Most furnace replacements Done performs are completed in a single day — typically four to eight hours from the time the technician arrives to the time your new system is running and the old equipment is removed. The exact timeline depends on a few variables: the complexity of your installation, whether any ductwork modifications are needed, and whether the new equipment requires a different venting configuration. Done will give you a clear time estimate when your installation is scheduled.

What Happens on Installation Day

The process begins with the technician shutting down and disconnecting the old furnace, draining any condensate lines (on high-efficiency models), and safely capping the gas line before removal. The old unit is then removed from the space. The new furnace is positioned, gas and electrical connections are made, the flue or PVC condensate venting is run (the configuration depends on whether you are moving from a standard-efficiency to a high-efficiency unit), and the system is connected to your existing ductwork and thermostat wiring. Before leaving, the technician performs a full startup sequence: verifying ignition, checking gas pressure, measuring supply and return air temperatures, confirming all safety controls function, and cycling the system several times to confirm stable operation.

Factors That Can Extend the Timeline

Certain situations add time to an installation. Switching from a standard-efficiency furnace (which uses a metal flue) to a high-efficiency condensing furnace (which vents through PVC pipe through an exterior wall) requires routing new venting — that can add time depending on where the nearest exterior wall is relative to the mechanical room. If the ductwork needs to be modified to accommodate a different cabinet size or to correct long-standing airflow issues, that is additional work. Older Denver-area homes sometimes have tight utility room configurations or non-standard installations that require more time to adapt. Done will identify these factors during the pre-installation assessment so you are not surprised on the day.

The Pre-Installation Assessment

Before scheduling an installation, Done performs a load calculation and site assessment. This step ensures the replacement furnace is correctly sized for your home — not just matched to the BTU rating of the old unit, which may have been improperly sized from the start. At Denver’s 5,280-foot elevation, altitude de-rating affects how gas appliances perform, and proper sizing must account for that alongside your home’s square footage, insulation levels, window area, and layout. Getting the size right the first time means the system runs efficiently, maintains comfort, and does not short-cycle.

Permits and Inspections

Furnace replacements in Colorado require a mechanical permit, and the installation must be inspected by a local building department inspector. Done handles the permitting process — you do not need to pull permits yourself. The inspection is typically scheduled within a few days of installation and confirms that the gas connections, venting, and electrical work meet code. This step matters: a permitted and inspected installation is documented, which protects you if you sell the home and ensures the work meets Colorado safety standards.

  • Typical installation time: 4–8 hours, completed in a single visit
  • Venting changes (standard to high-efficiency) may add time
  • Ductwork modifications add time if needed
  • Done handles permitting — no action required from the homeowner
  • Inspection follows within days and is part of a code-compliant install

Scheduling: How Soon Can Done Get Started?

Availability varies with the season. Done books installations year-round, but peak demand in fall — when Front Range homeowners realize their furnace did not survive the off-season — means lead times can stretch. If you know your system is aging, scheduling a replacement assessment before the heating season puts you ahead of the rush. Care Club members receive priority scheduling. For more information on what a new system involves, visit our furnace installation page or explore financing options to plan the investment on your terms.