Furnace installation, including the parts nobody quotes

What is furnace installation?

Furnace installation means removing the old unit and fitting a new one that burns gas, propane or oil - or uses electric elements - and distributes heat through your ducts. A straightforward swap takes four to eight hours. What changes the price is not the furnace: it is the venting, the gas line, the condensate drain a high-efficiency unit needs, and whether your existing ductwork can handle the airflow.

Signs you need furnace installation

• The furnace is past fifteen years old and has needed a part every winter for the last two or three
• The heat exchanger has been reported as cracked - this is a replacement conversation, not a repair one
• The burner flame is yellow or lazy rather than steady blue, or there is soot around the unit
• The system runs constantly in cold weather and still cannot reach the thermostat setting
• It short-cycles: fires, runs briefly, shuts off, repeats - often a sign it was oversized when installed
• Your gas bill has climbed year on year with no change in the weather or your thermostat habits
• The furnace is loud in a new way - booming on ignition, or a rattle that started this season
• Your carbon monoxide alarm has gone off, or a technician has red-tagged and shut the unit down

Seeing something not on this list? Browse hvac problems by symptom →

How furnace installation actually works

1. 1

   Heat loss calculation and sizing

   A Manual J heat loss calculation gives the BTU output your house actually needs at your local design temperature. Furnaces are commonly sized by rule of thumb, which is why so many homes have units that produce far more heat than they can ever use.

   Watch forSizing by matching the old furnace's nameplate. Houses get insulated, windows get replaced, additions get built - the old number often stopped being right years ago, and it may never have been right.

2. 2

   Fuel, efficiency tier and AFUE

   AFUE is the percentage of fuel energy that becomes heat in your house. Condensing furnaces sit in the 90s; older non-condensing designs sit in the 80s. The higher tier costs more up front and needs different venting - it is not simply a better version of the same box.

   Watch forA high-efficiency furnace quoted at the same install price as a standard one. Condensing units vent through PVC to a sidewall and produce acidic condensate that needs a drain. If neither appears in the scope, either the quote is incomplete or the venting plan is wrong.

3. 3

   Ductwork compatibility - the hidden cost

   The furnace has to push its rated airflow through your existing ducts. If the return side is undersized - very common in older homes with a single central return - the new unit will run at high static pressure, be noisy, trip on limit, and wear out prematurely.

   Watch forNo static pressure reading taken during the site visit. It takes minutes and it is the single best predictor of whether the new furnace will be quiet and long-lived. Duct modification found after the contract is signed is the most common change order on this job.

4. 4

   Venting and combustion air

   How exhaust leaves and how combustion air arrives are both code-governed. Replacing an old atmospheric furnace with a condensing one often orphans a water heater that shared the chimney, which then needs its own venting solution.

   Watch forThe orphaned water heater. Pulling a furnace off a shared chimney flue leaves the water heater venting into a flue now far too large for it, which causes condensation and backdrafting. A competent installer raises this before the job, not after.

5. 5

   Removal, gas line and electrical

   The old furnace comes out, the gas line is reconnected and leak-tested, and the electrical supply and condensate route are set up. On oil systems the tank and line are checked as part of this.

   Watch forNo leak test performed and shown. Gas connections are tested with a manometer or solution, not by smell.

6. 6

   Set-up, airflow and blower configuration

   Blower speed is set for the actual duct system, gas pressure is adjusted to the manufacturer's manifold specification, and temperature rise across the heat exchanger is measured and brought inside the range printed on the unit.

   Watch forTemperature rise never measured. Running outside the specified range overheats the heat exchanger and shortens its life - and the heat exchanger is the part whose failure ends the furnace.

7. 7

   Combustion testing and commissioning

   A combustion analyser is put in the flue to read carbon monoxide, oxygen and draft. This confirms the unit is burning cleanly and venting properly. It is the step that separates a furnace installation from a furnace delivery.

   Watch forNo combustion analyser on site. This is the safety-critical measurement on the whole job, and a crew without the instrument cannot have taken it. Ask for the readings on the paperwork.

8. 8

   Permit, inspection and handover

   Gas appliance replacement is permitted work almost everywhere, and the inspection covers venting and gas connection. You should be shown the filter location and size, the shutoff, and how the thermostat is configured for the unit's staging.

   Watch forAn installer who suggests going without a permit. On combustion equipment, the inspection is not paperwork - it is an independent check on venting that could otherwise put carbon monoxide in your house.

What does furnace installation cost?

A new furnace installed typically runs $3,500 to $10,000. The unit itself is the smaller part: efficiency tier, new venting for a condensing model, gas line or electrical work, and any duct modification are what decide where in that range your quote lands.

See the full furnace installation cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →