A Comprehensive Visual Handbook for High-Efficiency Condensing Boilers

Identifying the type of furnace you own simplifies the process of troubleshooting and repairs. In residential settings, the primary categories of gas furnaces are conventional furnaces and the more modern high-efficiency condensing furnaces. The technology employed in these furnaces significantly influences their energy efficiency, which is indicated by their AFUE (Annual Fuel Utilization Efficiency) ratings. Condensing furnaces are the only type capable of attaining AFUE ratings exceeding 90 percent, with certain models achieving ratings as high as 98 percent.

What Sets a High-Efficiency Condensing Furnace Apart

Although the condensing furnace features distinct design elements and maintenance requirements, it shares certain characteristics with traditional furnaces. Similar to a conventional furnace, it draws in cool air from the home and filters it, utilizes a circulating fan to move the air, and is equipped with a gas burner that has electronic ignition, albeit condensing furnaces often incorporate a greater number of electronic controls.

Single Heat Exchanger Compared to Dual Heat Exchangers

The primary distinction between a traditional furnace and a condensing furnace lies in their management of exhaust gases produced during combustion. Both furnace types feature a primary heat exchanger, which includes a combustion chamber where heat generated by gas burners is transferred to the air that circulates before being distributed in the home. In a traditional furnace, the hot exhaust gases from the combustion chamber are expelled directly through a metal flue to the outside. Since these gases remain at a high temperature, a significant amount of heat is lost in the process.

In contrast, a condensing furnace utilizes a secondary heat exchanger to capture a significant portion of the residual heat from the combustion exhaust gases. As these gases cool down, they condense, resulting in the formation of water and carbon dioxide, which together create carbonic acid. The resulting water, known as condensate, is expelled through a drainage pipe, while the leftover flue gases are released outside through a plastic pipe. The use of plastic for the venting pipe highlights the reduced temperature of the gases by the time they are expelled.

Air Purification and Distribution

A condensing furnace typically functions similarly to a traditional furnace by circulating air throughout your home. However, high-efficiency models frequently include advanced technology.

Heat Recovery Ventilator (HRV) is a system designed to improve indoor air quality by exchanging stale indoor air with fresh outdoor air while recovering heat from the outgoing air to warm the incoming air.

Similar to a conventional furnace, a condensing furnace continuously circulates and reheats the return air from your home. While the majority of this air is recirculated, a small portion of fresh air is typically drawn in through gaps around windows, doors, and other openings. In homes that are well-sealed, an additional fresh-air intake may be beneficial, allowing outdoor air to be introduced into the furnace and helping to avoid the creation of a partial vacuum. A heat-recovery ventilator (HRV) is an optional, separate unit that functions as an air-to-air heat exchanger. It utilizes the stale, warm air from inside the home to pre-heat the incoming fresh air from outside before it reaches the furnace.

Air Purifier

The majority of condensing furnaces, along with certain traditional models, utilize advanced air filtration systems to enhance the quality of indoor air. To maintain their effectiveness, these filters must be replaced on a regular basis. There are various types of filters to choose from, tailored to meet specific requirements. For instance, if a family member has allergies, filters specifically engineered to trap common allergens can be utilized.

Electric Fan Motor

Condensing furnaces can be equipped with either of two types of blower motors: a conventional permanent split capacitor (PSC) motor, which is typically seen in standard furnaces, or a variable-speed, direct-current electronically commutating motor (ECM). The ECM is utilized in two-stage or modulating furnaces and offers greater energy efficiency compared to a standard motor.

Fan Enclosure

High-efficiency furnaces typically feature insulated blower compartments to help maintain heat, whereas traditional furnaces generally lack this insulation.

Combustion of Fuels

Condensing furnaces share similarities with traditional furnaces in terms of fuel combustion, featuring essential elements such as a gas burner, electronic ignition, and a combustion chamber. Nonetheless, they differ in their methods of air delivery for combustion and in the technology used for burner gas valves.

Gas Control Valve

Traditional furnaces typically feature a single-stage burner gas valve, meaning they operate with just one on setting. In contrast, condensing furnaces frequently incorporate at least a two-stage (dual-stage) burner gas valve, equipped with electronic controls that enable the burner flame to operate at either a high or low setting, depending on the heating demand. The most efficient systems utilize a modulating (variable-capacity) gas valve along with an electronic control system for the burner, combined with an ECM-type blower motor. This setup allows for precise adjustments to both the burner setting and blower motor speed, resulting in enhanced temperature regulation and energy efficiency.

Electronic Ignition System

Condensing furnaces consistently utilize electronic ignition systems to ensure optimal efficiency and dependability. While older conventional furnaces might rely on a standing pilot system, many of the newer models are equipped with electronic ignition.

Air Intake for the Combustion Chamber

In contrast to traditional furnaces, a condensing furnace typically features a sealed combustion chamber and utilizes direct-vent combustion air. The air needed for combustion is typically sourced from outside the home and directed to the furnace via a pipe. This design prevents the furnace from using air that has already been warmed by its operation. However, condensing furnaces can also be set up in a single-pipe, non-direct-vent arrangement, where the combustion air is sourced from the interior of the home. This setup is frequently seen when the furnace is situated in an unheated area, like a utility room or basement.

Heat Transfer Devices

A condensing furnace distinguishes itself from a traditional furnace by effectively harnessing valuable heat generated during the combustion of fuel.

Main Heat Exchanger

The main heat exchanger in a condensing furnace resembles that found in a traditional furnace. It consists of a network of steel tubes that are specially coated.

Secondary Heat Recovery Condenser

In condensing furnaces, a secondary heat exchanger consists of narrow tubes that capture exhaust gases after they pass through the primary heat exchanger. This process allows for additional heat extraction, cooling the gases to the extent that they condense into water and carbon dioxide. The water and carbon dioxide form a mildly acidic condensate known as carbonic acid, necessitating the use of stainless steel for the secondary heat exchanger to prevent corrosion.

Condensate Discharge Pipe

The condensate of carbonic acid from the secondary heat exchanger is channeled through a PVC pipe and is typically released into a floor drain.

Exhaust Ventilation System

The temperature of flue gas emissions from a condensing furnace differs significantly from that of a traditional furnace. In a condensing furnace, the flue gas is comparatively cool, allowing for the use of a plastic vent pipe for exhaust, eliminating the need for a metal chimney vent.

Airflow-Generating Fan

Condensing furnaces, similar to various traditional furnaces, utilize a fan to create a draft along with a pressure switch.

Plastic Vent for Flue Gases

The exhaust gases produced by a condensing furnace can be discharged via a PVC, ABS, or CPVC pipe due to their relatively low temperature, typically around 100°F or lower. These gases typically exit through a wall of the house, positioned at least 12 inches above the ground or above the expected snow accumulation.