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Discover how your furnace works with our complete guide. Learn the heating cycle, main components, types of furnaces, and Denver-specific considerations for optimal home comfort.
How Does a Furnace Work? A Complete Guide for Denver Homeowners
Categories: Furnace Systems, Home Heating, HVAC Education
Your furnace keeps your Denver home comfortable through harsh Colorado winters, but have you ever wondered how it actually works? Understanding your furnace's operation helps you recognize problems early, communicate effectively with HVAC technicians, and make informed decisions about maintenance and replacement.
At MoJo Your Homes, we believe educated homeowners make better decisions about their home comfort systems. In this comprehensive guide, we'll walk you through exactly how a furnace works, from the moment your thermostat signals for heat to the warm air flowing through your vents.
Watch our video explanation of how furnaces work
What Is a Furnace?
A furnace is the heart of your home's heating system. It's an enclosed unit that generates heat and distributes it throughout your entire home using a forced air system. Unlike space heaters that warm individual rooms, a furnace is permanently installed as part of a central heating system designed to maintain comfortable temperatures in every room.
Furnaces are the most popular heating system in the United States, with 48% of homes using them as their primary heat source. In Denver and throughout Colorado, natural gas furnaces dominate because of the region's abundant natural gas supply, cold winters, and the efficiency of modern gas heating technology.
The Main Components of a Furnace
To understand how a furnace works, you need to know its key components. Modern furnaces are sophisticated systems with multiple parts working together to heat your home safely and efficiently:
1. Thermostat
The thermostat is your furnace's command center. Installed on your living room wall away from direct sunlight and drafts, it monitors your home's temperature and tells the furnace when to turn on and off. When the temperature drops below your set point, the thermostat sends a low voltage signal to the furnace control board to begin the heating cycle.
Modern smart thermostats offer additional features like scheduling, remote control via smartphone apps, and learning algorithms that optimize comfort and efficiency.
2. Control Board
The control board is the furnace's brain. This small circuit board receives signals from the thermostat and orchestrates the entire heating cycle, controlling when the igniter activates, when the gas valve opens, and when the blower motor turns on. In newer furnaces, integrated control boards also manage variable speed blower operation and multi stage heating.
3. Gas Valve
The gas valve controls the flow of natural gas or propane into your furnace. When the control board signals for heat, the gas valve opens to allow fuel to flow to the burners. It precisely regulates how much gas enters the system based on your furnace's capacity and your home's heating needs.
The gas valve also serves as a critical safety component. If the flame sensor doesn't detect a flame within a few seconds, the control board shuts the gas valve to prevent dangerous gas buildup.
4. Igniter
Modern furnaces use either a hot surface igniter or an electronic ignition system instead of the old standing pilot light found in older models. Hot surface igniters work like a car's cigarette lighter, glowing red hot when electricity passes through them. When the gas valve opens, the igniter lights the gas to begin the combustion process.
Electronic ignition systems use a spark, similar to a car's spark plug, to ignite the gas. Both systems are more efficient and safer than standing pilot lights, which burn gas continuously even when your furnace isn't heating.
5. Burners
The burners are where combustion occurs. Natural gas or propane mixes with air and ignites, creating controlled flames that generate heat. Modern furnaces typically have multiple burners arranged in a row, all igniting simultaneously when the system calls for heat.
The burner assembly sits below the heat exchanger and includes the flame sensor, which monitors combustion to ensure safe operation.
6. Flame Sensor
The flame sensor is a critical safety device. It's a metallic rod positioned near the burners that generates a small electrical current when exposed to flame. If the sensor doesn't detect flame within a few seconds after the gas valve opens, it signals the control board to shut off the gas valve immediately.
Dirty flame sensors are one of the most common causes of furnace problems. When they become coated with soot or corrosion, they can't detect flames properly, causing the furnace to shut down even though combustion is occurring normally.
