What does replacing a furnace involve?

Replacing a furnace is a one-day job for most houses, done by a licensed HVAC contractor under a permit. The contractor works out what size the house needs, pulls the permit, takes the old furnace out, connects the new one to your ductwork, gas or electric supply, venting and condensate drain, then starts it up, tests it and has it inspected. Knowing each part helps you compare quotes and spot one that skips something.

Sizing the new furnace

A proper replacement starts with the house, not the old furnace's label. ENERGY STAR says a contractor should size the heating and cooling system based on the size of your house, level of insulation, and windows,^[1] and that a good contractor will inspect the duct system for air leaks and insulation and measure airflow to make sure it meets the manufacturer's specifications.^[1] Bryant makes the same point: a reputable installer should size the equipment based on the home's heating requirements rather than simply replacing the old furnace with the same size.^[3]

Size matters because a furnace that is too big is not a bonus. Carrier warns that without a professional load calculation you risk a unit that is too large or too small, which leads to short cycling, uneven heating, high energy bills, and premature wear.^[2] If you have added insulation or replaced windows since the old furnace went in, the right size may now be smaller.

Choosing the furnace

The main choice is efficiency, shown as an AFUE rating. Carrier's gas furnaces range from 80% AFUE to as high as 98.5% AFUE, depending on the model.^[2] Going up to a high-efficiency model can change the installation as well as the price: Bryant notes that its high-efficiency condensing furnaces require model-specific venting and condensate configurations.^[3] Bryant says installation requirements can also vary significantly between standard and condensing equipment,^[3] and that ductwork modifications, venting upgrades, or gas line adjustments can increase total costs.^[3] Ask each contractor what venting and drain work the model you choose needs.

You will also choose between single-stage, two-stage and modulating models, which differ in how evenly they heat and in cost. For what each option does to the price, see how much a new furnace costs in Virginia.

The permit

Virginia's building code exempts the replacement of mechanical appliances from a permit only when the equipment is not fueled by gas or oil,^[4] so a new gas, propane or oil furnace needs one. When a furnace dies in January, the work does not have to wait: applications for emergency equipment replacement must be submitted by the end of the first working day after the day the work starts.^[4] A like-for-like electric furnace may fall under the exemption, but the exemption does not excuse the work from the code's other requirements.^[4]

Localities handle the paperwork their own way. Fairfax County lists new, replacement, repair or conversion of HVAC systems among the projects that need a residential mechanical permit,^[5] and says that once the permit is issued and work begins, the county must be notified for inspections.^[5] Spotsylvania County's mechanical permit packet, updated in August 2025, lists an HVAC or furnace replacement at $147.00,^[6] and asks for a Virginia state contractor's licence or a landownership affidavit.^[6] More detail is in do you need a permit to replace an HVAC system in Virginia.

Removing the old furnace

On the day, Carrier describes the sequence this way: the installer turns off the power and gas to the existing unit, disconnects it from the ductwork, fuel lines, electrical connections and exhaust venting, and removes and disposes of it.^[2] On a replacement, the dealer also evaluates whether ductwork, venting, electrical connections or other components need to be modified to support the new system.^[2] Those modifications are where quotes for the same furnace differ most, so ask each contractor to list them.

Setting and connecting the new one

The new furnace goes in the old one's place, or a new location if the house needs it. Bryant's description of the work is that gas, electrical, venting, condensate and duct connections are completed as required for the specific furnace.^[3] In Virginia, work for the public for pay in the electrical and heating, ventilation and air conditioning trades needs a tradesman's licence.^[8][9] Bryant says its "installation instructions specify equipment requirements, clearances, connections, and startup procedures for the applicable furnace model."^[3]

Start-up, testing and inspection

A furnace is not finished when it lights. In Bryant's account, the technician starts and tests the furnace, verifies airflow and operating temperatures, and checks safety controls and combustion-related measurements.^[3] Carrier adds that the technician confirms that the venting system is free of blockages and directs gases safely outside.^[2] Then the building department inspects the work under the permit.

Before the crew leaves, they should show you how to run the thermostat and change the filter, and register the warranty. Registration matters: Carrier's furnace warranty, for example, gives the original owner a 10-year parts limited warranty upon timely registration, and 5 years if the furnace is not registered within 90 days.^[2] Ask who registers it, and keep the paperwork.

How long it takes

Bryant says most replacements take one day, though complex installations may take longer.^[3] Carrier puts a standard installation at 4 to 8 hours and a complex one with significant ductwork or gas line changes at 10 to 15 hours.^[2] Plan for a day without heat, and ask when the inspection will be booked.

What the contract should cover

Virginia's Board for Contractors requires a contractor doing residential work, other than a subcontractor or someone on a routine maintenance or service contract, to use a legible written contract, signed by both the customer and the licensee or the licensee's agent before work starts or payment is accepted.^[7] Among its minimum terms are when work begins and the estimated completion date, the total cost, the payment schedule and down payment, the materials and work, a statement that the contractor will comply with local permit, inspection and zoning requirements, and the contractor's name, address, licence number and class.^[7] ENERGY STAR adds a written proposal specifying project costs, model numbers, job schedule and warranty information.^[1] The Board's list of prohibited acts includes "Failure to obtain a building permit or applicable inspection, where required".^[7]

When to call a licensed pro

Every part of a furnace replacement is a job for a licensed HVAC business. Carrier is plain about why: "Furnace installation involves gas lines, high-voltage electricity, and exhaust venting. Mistakes can result in gas leaks, carbon monoxide exposure, or fire."^[2] Under Virginia law, no individual may work as a tradesman without a licence, and the Code's definition of a tradesman includes anyone doing heating, ventilation and air conditioning work for the public for pay.^[8][9]

A contractor's licence class limits the size of contract it may take: Class C covers a single contract over $1,000 but less than $30,000,^[10] and Class B covers a single contract of $30,000 or more but less than $150,000.^[10] Compare the class with the total on your quote. The same section says an HVAC contractor's licence requires a master tradesman licence.^[10] Check the business on DPOR's License Lookup^[11] before you sign, or follow how to check if a contractor is licensed in Virginia.

Call for quotes when the furnace is old and failing, a technician has found a safety problem, or repairs keep coming back. If you are not sure it is time, read when it is time to replace a furnace. If you smell gas or have carbon monoxide symptoms, Columbia Gas of Virginia tells its customers to "go outside and call 911" and then call the gas company.^[12]

Getting quotes in Virginia

Loclis lists DPOR-licensed HVAC businesses with their licence number and class. Look up HVAC businesses in Blacksburg, browse furnace replacement, or read more heating and cooling answers.

Describe your furnace and house once and licensed local HVAC businesses quote the replacement.