What is a Furnace and How Does it Work?

A home furnace produces heat and distributes heat throughout a space using air registers or grills.  Furnaces use either ducted warm-air or a forced warm-air distribution system to warm the house. A furnace can be powered by electricity, fuel, or natural gas.

A furnace works by burning it’s fuel and circulating heated air throughout your home. Furnace gets its name from the Greek word “fornax”, meaning oven.

How Does a Furnace Work?

Gas and oil furnaces differ by model, but operate with similar principles: the heat coming from a furnace is generated by gas or oil burner systems, fueled by natural gas, propane, or oil. Forced air furnaces are mechanically configured to send heated air up, down or sideways. The configuration selected depends on the application. Gas and oil furnaces pass room air through a heat exchanger.

Electric furnaces operate differently, using one of two main designs; straight-heat and backup or emergency heat. The straight-heat electric furnace produces heat using thin metal strips, called heat strips, which are electrically-energized to a red-hot state. The backup or emergency heat design, commonly used in split systems, utilizes the same heat strip concept when conditions drop below heat pump-capable temperatures. Both electric furnace designs pass room air across these metal strips to heat air.

How Does Furnace Airflow Work?

Gas and oil furnaces have two completely separated airflow systems intersected but divided at the heat exchanger:

  1. A combustion air and exhaust system on one side of the heat exchanger unit
  2. The other side of the heat exchanger unit (structure environment side), where room air passes.

The first air system (for the combustion side) uses either convection or an induced-draft fan motor. For conventional convection (non-induced) exhaust, natural hot gasses rise, exiting through the exhaust. All exhaust piping or flueing must maintain an upward slope and/or straight-up design; the exhaust must always allow for heat to rise.

The induced-draft systems draw air into the burner assembly using a fan, through the heat exchanger, and out of the structure through the exhaust.

Both heated air processes saturate the heat exchanger with heat, transferring heat into the structure-side environment air.

Exhaust air presents a danger to humans and animals by carrying carbon monoxide and other toxic fumes, and must be entirely removed from the structure.

The second air system uses a furnace blower fan to draw air through the furnace return vent intake, forcing it past the heat exchanger, and through the home’s venting duct system. The room air is then drawn back through the return vent intake, completing the cycle, and the airflow cycle continues. Once the room air has warmed sufficiently to the requested temperature, the wall thermostat triggers and signals the furnace to turn off until the room temperature decreases.

This combustion air process can be broken down into four sequence steps:

  1. Pre-ignition: found in induced draft (high efficiency) systems
  2. Ignition and combustion
  3. Combustion air circulation
  4. Temperature control

If any of the below furnace system checkpoint sequences fail three consecutive times, most control modules will enter a lockout phase; preventing further attempts – requiring a technician to investigate and reset the furnace controller.

What Is the Furnace Pre-ignition Sequence?

During pre-ignition, for induced exhaust, heat is induced by fans (induced draft), and does not require an exhaust system that allows for natural heat rising.

Once the furnace is called for at the thermostat (beginning a chain of micro-relay logic sequences), the pre-ignition (found in induced draft systems) begins. First, the induced draft fan blows the combustion chamber clean and free of any possible residual fuels or explosives. This safety process can take several seconds. During this time, a pressure switch in the exhaust has to confirm no restrictions in the exhaust routing.

After the unit controller has acknowledged and approved pre-ignition, it moves on to pilot or hot surface igniter check. A burning pilot is confirmed with a thermocouple, thermopile or infrared detector. A hot surface igniter is confirmed by the controller module reading electrical resistance changed by heated air. Once confirmed, the sequence moves on to opening the gas valve to add fuel.

What Is the Furnace Ignition and Combustion Sequence?

The fuel contacts the igniter device and the burner fires up. The controller module confirms burner flame within a second or two and allows the furnace to continue on.

What Is the Furnace Combustion Air Circulation Sequence?

Once the combustion chamber reaches the correct temperature the fan switch will turn on the furnace blower motor to begin warming the structure.

What Is the Furnace Temperature Control Sequence?

Once the requested temperature is achieved in the structure, the thermostat will signal the controller to close the gas valve and reset for the next heat request sequence to begin. When employing a smart or programmable thermostat, the process remains, and is scheduled into many customized automated operation commands.

Are Furnaces in All Houses?

Most homes in the United States are heated with either furnaces or boilers. Some homes in the south of the United States have local jurisdictions requiring installation of a heated air system. These requirements are most common in residential rental properties, for health and safety reasons. Former warming methods in these structures were plug-in space heaters, and portable ductless oil and gas heaters.

Where Is the Furnace in My House?

Furnaces are most commonly found in the center of your home, in a closet-like section of the house, located in the basement or basement crawl space, at ground-level, or in the attic.

What Is a Furnace Filter?

A furnace filter blocks impurities in the air from entering the furnace. The filter is a consumable item requiring replacement in regular intervals. This keeps the internal components, such as the heat exchanger and various temperature sensing devices clean; prolonging the furnace lifespan.

What Furnace Do I Have?

Your furnace is going to be electric, gas, or oil-operated. Look for electrical conduits or gas piping. Additional system details will be found on any machine tag attached to the cabinet of the unit. The machine tag on your unit will typically state the system’s BTU’s, SEER or SEER2 rating. Without an identification or data tag, services may be requested from a professional technician for identifying other specifying attributes. This visit would be a good time to discuss replacement costs and remaining lifespan of your unit.