7. Heat Exchanger
The heat exchanger is where the magic happens. It's a series of metal tubes or chambers that separate the combustion gases from the air that circulates through your home. Heat from the burning fuel warms the metal of the heat exchanger, and then air from your home passes over the outside of these hot metal surfaces, absorbing the heat.
This design is crucial for safety. It allows heat transfer while keeping dangerous combustion gases (including carbon monoxide) completely separated from your home's air supply. High efficiency furnaces use secondary heat exchangers to extract even more heat from the combustion gases before they're vented outside.
A cracked heat exchanger is a serious safety issue because it can allow combustion gases to mix with your home's air. This is why annual furnace inspections are essential.
8. Blower Motor and Fan
The blower motor drives a large fan that circulates air through your home. Once the heat exchanger reaches the proper temperature, the blower turns on and pulls cool air from your home through return ducts, pushes it across the hot heat exchanger, and forces the now warmed air through your supply ducts to heat every room.
Modern furnaces use variable speed blower motors that can adjust their speed based on heating demand. This provides better comfort, quieter operation, and improved efficiency compared to older single speed motors.
9. Draft Inducer Fan
The draft inducer fan (also called the venter motor) is a separate fan that runs before and during the combustion process. It draws air into the burner assembly to support combustion and pulls combustion gases through the heat exchanger and out the vent pipe.
This ensures proper combustion and prevents dangerous gases from backing up into your home. In older furnaces, combustion gases rose naturally through the heat exchanger. Modern furnaces use induced draft for better control and efficiency.
10. Ductwork and Vents
While not technically part of the furnace itself, the ductwork system is essential to operation. Supply ducts carry heated air from the furnace to rooms throughout your home via registers or vents. Return ducts bring cool air back to the furnace to be reheated. Properly sized and sealed ductwork is critical for efficient operation.
11. Flue Pipe / Vent
The flue pipe safely exhausts combustion gases outside your home. Standard efficiency furnaces (80% AFUE) use metal flue pipes, often connected to an existing chimney. High efficiency furnaces (90%+ AFUE) use PVC pipes because their combustion gases are cooler, having extracted more heat before venting.
12. Air Filter
The air filter protects your furnace components and improves indoor air quality by capturing dust, pollen, pet dander, and other particles. Located where return air enters the furnace, filters should be checked monthly and replaced when dirty. Clogged filters restrict airflow, reducing efficiency and potentially causing the furnace to overheat.
How Does a Furnace Work? The Complete Heating Cycle
Now that you understand the components, let's walk through the complete heating cycle step by step. This process repeats thousands of times each heating season to keep your Denver home comfortable:
Step 1: Thermostat Signals for Heat
The cycle begins when your thermostat detects that your home's temperature has dropped below your set point. The thermostat sends a low voltage signal (typically 24 volts) to the furnace control board, telling it to begin the heating cycle.
Step 2: Control Board Initiates Start Sequence
When the control board receives the signal from the thermostat, it begins a timed ignition sequence. This sequence happens in a specific order to ensure safe operation. The entire process from thermostat signal to warm air typically takes 45 to 90 seconds.
Step 3: Draft Inducer Fan Activates
First, the control board turns on the draft inducer fan. This fan runs for 30 to 60 seconds before ignition, drawing air through the burners and establishing proper draft. The system includes a pressure switch that confirms adequate airflow before allowing ignition to proceed.
This pre purge cycle clears any residual gas from previous cycles and ensures proper combustion air supply. If the pressure switch doesn't sense adequate airflow, the furnace won't proceed to ignition, preventing unsafe operation.
Step 4: Igniter Heats Up
Once the draft inducer establishes proper airflow, the control board energizes the hot surface igniter. The igniter glows bright orange or white hot, reaching temperatures of 2500°F or higher. This typically takes 15 to 30 seconds.
Step 5: Gas Valve Opens
When the igniter reaches proper temperature, the control board opens the gas valve. Gas flows to the burners and immediately ignites when it contacts the hot surface igniter. You'll hear a distinctive "whoosh" sound as the burners light.
