Furnace Installation and Replacement: What to Know Before You Replace an Old Furnace
Furnace installation is the job of setting a new furnace in place and connecting it to your fuel supply, venting, ductwork, wiring and thermostat, then testing it so it heats safely. A good install starts weeks before install day, with the right size, the right efficiency and a clear written quote.
This article covers what most homeowners never get told: why bigger is not better, how your ducts and chimney affect the process, how to read an estimate line by line and what to check in the first week. Use it to learn what a quality, properly installed system looks like.
What furnace installation and furnace replacement cover.
People use "furnace installation" and "furnace replacement" as if they mean the same thing. They overlap. A replacement takes out an old furnace and puts a new one in its spot. An installation can also mean a home that never had a furnace, a new addition or a house switching from another kind of heating.
Either way, the furnace is only one piece. Replacing gas furnaces means connecting to five other things: the fuel supply (natural gas, propane, oil or electricity), the vent that carries exhaust outside, the supply and return ductwork, the electrical circuit and the thermostat. If any one is wrong, a brand new furnace can run badly or unsafely. Check the gas lines too.
That is why planning matters more than the brand name on the cabinet. Most problems blamed on a "bad furnace" trace back to sizing, airflow or venting choices made before the install. It is important to make sure each is handled properly.
Signs it is time to replace your old furnace.
Age plus a big repair
ENERGY STAR suggests thinking about replacement when a furnace is more than 15 years old and needs a repair [ENERGY STAR, energystar.gov]. A large repair on an aging unit often just delays the bigger bill.
Rising gas bills
If heating bills keep climbing while your habits stay the same, the furnace may be losing efficiency. Compare the same winter months year over year.
Uneven rooms and short cycles
Cold and warm rooms, or a furnace that turns on and off every few minutes, can point to a worn part, an airflow problem or a unit that was the wrong size from the start. If the furnace runs but the air feels cold, read why a furnace blows cold air.
Safety warnings
A cracked heat exchanger, a yellow flickering flame, soot near the unit or a carbon monoxide alarm are serious. If your alarm sounds, get everyone outside and call 911 or your gas company from outside the house. Learn more about carbon monoxide and gas appliances and the NFPA heating equipment safety tips.
Weighing a full system swap instead? See what HVAC replacement costs before you decide.
Why bigger furnaces are not better.
Furnaces are sized by how much heat they put out, measured in BTUs per hour. In practice, an oversized furnace heats the house in quick bursts, shuts off, cools down and starts again. That on and off pattern is called short cycling. It wears parts faster, leaves cold spots and wastes fuel.
The right way to size a furnace is a heat loss calculation. The common method in the US is called Manual J, published by the Air Conditioning Contractors of America (ACCA). The pro measures your rooms and looks at your insulation, windows, air leaks and local winter weather.
Be careful with an estimate that sizes the new unit from the label on the old unit. It may have been oversized from the start, and you may have added insulation or windows since. A tighter home usually needs a smaller furnace.
Ask each pro who gives you a quote how they sized the unit. A clear answer with the numbers written down is a good sign of a qualified, careful installer.
How to choose an energy efficient new furnace: brands, comfort and what to consider.
Many furnace brands sell similar models, so compare the available efficiency tiers, comfort features and noise level rather than the name on the cabinet. The average household sees lower heating bills as efficiency rises, and a correct BTU rating keeps every room comfortable. Plan for filter changes and yearly cleaning once the unit is in.
AFUE, in plain words
AFUE stands for annual fuel utilization efficiency. It is the share of fuel that turns into heat in your home. A 90 percent AFUE furnace turns 90 cents of every fuel dollar into heat, and the rest goes out the vent [U.S. Department of Energy, energy.gov]. The DOE lists high efficiency furnaces at 90 to 98.5 percent AFUE [U.S. Department of Energy, energy.gov]. You can compare ENERGY STAR furnace efficiency ratings before you ask for quotes.
80 percent or high efficiency?
An 80 percent furnace costs less up front and vents hot exhaust through a metal flue, often into a chimney. A high efficiency (condensing) furnace pulls extra heat out of the exhaust, so it vents through plastic pipe and makes water that needs a drain. Cold climates favor high efficiency. Mild climates may not burn enough gas to earn back the difference.
