A furnace is a forced-air heating system that generates heat by burning fuel — typically natural gas or propane — and then distributes that heat throughout your home by blowing warmed air through a network of ducts and vents. It’s the most common type of heating system in Denver and across the Front Range, found in the vast majority of homes built in the region over the past several decades. When you set your thermostat and feel warm air coming from your floor or ceiling registers, that’s almost certainly a furnace doing the work.

How a Furnace Works, Step by Step

Understanding the basic sequence helps homeowners recognize when something is off. Here’s what happens when your thermostat calls for heat:

  • The thermostat sends a signal to the furnace’s control board, which initiates the startup sequence.
  • On modern furnaces, an inducer motor starts first, pulling fresh air into the combustion chamber and verifying the flue is clear — a safety check before any gas flows.
  • The gas valve opens and the igniter (either a hot surface igniter or electronic spark) lights the burners.
  • Combustion happens inside the heat exchanger — a sealed metal chamber that transfers heat to the air passing over it without allowing combustion gases to mix with the air in your ducts.
  • Once the heat exchanger reaches operating temperature, the blower motor turns on and pushes air from your home’s return ducts, over the heat exchanger, and out through the supply ducts to every room.
  • When the thermostat is satisfied, the gas valve closes, the burners go out, and the blower runs a little longer to extract remaining heat from the exchanger before shutting off.

Single-Stage, Two-Stage, and Variable-Speed Furnaces

Not all furnaces operate the same way. Older and entry-level systems use a single-stage gas valve — it’s either fully on or off. Two-stage furnaces have a high and low fire setting, running at lower capacity during mild weather and ramping up only when needed, which is more efficient and more comfortable. Variable-speed or modulating furnaces go even further, adjusting output continuously between roughly 40% and 100% of capacity, similar to how a dimmer switch works compared to an on/off switch. Variable-speed systems are significantly more efficient and tend to maintain steadier temperatures with less noise.

In Denver’s climate, where you might need minimal heat on a 45°F fall evening and maximum heat during a February cold snap that drops to -10°F, a two-stage or variable-speed furnace makes a meaningful difference in both comfort and operating cost.

Furnace Efficiency: What AFUE Means

Furnace efficiency is measured in AFUE — Annual Fuel Utilization Efficiency. An 80% AFUE furnace converts 80 cents of every dollar of gas into usable heat; the remaining 20% goes up the flue. A 96% AFUE furnace wastes only 4 cents of every dollar. High-efficiency (90%+ AFUE) furnaces achieve this by condensing the water vapor in flue gases and extracting additional heat from it before exhausting a cooler, damp flue gas — which is why they use PVC venting pipes rather than metal flues. In a cold climate like Colorado’s, the fuel savings from a high-efficiency unit add up quickly over a season.

Furnace vs. Other Heating Options

A furnace is one of several ways to heat a home. Boilers heat water and distribute it through radiators or radiant floor systems. Heat pumps move heat from outdoor air into the home using refrigerant, without burning fuel at all. Ductless mini-splits are a type of heat pump that serves individual rooms or zones without ductwork. Each has advantages depending on the home’s construction, existing infrastructure, and the homeowner’s priorities.

Gas furnaces remain popular in Denver largely because natural gas has historically been an affordable fuel source in Colorado, the infrastructure is already in place in most neighborhoods, and forced-air systems integrate easily with central air conditioning — so one duct system handles both heating and cooling.

When to Call a Professional

Furnaces involve gas, high-voltage electricity, and combustion — three things that require a licensed professional for anything beyond basic filter changes. If your furnace won’t start, is producing strange smells or sounds, is short-cycling, or isn’t heating evenly, those are signs to call for service rather than investigate on your own.

Done’s heating team services and repairs all makes and models of furnaces across the Denver Front Range. Whether you need routine furnace maintenance or a furnace repair, we’re ready to help — with upfront pricing and no-surprise billing.