Step 6: Flame Sensor Confirms Ignition
Within a few seconds, the flame sensor must detect flame and send a signal back to the control board. If the sensor doesn't detect flame (either because ignition failed or the sensor is dirty), the control board immediately closes the gas valve to prevent unburned gas from accumulating.
The furnace will typically attempt this ignition sequence three times. If flame isn't established after three tries, the system locks out and requires manual reset. This prevents repeated attempts that could create dangerous conditions.
Step 7: Heat Exchanger Warms Up
With the burners burning steadily, the flames heat the metal heat exchanger. Heat from combustion transfers into the metal tubes or chambers, raising their temperature to 140°F to 170°F or higher depending on furnace design.
Meanwhile, the draft inducer continues running, pulling combustion gases through the heat exchanger and out the vent pipe. These gases contain carbon monoxide, carbon dioxide, and water vapor, which is why proper venting is essential.
Step 8: Blower Motor Starts
The control board monitors the heat exchanger temperature using a limit switch. Once the heat exchanger reaches proper temperature (usually 120°F to 140°F), the control board turns on the blower motor.
This delay (typically 30 to 60 seconds) prevents the blower from circulating cool air before the heat exchanger is warm enough. You want hot air, not just moving air.
Step 9: Air Circulation Begins
The blower fan pulls cool air from your home through return ducts and into the furnace. This air passes through the filter (removing particles), flows across the hot heat exchanger (absorbing heat), and is pushed through supply ducts into your rooms via supply vents or registers.
The temperature of this supply air is typically 120°F to 140°F depending on outdoor temperature, furnace capacity, and other factors. Modern variable speed blowers adjust their speed based on heating demand, providing better comfort and quieter operation.
Step 10: Cycle Continues Until Temperature Satisfied
The furnace continues running with burners firing and blower circulating until your thermostat senses that your home has reached the set temperature. At this point, the thermostat stops sending the heating signal to the control board.
Step 11: System Shut Down
When the thermostat is satisfied, the control board closes the gas valve immediately, extinguishing the burner flames. However, the blower continues running for 60 to 90 seconds to extract remaining heat from the heat exchanger. This "blower delay" prevents wasted heat and helps cool the heat exchanger.
The draft inducer typically shuts off shortly after the burners extinguish. The entire system then sits idle until the thermostat calls for heat again.
Step 12: Cycle Repeats as Needed
As your home naturally loses heat to the cold Denver outdoors, the temperature gradually drops. When it falls below your set point, the thermostat signals for heat again, and the entire cycle repeats.
During mild weather, your furnace might cycle every 15 to 30 minutes. During extreme cold, it may run almost continuously with minimal off time.
Types of Furnaces: How They Differ
While the basic operation remains similar, different furnace types have specific characteristics that affect performance, efficiency, and operation:
By Fuel Type
Natural Gas Furnaces
Natural gas furnaces are by far the most common in Denver, powering 48% of American homes and an even higher percentage in Colorado. They burn natural gas (primarily methane) delivered through utility lines to your home.
Advantages:
Lower operating costs compared to electricity or propane
Powerful heating capacity ideal for Denver winters
Modern models achieve 95% to 98% efficiency
Reliable fuel supply through utility infrastructure
Faster heating than heat pumps or electric
Best for: Denver homeowners with natural gas access who want powerful, cost effective heating.
Propane Furnaces
Propane furnaces operate identically to natural gas models but burn propane stored in an on site tank. Common in rural areas without natural gas infrastructure.
Advantages:
Similar heating power to natural gas
Works anywhere propane delivery is available
Reliable even during utility outages (if tank has fuel)
Disadvantages:
Higher fuel costs than natural gas
Requires tank refilling and monitoring
Propane prices fluctuate seasonally
Electric Furnaces
Electric furnaces use heating elements (like giant versions of toaster elements) instead of combustion. When energized, these elements glow red hot and directly heat air passing over them.