Single stage, two stage or modulating
A single stage furnace has one setting: full blast. A two stage furnace adds a low setting it uses most of the time, which means longer, quieter, more even heating. A modulating furnace adjusts in small steps to match what the house needs. Each step up costs more and adds comfort.
Blower motor and fuel type
A variable speed blower moves air gently and uses less electricity than an older single speed motor. On fuel, most homes stay with what they have. Some pair a gas furnace with a heat pump, so the heat pump handles mild days and the furnace takes over in deep cold. That setup also handles cooling.
Should the AC be replaced at the same time? If your central air conditioner is about as old as the furnace, it is worth asking. The furnace blower moves the air for both systems, and the AC coil sits on top of the furnace. Matched equipment tends to run better, and doing both in one visit saves a second round of labor. If the AC is fairly new, ask the pro to confirm the new blower fits your existing coil.
Gas furnace ductwork, venting and wiring: the parts you do not see.
Ductwork. Ducts carry warm air to your rooms and bring air back to the furnace. Many installs reuse the existing ducts, which is often fine. But a stronger blower can expose ducts that are too small, crushed or leaking. Ask the pro to measure static pressure, a quick test of how hard the blower has to push. High static pressure means the ducts are fighting the furnace.
Venting. Switching from an 80 percent furnace to a high efficiency one changes the vent. The new plastic pipes usually run out a side wall or the roof. If your old furnace shared a chimney with a gas water heater, taking it off that chimney can leave the water heater venting into a flue that is now too big for it. That can cause poor draft and condensation. A careful pro checks this and may add a chimney liner to meet building code.
The drain. High efficiency furnaces make water. It needs a floor drain, a laundry sink or a small pump. In an attic or unheated garage, the drain line has to be protected from freezing.
Gas and electrical. A different input rating may need bigger gas lines. The furnace also needs its own circuit and a shutoff switch nearby. A two stage or modulating furnace may need extra thermostat wires.
Each item belongs in the quote, as either included or not needed.
How a professional HVAC crew installs a gas furnace.
Protect the path. The crew lays floor covers from the door to the furnace and shuts off gas and power to the old unit.
Remove the old furnace. They disconnect the gas line, vent, ducts and wiring, then carry the old unit out. The AC coil on top of the furnace may need to come off and go back on.
Set and level the new unit. The new furnace goes on a level base so the water inside drains correctly.
Make the connections. The installer seals the ductwork connections, runs the venting, connects the gas line with a new shutoff valve, wires the circuit and thermostat and sets up the drain.
Leak test. Every gas joint is checked for leaks before the furnace is ever lit.
Commission it. This separates a good install from a fast one. The installer checks gas pressure, measures the temperature rise against the range on the label, measures static pressure and runs a combustion test to confirm clean burning and proper performance.
Walk you through it. Before leaving, the pro should show you the filter, the thermostat and the gas and power shutoffs, then clean up.
Ask for the commissioning readings in writing. A pro who measured them will have no trouble handing them over.
Furnace installation cost: how to compare quotes and check each price.
Two quotes for the same house can look very different, and the lower price is not always the better deal. Put them side by side and make sure each lists:
- The exact brand and model number, the AFUE rating and the heat output in BTUs
- How the unit was sized, ideally with a load calculation attached
- Venting, drain, gas line and electrical work, each marked included or not needed
- Any duct repairs or changes, plus a static pressure test
- The permit and inspection, and who arranges them
- Haul away of the old furnace
- The thermostat, if a new one is part of the job
- The start date, the expected finish and the payment schedule
For the actual cost, our HVAC replacement cost guide covers price ranges and labor. The cheapest estimate often leaves out venting or the permit, and those come back as add-ons.
Questions worth asking: How did you size this furnace? Will you test static pressure? Who pulls the permit? What experience do you have with my kind of system? Ask the pro for their license number and proof of insurance, then check the license with your state or city yourself.
Red flags: no written quote, pressure to sign the same day, a size picked from the old label and a request to pay in full before work starts. You pay the pro directly, so get the payment terms in writing.
