over a 60% AFUE furnace. Therefore, modern furnaces are efficient in providing consistent and rapid heating, making them a popular choice for many households, especially in colder climates.

Pros and cons

Here is a breakdown of the pros and cons to consider, for using a furnace to heat your home:

Pros

1. Efficient heating: Furnaces are efficient in providing quick and consistent warmth, especially in colder climates. They can quickly raise the indoor temperature.
2. Wide availability and adaptability: Furnaces are commonly available and can be fueled by natural gas, propane, oil, or electricity, offering homeowners various options based on their location and preferences.
3. Even heating: With the help of ducts and vents, furnaces distribute heated air evenly throughout the house, ensuring a more consistent temperature across different rooms.
4. Cost-effective: In areas where natural gas is abundant and affordable, gas furnaces can provide very cost-effective heating solutions.

Cons

1. Air quality: Furnaces can potentially reduce indoor air quality due to the recirculation of air. Without proper maintenance and filtration, dust, allergens, and other particles can be created.
2. Initial cost: Installation of a furnace can be expensive, especially if there’s no existing ductwork. The initial costs could be higher compared to other heating options, however, it is generally cheaper compared to a new boiler cost.
3. Environmental impact: Furnaces running on fossil fuels like natural gas or oil contribute to carbon emissions and environmental impact, though modern models are more energy-efficient.
4. Noise: Some furnace models can produce noise while operating, especially older units. This noise can be bothersome in certain settings, particularly in smaller homes or if the furnace is located near living spaces.

Therefore, choosing a heating system depends on many factors, including climate, budget, available fuel sources, and personal preferences. Whilst furnaces offer efficient and rapid heating, it’s essential to weigh the pros and cons to decide if it’s the right heating option for your home. Regular maintenance and proper insulation can help mitigate some of the drawbacks associated with furnace heating.

Written by Billy Guteng. February 2, 2024

Billy is a content writer for Kukun. He was born in France, and has lived in multiple cities around the world ever since. He loves art, design and architecture as well as construction and real estate.