Advantages:
Lower upfront cost ($2,500 to $4,500 installed)
No combustion means no venting required
No carbon monoxide risk
Longer lifespan (20 to 30 years vs 15 to 20 for gas)
100% of electrical energy converts to heat
Disadvantages:
Much higher operating costs in Denver where electricity is expensive
Slower heating than gas furnaces
Less effective in extreme cold
No heat during power outages
Best for: Homes without natural gas access and mild climates. Not ideal for Denver's cold winters unless electricity rates are unusually low.
Oil Furnaces
Oil furnaces burn heating oil stored in an on site tank. Once common in the Northeast, they're rare in Denver but some older homes still have them.
Advantages:
Powerful heating capacity
Works anywhere oil delivery available
Disadvantages:
Higher fuel costs than gas
Messy to maintain
Requires oil tank and regular deliveries
Less efficient than modern gas furnaces
By Operating Stages
Single Stage Furnaces
Single stage furnaces have one setting: full blast. When they turn on, they operate at 100% capacity until the thermostat is satisfied, then shut off completely. This on/off operation is less efficient and creates temperature fluctuations.
Best for: Budget conscious homeowners in mild climates with smaller homes.
Two Stage Furnaces
Two stage furnaces have two capacity settings: low (typically 65% to 70% capacity) and high (100% capacity). During moderate cold, they operate on low stage, providing more consistent heat with less cycling. On extremely cold days, they ramp up to high stage.
Advantages over single stage:
More consistent temperatures
Better humidity control
Quieter operation on low stage
10% to 15% better efficiency
Best for: Most Denver homeowners seeking balanced performance and efficiency.
Modulating Furnaces
Modulating furnaces can adjust their output in 1% increments from minimum to maximum capacity. They continuously fine tune heating output to match your home's exact needs, rarely cycling completely off.
Advantages:
Maximum comfort with minimal temperature fluctuation
Quietest operation
Best efficiency (often 95%+ AFUE)
Superior air circulation and filtration
Disadvantages:
Highest upfront cost
More complex controls and maintenance
Best for: Homeowners prioritizing ultimate comfort and efficiency regardless of cost.
By Efficiency Rating
Standard Efficiency (80% AFUE)
Standard efficiency furnaces achieve 80% AFUE (Annual Fuel Utilization Efficiency), meaning 80% of fuel energy becomes heat in your home while 20% escapes up the flue. These use metal vent pipes and often connect to existing chimneys.
High Efficiency (90% to 98% AFUE)
High efficiency furnaces (also called condensing furnaces) extract so much heat that combustion gases cool below the water vapor condensation point. They achieve 90% to 98% efficiency, wasting only 2% to 10% of fuel energy.
These models use PVC vent pipes because combustion gases are cool enough. They also produce condensate (water) that must drain properly. While they cost $1,500 to $2,500 more upfront, Denver homeowners typically recover this cost within 5 to 10 years through lower gas bills.
Denver Specific Furnace Considerations
Operating a furnace in Denver presents unique challenges that affect performance, efficiency, and longevity:
High Altitude Effects (5,280 Feet)
Denver's elevation means thinner air with less oxygen. This affects combustion appliances significantly:
Altitude Adjustments Required: Gas furnaces must have burner orifices adjusted or replaced for proper combustion at altitude. Without adjustment, flames burn inefficiently with incomplete combustion.
Reduced BTU Output: Furnaces produce 4% less heat per 1,000 feet of elevation. At 5,280 feet, a furnace rated for 100,000 BTU produces about 79,000 BTU effective output.
More Complex Venting: Thinner air affects draft and venting performance. Proper venting becomes more critical.
Installation Requirements: Denver building codes require altitude specific installation procedures and equipment adjustments.
