HVAC · Service

Furnace installation, including the parts nobody quotes

A furnace burns fuel inside your house. How it is vented, how it is commissioned, and whether your ducts can carry what it produces matter more than the badge on the cabinet — and they are where quotes quietly diverge.

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In short

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.

Triggers

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 →

The job

How furnace installation actually works

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 for: Sizing 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.
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 for: A 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.
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 for: No 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.
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 for: The 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.
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 for: No leak test performed and shown. Gas connections are tested with a manometer or solution, not by smell.
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 for: Temperature 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.
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 for: No 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.
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 for: An 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.

Money

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 →

Straight answer

When furnace installation is the wrong call

Everyone else ranking for this is paid when you say yes. Here's when you shouldn't.

The furnace is under twelve years old and the fault is an ignitor, flame sensor or inducer motor. These are ordinary wear parts on a healthy furnace. A replacement pitch on the back of one of them deserves a second opinion.
Your AC is also close to the end of its life. The evaporator coil sits on top of the furnace. Doing them separately means paying twice for the same access and losing the matched-system efficiency — price the pair together first.
You are strongly considering electrifying. If a heat pump is on your horizon, installing a new furnace now commits you for fifteen years or wastes most of that spend. Get both quoted side by side before you decide, including a dual-fuel option.
Your return ducts are undersized and duct work is not in the budget. A new furnace on a starved return will be loud, will trip its high limit, and will fail early. You will blame the furnace. Fix the return or accept that outcome knowingly.
The house is genuinely cold because it leaks, not because the furnace is weak. Air sealing and attic insulation often cost less than the size upgrade you are being sold, and they reduce the load permanently rather than paying to overcome it every winter.
You are quoted a bigger unit because the house "never feels warm enough". Oversizing produces short bursts of hot air, big temperature swings and cold spots — the exact complaint it is being sold to fix. Insist on the load calculation first.

Your options

Types of furnace installation

Gas furnace

The default where natural gas is available. Efficiency tier is the main decision.

Heat pump instead

Heats and cools from one system. Worth pricing against a furnace before you commit.

Heat pump vs furnace and AC

The comparison, side by side.

Compare hvac options side by side →

Before you sign

What to check when hiring for furnace installation

What is my measured static pressure, and will this furnace run inside its rated range?

The question that surfaces duct problems before they become a change order. A contractor who has measured it will have a number ready.
If this is a condensing furnace, where does the new vent terminate and where does the condensate drain?

Both are real work and both get left out of thin quotes. Sidewall termination has clearance rules; condensate needs a drain or a pump.
Does removing the old furnace orphan my water heater on the chimney?

A genuinely common oversight with a real safety consequence. If the answer is yes, the fix needs to be in the quote, not discovered later.
Will you perform a combustion analysis and give me the readings?

The safety measurement on a fuel-burning appliance. If a company does not own a combustion analyser, that answers a lot of other questions too.
What temperature rise will you set, and what does the unit specify?

Shows whether the installer commissions to the manufacturer's specification or just turns it on and leaves.
Is the permit included, and who schedules the inspection?

On combustion equipment the inspection is an independent check of venting. It is the last thing you should be skipping to save money.
What is the labour warranty, and what does it exclude?

Heat exchangers often carry long or lifetime parts warranties, but the labour to replace one is where the real cost sits. Read that clause before signing.

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Local pros

Furnace Installation near you

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Related work

Often done at the same time

Furnace Repair

When it is a part, not the furnace

HVAC Replacement

Doing heating and cooling together

Heat Pump Installation

The alternative worth pricing

Ductwork Installation

If the ducts cannot keep up

All hvac services →

Go deeper

Guides & terms

Guides

HVAC Sizing Guide

All hvac guides →

Terms on this page

AFUE

Full hvac glossary →

FAQ

Furnace Installation — questions people ask

Still have a question? Talk to our team →

How long does a furnace installation take?

A like-for-like replacement in an accessible basement or utility room is usually four to eight hours. Going from a standard-efficiency unit to a condensing one adds time because new PVC venting has to be routed to an exterior wall and a condensate drain established. Duct modifications, a gas line change or an awkward attic or crawlspace location can push the job into a second day.

What AFUE rating should I choose?

AFUE is the share of fuel energy that ends up as heat in the house. Condensing furnaces sit in the 90s, older designs in the 80s, and many jurisdictions now effectively require the higher tier. The payback on the top tiers depends on your heating season length and your gas price — in a long cold winter it is easy to justify, in a mild climate the extra installation work for condensing venting may outweigh the fuel saving.

Why is one furnace quote thousands more than another?

Usually not the furnace. Look for what one quote includes and the other does not: new venting, a condensate pump, gas line resizing, a new return duct or trunk modification, electrical work, permit fees, and the length of the labour warranty. Ask each contractor to itemise. A cheap quote that omits duct work is not cheaper — it just defers the bill to a change order.

What size furnace do I need?

A Manual J heat loss calculation gives you the answer, based on your insulation, air leakage, window area and local design temperature. Oversizing is the standard failure here. An oversized furnace satisfies the thermostat fast, shuts off, and repeats — producing wide temperature swings, cold rooms, more wear from constant cycling, and the very "it never feels warm" complaint that gets blamed on the unit being too small.

Do I need a permit to replace a furnace?

In almost all US jurisdictions, yes. Gas appliance replacement is permitted work, and the inspection focuses on the things you cannot check yourself: venting, combustion air, gas connection and clearances. Contractors normally pull it and include it in the price. Because the failure mode here is carbon monoxide rather than inconvenience, this is not a corner worth cutting.

Should I replace the furnace and AC at the same time?

Price it, at least. The indoor evaporator coil sits directly on the furnace, so a technician replacing one is already working in the space the other occupies — doing them together saves a second round of labour and access. It also lets you install a properly matched system rather than pairing new equipment with old. If the AC has real life left, splitting them is defensible; if it is the same age, it usually is not.