What Furnace Do I Need?

When choosing a furnace, an HVAC company can perform an energy study on your home to first acquire the needed BTU’s. Once the necessary furnace size is known, system design needs to be selected from forced or central air, or boiler system. Finally, furnace efficiency will need to be addressed.

Furnace Efficiency and AFUE Rating

The AFUE rating is an important factor to consider when shopping for a new heating system or upgrading your existing one. A higher AFUE rating means that the furnace is more efficient at converting fuel into heat, which can translate into lower energy bills and a smaller carbon footprint.

When shopping for a new furnace, it’s important to look for products with a high AFUE rating. Most furnaces have an AFUE rating of around 80-95%, with higher-efficiency models on the market reaching ratings of up to 98%. However, it’s worth noting that the initial cost of a high-efficiency furnace may be higher than a standard efficiency model.

In addition to the AFUE rating, there are other factors that can impact the efficiency of a furnace. These include the type of fuel it uses, the size of the furnace, and the age and condition of the unit. It’s important to consider these factors when evaluating the overall efficiency of a furnace, as well as any potential rebates or incentives that may be available for energy-efficient products.

In conclusion, the AFUE rating is a measure of the efficiency of a furnace in terms of how well it converts fuel into heat. When shopping for a new furnace, it’s important to look for products with a high AFUE rating and consider other factors that can impact efficiency. Investing in a more efficient furnace can save you money on energy costs and reduce your carbon footprint.

What Is a Furnace’s Lifespan?

15 to 20 years is commonly considered to be the lifespan of a well-maintained furnace. As a furnace gets older, part locating and component replacement becomes more challenging for maintenance; often before the furnace itself reaches end-of-life.

How Much Does a Furnace Cost?

Once an energy study for your home has been performed and the furnace size has been determined, the cost of a new or replacement furnace will vary depending on the manufacturer and design. Furnace system pricing varies between economy, mid-range, and high-end furnace models, as well as region. These decisions can be made easier by watching online discount and auction sites. Once a furnace has been selected, and pricing determined, an accurate estimation can be made by doubling the system cost to cover labor and installation.

What Is the Difference Between a Furnace and Boiler?

Both a furnace and boiler have the same fuel-burning systems, with the biggest differences being in safety control.

A furnace dispenses warm air from a central location through ductwork to targeted spaces within a structure. A boiler dispenses hot water from a central location through pipes to radiators which release heat directly into target spaces through convection. A furnace heats air, while a boiler heats water.

What Is the Difference Between a Furnace and a Heat Pump?

Both a furnace and heat pump heat air within a targeted space by passing heat through a heat exchanger. A furnace uses fuel and flame to warm a heat exchanger that transfers heat into passing air. A heat pump uses refrigerant to collect heat, which warms the heat exchanger, transferring heat into passing air, however a heat pump itself does not generate heat.

What Is the Difference Between a Furnace and an Air Conditioner?

A furnace and an air conditioner are climate controllers within a space, however, a furnace heats air for a space, while an conditioner cools air for a space.

What Is the Difference Between a Furnace and Central Heating?

A furnace is the heart of a central heating and air system. However, a furnace alone does not have central heater’s ductwork, air registers, or distribution system. Central heating, which a furnace is just one of many unique systems grouped under the “heating system” umbrella with, is itself a large concept that describes many physical appliances and components that work together in a network of heating, ventilation, ducting and air conditioning systems.

What Is Furnace Servicing?

System checkups should be performed on a bi-annular per-season basis. Changing a filter in regular intervals, more frequently (every 90 days) is most important. Furnace maintenance should be addressed upon discovering any unusual noises or smells.

What Is a Furnace Tune-Up?

Preventative maintenance should be performed annually, including:

  • Fan blade cleaning
  • Motor-bearing oiling
  • Burner flame adjustment
  • Thermocouple or thermopile cleaning
  • Pilot flame or pilot light inspection
  • Electrical connections inspection
  • Overall visual inspection (inside and out)
  • Duct cleaning may occur every 5 to 10 years

These furnace tune-up maintenance steps will keep your furnace running efficiently and safely all season.

Replacing a Furnace: Upgrading to an Energy-Efficient Heat Source for your HVAC System

The furnace is an integral part of your home’s HVAC system, providing heat during the colder months. When it’s time to replace your furnace, there are several factors to consider to ensure that you choose a product that will provide efficient and reliable heat for your home.

  1. Cost: The cost of a new furnace can vary depending on the size and efficiency of the unit, as well as the complexity of the installation process. It’s a good idea to research different products and get multiple estimates to determine the best option for your budget.
  2. Energy efficiency: A furnace with a higher AFUE rating indicates higher efficiency and a higher percentage of the furnace’s fuel being converted into usable heat.
  3. Size of the unit: The size of the furnace you need will depend on the size of your home and the local climate. It’s important to choose a furnace that is appropriately sized for your home in order to ensure efficient operation and avoid issues with under- or over-heating.
  4. Installation: Proper installation is essential for the efficient operation of your furnace. Work with a reputable HVAC professional who can properly size the unit for your home and install it according to industry standards.

By considering these factors, you can choose a furnace that will effectively and efficiently heat your home and provide reliable comfort for years to come.

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