Extreme Temperature Swings
Denver experiences dramatic temperature fluctuations, sometimes 40 to 50 degrees in a single day. Your furnace must handle:
Frequent cycling as temperatures rapidly change
High demand during sudden cold snaps
Extended runtime during sustained below zero periods
Variable load that two stage or modulating furnaces handle better than single stage
Dry Climate Challenges
Denver's low humidity (often 20% to 30% in winter) combines with forced air heating to create extremely dry indoor conditions:
Static electricity issues
Dry skin, irritated sinuses, respiratory discomfort
Damage to wood furniture and flooring
Solution: Most Denver homes benefit from whole home humidifiers integrated with the furnace, adding moisture during the heating cycle.
300+ Days of Sunshine
Intense UV exposure affects outdoor components. While furnaces are indoor appliances, AC condensers and heat pumps sharing the same air handler face accelerated degradation from Denver's intense sun.
Seven Month Heating Season
Denver typically requires heating from October through April, accumulating significant runtime hours. This extended heating season makes efficiency especially important for operating cost control.
Common Furnace Problems and What Causes Them
Understanding common furnace issues helps you recognize problems early:
Furnace Won't Turn On
Common causes:
Thermostat batteries dead or wiring issue
Circuit breaker tripped or blown fuse
Furnace power switch accidentally turned off
Dirty flame sensor preventing ignition
Pressure switch not detecting proper draft
Furnace Runs But No Heat
Common causes:
Pilot light out (older furnaces)
Faulty igniter not lighting burners
Gas supply issue or closed manual valve
Tripped rollout limit switch
Extreme clogged filter restricting airflow
Furnace Short Cycles (Turns On and Off Frequently)
Common causes:
Dirty air filter restricting airflow
Thermostat poorly located near heat source or draft
Oversized furnace for home size
Failing limit switch
Clogged condensate drain (high efficiency models)
Strange Noises
Rumbling: Can indicate delayed ignition (gas buildup before ignition), dirty burners, or issues with the burner flame pattern.
Squealing: Usually indicates blower motor bearings failing or belt slipping (on belt drive blowers).
Banging: May indicate ductwork expanding and contracting, or more seriously, a cracked heat exchanger.
Whistling: Often indicates duct leaks or gaps allowing air to escape.
Yellow or Orange Burner Flame
A properly adjusted gas furnace flame should be mostly blue with perhaps small yellow tips. A predominantly yellow or orange flame indicates incomplete combustion, potentially producing dangerous carbon monoxide. This requires immediate professional attention.
Furnace Maintenance: Keeping Your System Running Efficiently
Regular maintenance extends your furnace's lifespan, improves efficiency, and prevents breakdowns:
Monthly Tasks (DIY)
Check and replace air filter: In Denver's dusty environment, check monthly during heating season. Replace when dirty, typically every 1 to 3 months.
Visual inspection: Look for obvious issues like leaks, rust, or unusual accumulations around the furnace.
Listen for unusual sounds: Familiarize yourself with your furnace's normal operation so you notice changes.
Annual Professional Maintenance (Fall, Before Heating Season)
A comprehensive professional tune up should include:
Complete system inspection
Burner cleaning and adjustment
Flame sensor cleaning
Heat exchanger inspection for cracks
Blower motor and wheel cleaning
Belt inspection and adjustment (if applicable)
Thermostat calibration
Gas pressure testing
Combustion analysis to verify proper operation
Carbon monoxide testing
Electrical connection inspection and tightening
Condensate drain cleaning (high efficiency models)
Vent pipe inspection for proper draft
In Denver, annual furnace maintenance typically costs $150 to $250 and can prevent 90% of furnace failures. Many HVAC companies offer maintenance plans that include annual service plus discounts on repairs.
Furnace FAQs
How does a gas furnace work step by step?
A gas furnace works in this sequence: (1) Thermostat signals for heat, (2) Control board activates draft inducer fan, (3) Igniter heats up, (4) Gas valve opens and burners ignite, (5) Flame sensor confirms ignition, (6) Heat exchanger warms up, (7) Blower motor starts circulating air, (8) Warm air flows through ducts to your home, (9) Process continues until thermostat is satisfied, (10) Gas valve closes but blower runs briefly to extract remaining heat. The entire cycle from thermostat signal to warm air takes 45 to 90 seconds.