Getting your home ready for the install.
Pick your timing
If the old furnace still runs, book in early fall or spring, when HVAC pros are less swamped than in a cold snap.
Clear a path
Move stored items away from the furnace and the route to it. The crew needs room to carry the new unit in and the old unit out.
Plan for a cold day
The heat is off during the work. On a cold day, plan a warm room for babies, older family members and pets, away from the work area.
Gather your papers
Have the quote, the permit number and any rebate forms ready. Many utilities offer rebates on efficient equipment, so check your utility's website before you choose a model.
What does a normal first week with your new furnace look like?
A burning smell on the first runs is common. New parts have a factory coating that burns off within a few heating cycles. A rotten egg smell is different and can mean gas. Leave the house and call your gas company from outside.
Watch how it runs. A furnace that clicks on and off every few minutes, bangs, squeals or leaves rooms colder than before is worth a call back to the installer.
Check the drain. On a high efficiency unit, look for water pooling around the base or the pump in the first days. A small leak is easier to fix early.
Register the furnace. Many manufacturers ask you to register a new furnace within a set time to get full parts coverage. Do it the first week, and keep the quote, invoice and permit sign off in one folder.
Test your alarms. The U.S. Consumer Product Safety Commission recommends a carbon monoxide alarm on every level of the home and outside sleeping areas [U.S. Consumer Product Safety Commission, cpsc.gov]. A new furnace is a good reminder to test them.
Maintenance and safety that keep your heating equipment running well.
Check the filter monthly. ENERGY STAR advises checking the filter every month and changing it at least every 3 months [ENERGY STAR, energystar.gov]. A clogged filter chokes airflow and makes the furnace work harder.
Book a yearly service visit. ENERGY STAR recommends a heating check up each fall before cold weather starts [ENERGY STAR, energystar.gov]. A tune up lets a pro catch a weak part before it fails on a freezing night and extends the life of the unit.
Keep the area clear. Do not store paint, gas cans or chemicals near the furnace, and keep the space around it open.
Keep vents open outside. For a high efficiency furnace, clear snow, leaves and nests away from the plastic intake and exhaust pipes on the outside of the house.
Furnace installation questions.
How long does furnace installation take?
A like for like swap of a gas furnace in the same spot is often done in one day. Plan for longer if the job changes the venting, moves the unit or reworks ductwork. Ask the pro for a start and finish date in the written quote so you can plan around it.
What is the difference between furnace installation and furnace replacement?
Replacement means taking out an old furnace and putting a new one in its place. Installation is the wider term. It also covers homes that never had a furnace, new additions and new construction. Most homeowners searching for furnace installation are replacing an old furnace.
What happens to my old furnace?
The installer disconnects it, carries it out and usually hauls it away. Most of a furnace is metal and can often be recycled. Check that haul away is in your quote.
Can my existing ductwork be reused?
Often, yes. If the ducts are the right size, sealed and in decent shape, a new furnace can connect to them. Ask the pro to measure static pressure, which shows whether air moves through the ducts easily. If the ducts are too small or leaky, fixing them first protects the new unit.
Will a gas furnace run during a power outage?
No. A modern gas furnace needs electricity for its igniter, controls and blower fan. Without power it will not heat. If outages are common where you live, ask an electrician about a generator with a proper transfer switch. Never plug a furnace into a generator with an extension cord.
What AFUE rating should I get?
In a cold climate with long winters, a high efficiency furnace of 90 percent AFUE or more usually pays off over time. In a mild climate, an 80 percent furnace can make sense because you burn less gas each year.
Do I need a permit for furnace installation?
In many areas, yes. Replacing a gas furnace often needs a mechanical permit, and an inspector checks the gas line, venting and wiring afterward. Rules vary from place to place, so ask the pro who pulls the permit and call your local building department if you are not sure.
Can I install a furnace myself?
It is not a good DIY project. The work involves gas piping, venting that carries carbon monoxide out of the house and electrical wiring, and most areas require a permit and inspection. Some manufacturers will not cover a unit a homeowner installed.
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