If you need to replace your home’s furnace, consider installing a sealed combustion unit instead of a conventional one. Sealed combustion furnaces are more commonly referred to as condensing furnaces, and they have several significant advantages over traditional heating systems. They are safer and more energy efficient, and they provide consistent heating. In this article, we’ll provide a full overview of how condensing furnaces work and explore how they are superior to conventional ones.
An Overview of Draft-Induced Furnaces
Since 1992, when the U.S. Department of Energy introduced energy efficiency standards for furnaces, the majority of residential heaters have been draft-induced models. Earlier ones, known as natural draft furnaces, relied on gravity and the natural rise of heat to move hot combustion gasses through the heat exchanger and vent them outside. In these systems, the rising exhaust created a small amount of negative pressure, drawing air into the furnace so the flames could burn hot and ensure gas combustion.
However, natural draft furnaces had a major drawback: their heat exchangers couldn’t be too restrictive, or the exhaust gasses wouldn’t vent properly. As a result, they moved quickly through the heat exchanger, limiting the amount of heat captured and causing significant energy waste. This inefficiency meant natural draft furnaces were typically only 60-70% efficient.
To meet new efficiency standards, manufacturers redesigned heat exchangers to be more restrictive, allowing the exhaust gasses to stay inside longer and transfer more heat. This redesign required the addition of a draft inducer, which creates negative air pressure to ensure proper venting and prevent fumes from escaping into the building. The draft inducer also draws more air into the combustion chamber, enabling the flames to burn hotter and further improving energy efficiency.
What Is a Sealed Combustion Furnace?
Conventional furnaces have an open combustion chamber, meaning the inducer pulls air from the surrounding indoor space into the furnace. A drawback of this design is that some heat escapes through the open chamber. In contrast, condensing furnaces feature a sealed combustion chamber, preventing any indoor air from entering. Instead, the furnace is connected to a PVC air intake pipe that extends outside the building, allowing the inducer to pull air from outside rather than from inside the home.
Another drawback of an open-combustion furnace is that it creates negative air pressure inside your home by continually pulling air from your living spaces and venting it outside. This negative pressure causes outside air to seep in through gaps in the building’s exterior. As a result, conventional furnaces tend to heat more slowly since cold outdoor air is constantly entering your home while the furnace operates. Additionally, this type of heating system reduces indoor humidity levels, making homes with conventional furnaces feel overly dry during the winter because the incoming outdoor air contains less moisture.
A sealed combustion furnace resolves both of these issues by pulling air directly from outside, preventing the creation of negative air pressure in your home. This means cold, dry air isn’t drawn into your living spaces from outside. As a result, the furnace heats your home more quickly, using less energy over time due to shorter heating cycles. Additionally, it helps maintain indoor humidity levels, preventing your home from becoming overly dry.
Another key advantage of condensing furnaces is their typically longer lifespans compared to conventional models. Because they heat more efficiently and quickly, they run for fewer hours each winter. This reduced runtime results in less wear and tear annually, potentially lowering both repair needs and extending the furnace’s overall longevity.
Secondary Heat Exchangers
One key difference between sealed combustion furnaces and conventional units is that condensing models have two heat exchangers instead of one. Most modern traditional furnaces are about 80% efficient because a single heat exchanger can only absorb a limited amount of heat from the combustion gasses before they escape through the exhaust flue. This results in roughly 20% of the heat produced by the burners being wasted.
With a condensing furnace, the still-hot gasses flow out of the primary heat exchanger and through the secondary exchanger before finally entering the flue and flowing outside. The secondary heat exchanger allows the furnace to capture much more heat from the combustion gasses so that less heat is wasted. This process is why all condensing furnaces are at least 90% efficient, and some are over 98% efficient. That means a condensing furnace will reduce your heating costs by at least 10% compared to heating your home with a conventional furnace. In most cases, your total savings will be even higher. These additional savings are related to the sealed combustion design, which eliminates cold air from being drawn into your home through imperfections in the insulation of your walls and around your doors and windows.
Sealed combustion furnaces are usually referred to as condensing furnaces because condensation forms inside the secondary heat exchanger as the furnace runs. When natural gas burns, both carbon dioxide and water vapor are produced. In a conventional furnace, the air flowing through the heat exchanger remains hot enough that the water vapor flows straight outside. Alternatively, a condensing furnace captures so much heat from the exhaust that the water vapor cools to the point where it condenses into liquid. On average, a condensing furnace can produce five to seven gallons of water per day as a byproduct. That’s why a condensing furnace has a drain pan. It captures the water, which then drains into a homeowner’s sewer system.
Installing a Sealed Combustion Furnace
Switching from a traditional furnace to a condensing furnace involves several modifications to ensure optimal performance and safety. One of the primary ones is the installation of a drainage system. The condensate produced by this type of heating system is acidic, meaning it contains corrosive compounds that can damage your home and plumbing if not adequately managed. During installation, an HVAC professional will add a condensate drain line to direct the acidic water from the system’s drain pain to a suitable drainage point, such as a floor drain.
Another crucial modification is the adjustment of furnace ventilation. The sealed combustion system needs an intake pipe to bring in fresh air for combustion and an exhaust pipe to expel the gases produced during the heating process. A fan may also be needed to vent exhaust gasses, especially in scenarios where natural drafting is insufficient.
Additionally, upgrading your thermostat can further enhance the efficiency of your new condensing furnace. A programmable or smart thermostat can optimize the heating system’s performance by adjusting temperatures based on your schedule and preferences, leading to greater energy savings.
Contact the Pros
At DeWolfe Plumbing, Heating & Cooling, we are a family-owned business that has been serving the residents of Falmouth, MA for four generations. We install, maintain, and repair heating and cooling systems. Additionally, we can help with all your plumbing, indoor air quality, and ductwork needs. Contact us today to schedule an appointment with one of our experienced team members to enhance your home’s heating system.