What are the main components of a furnace?
The main furnace components are: thermostat (temperature control), control board (system brain), gas valve (fuel control), igniter (starts combustion), burners (where gas burns), flame sensor (safety device), heat exchanger (transfers heat to air), blower motor and fan (air circulation), draft inducer fan (exhaust control), ductwork (air distribution), flue pipe (exhaust venting), and air filter (air cleaning and system protection). All these parts work together to safely heat your home.
How long does a furnace last in Denver?
Furnaces in Denver typically last 15 to 20 years with proper annual maintenance. Gas furnaces generally last 15 to 20 years, while electric furnaces can reach 20 to 30 years. Denver's high altitude and extended heating season (October through April) can reduce lifespan by 10% to 15% compared to lower elevations without proper altitude adjustments and maintenance. Factors affecting lifespan include installation quality, maintenance frequency, usage patterns, and whether the furnace was properly sized for your home.
What does AFUE mean for furnaces?
AFUE (Annual Fuel Utilization Efficiency) measures how much fuel your furnace converts to heat versus what escapes as exhaust. An 80% AFUE furnace converts 80% of fuel to heat while 20% escapes up the flue. Modern furnaces range from 80% (standard efficiency) to 98% (high efficiency). For Denver homeowners, upgrading from an old 60% AFUE furnace to a 95% AFUE model can save $400 to $800 annually on heating costs during our 7 month heating season.
Why does my furnace need altitude adjustment in Denver?
Denver's 5,280 foot elevation means thinner air with less oxygen, affecting gas combustion. Furnaces designed for sea level burn inefficiently at altitude without adjustment, wasting fuel and producing incomplete combustion. Proper altitude adjustment involves changing burner orifices or adjusting gas pressure to ensure optimal combustion. Without adjustment, your furnace produces 4% less heat per 1,000 feet of elevation and may create dangerous carbon monoxide. All gas furnaces installed in Denver must be altitude adjusted per building code.
How much does furnace repair cost in Denver?
Common furnace repairs in Denver range from $150 to $800 depending on the issue. Flame sensor cleaning costs $150 to $250, igniter replacement runs $250 to $450, blower motor replacement costs $400 to $800, and gas valve replacement ranges from $350 to $650. Emergency service after hours or on weekends typically adds $100 to $200. Annual maintenance ($150 to $250) prevents 90% of these repairs and extends furnace lifespan by 3 to 5 years, making it a wise investment.
Sources & References
U.S. Department of Energy - "Furnaces and Boilers: Energy Efficiency and Operation Guide", Energy.gov, 2025
Carrier HVAC - "How Does a Furnace Work? Complete Heating System Guide", 2025
Trane Residential - "How Does a Furnace Work? Step-by-Step Heating Process", 2025
American Standard - "How Does a Furnace Work? Understanding Your Heating System", November 2024
HVAC.com - "Furnace Basics: How Your Heating System Works", 2025
Understanding Your Furnace Makes You a Better Homeowner
Now that you understand how your furnace works, from the thermostat signal through combustion, heat transfer, and air circulation, you're better equipped to maintain your system, recognize problems early, and make informed decisions about repairs and replacement.
Your furnace is a sophisticated piece of equipment with multiple safety systems and precisely orchestrated operations. While the basic principle is simple (burn fuel, heat metal, blow air over hot metal), the execution involves complex components working in perfect harmony to keep your Denver home comfortable through our challenging winters.
Regular maintenance keeps all these components functioning properly, prevents expensive repairs, and ensures your furnace operates safely and efficiently for its entire 15 to 20 year lifespan.
At MoJo Your Homes, we're committed to helping Denver Metro homeowners understand their HVAC systems. When you know how your furnace works, you can partner with us to keep it running optimally year after year.
Need furnace service, repair, or have questions about your system? Contact MoJo Your Homes for expert furnace service throughout the Denver Metro area, including Wheat Ridge, Aurora, Lakewood, Arvada, and surrounding communities.
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