Replacing Furnace and Air Conditioner Together: The Definitive Guide

September 2, 2023

Home owners are often faced with the decision to replace their furnace and/or air conditioning system. Usually many are not faced with having to install both at the same time. But, as a leading HVAC provider, we are asked frequently,

"Should I replace my air conditioner and furnace at the same time?"

The short depends. It depends on the age of your units, your local climate conditions, your home's age, the HVAC features or technology you want, and more. A "matched" system is probably the best choice but it needs to be right for your home and budget.

If your year-round comfort matters in both your home and wallet, this may be the guide for you to explore. It's a comprehensive guide that will show you the advantages to replacing both systems at the same time. Replacing both systems — even if one is still working — can save you energy and money over the long term.

Two New Systems = One Comfortable Home

Bonus Offer: Download our Replacing Your Furnace and Air Conditioner Together Go-to Guidebook as a PDF for free. Easily save it or print for reference anytime.

Here's what you'll learn in our Definitive Guide: Replacing Furnace and Air Conditioner Together...

Make sure to click on each of the informative links in the chapters to get access to the best resource.

Chapter 1. Meet Your HVAC System Today

Chapter 2. Biggest HVAC Mistakes

Chapter 3. Repair or Replace?

Chapter 4. Energy Efficiency

Chapter 5. Old vs. New Technology

Chapter 6. Know the Pitfalls to Avoid When Purchasing and Cost

Chapter 7. Advantages of Replacing Furnace and AC Together

Did you know? On average, a modern furnace will last 15 - 20 years; a central air conditioning system will last 12 - 15 years, depending on the quality and efficiency of the equipment.

How many years have you had your systems?


Chapter 1

Meet Your HVAC System Today

(Heating, Ventilation, and Air Conditioning)

Shopping around for a new HVAC unit can be overwhelming. There are different parts unknown to most, new technology always emerging, and different brands to choose from.

These resources will give you a heads up regarding the in's and out of your HVAC system and how it all works together.

HVAC: A Brief History  Your HVAC system ensures the health and comfort of your family throughout the year. For you history buffs, this article depicts the evolution of the HVAC system and the American ingenuity.

Heating and Air Conditioning [Video] The video displays the three types of HVAC systems for your home. From Split system, Packaged Unit to Heat Pump System, know which is best suited for you and your home.

HVAC Basics From American Standard The article answers your questions and guides you through the basics so every part of the purchasing process is comfortable for you.

Understand the Basic of Your Furnace Workings

Here's a quick summary of how a gas furnace operates:

1. Fuel is pumped into a furnace from the storage tank

2. The fuel (natural gas) is mixed with air and ignited

3. The flame heats air

4. The leftover gases are sent out of the home through a chimney or flue

5.  Ducts bring air from the house and move it past the heat exchanger

6.  The house air is heated and sent back through a filter (usually) and then through the ducts to the vents in the house

7.  Heated air is "forced" by the blower through the duct work and out through vents around the home, where you feel the heat.

Understand the Basic of Your Air Conditioning

Your air conditioning system provides cooling and humidity control for your home.

Here's a quick summary of how your air conditioner works:

1. Cooling systems pull heat out of your home with refrigerant

2. Supply ducts distribute cool air from the air handler to the different rooms of your house

3. Return ducts carry warm air back to the handler to be filtered and cooled

Understand Ventilation

Ventilation helps circulate and purify the air you are breathing.

Your HVAC system plays an important role in maintaining your home’s indoor air quality.

More resources:

Everything You Need to Know About How Central AC Works

How AC Works

Understanding Your HVAC System


Chapter 2

Biggest HVAC Mistakes You Must Avoid at All Cost

We've all been through the buying cycle and at the end we either dismiss the annual maintenance or think bigger is always better. Learn the pitfalls to eliminate any costly mistakes when it comes to your heating and cooling.

Biggest HVAC Mistakes From maintenance to over-sized heating systems. Don't pay for an appliance that's too big for its britches.

Major problems related to installing an over-sized furnace too large for the size of living space? You may have a condition called "short cycling".

Know your Furnace Size Before You Start to Avoid Problems Later

How do you find the right size furnace?

Sizing a furnace accurately is really the job of a qualified HVAC contractor. To recommend the right furnace size, the professional should perform a whole house evaluation.

The U.S. Standard for HVAC equipment sizing is the Manuel J Load Calculation from the Air Conditioning Contractors of America Association (ACCA), which takes into account many qualities of your home, including:

  • Square footage of the home
  • Measuring the insulation in the floors, ceilings, crawl spaces and walls
  • What type of insulation it is
  • Number of storm windows
  • Whether or not there is a tight seal for all the windows and doors
  • The degree of shelter around the outside of the home

The ACCA Standards evaluate heating and cooling load based on a large number of factors to include climate, home insulation, window size and grade, and several other important data variables.

Any contractor who uses a rule of thumb or “professional experience” to size your heating system is, at best, only guessing.

Costly HVAC Mistakes [Video] Aside from where you set your thermostat, you might think you don’t have much control over your HVAC costs. In reality, homeowners have quite a bit of influence on the cost of heating and cooling their homes. The video displays the five costliest HVAC mistakes homeowners make.


Chapter 3

Is it Time to Repair or Replace Your System?

You've probably been faced with this decision at one time or another.

Let's take a look at what to do with an inefficient air conditioner:

What to Do With an Inefficient A/C ...

Weighing your options with complete information will help make your decision about when to invest in a new air conditioning system. Consider the age of your unit and your indoor air quality.

Let's not forget this topic is not only about your A/C but covers both A/C and Furnace.

Repair Versus Replace While there’s no hard-and-fast rule, there are some important considerations to keep in mind as you make your decision.

Lennox hits on the top reasons to consider before you make a change.

  • age
  • outdated technology
  • repair cost
  • size and capacity
  • impact on the environment
  • system performance

HVAC: Repair vs Replace Your Heating and Cooling System [VIDEO] This video describes the key deciding factors in either replacing or repairing your heating and cooling system. Again age, technology, and efficiency all play a part.

Your air conditioning might still cool down your home, but it might not be doing it efficiently. The same issue can be found with an under-performing furnace. The age of your equipment is a key indicator as to whether it is time to upgrade both. Both work together as a "system".

Also keep in mind safety, the amount of repairs and breakdowns you are experiencing, and quality. (quality of the units and quality of the installation)


Chapter 4

How to Get More Energy Efficiency From Your HVAC System

The heating and air conditioning systems being manufactured today are highly efficient and use less energy than systems built just 10 - 15 years ago.

You can easily compare the efficiency of different air conditioning systems as you research by checking the SEER rating (Seasonal Energy Efficiency Ratio). New systems have a SEER number of at least 13, the minimum government standard for cooling systems.

The higher the SEER, the less energy used and less cost to operate.

The rating for furnaces is the annual fuel utilization efficiency (AFUE), a measure of seasonal performance. In general, AFUE of 90% or higher is recommended for a new, efficient furnace system.

Boilers and heat pump systems will have a slightly different recommended number.

Stronger Manufacturers' Energy Efficiency Standards for Residential Air Conditioners Go Into Effect "Homeowners who choose to buy more energy-efficient air conditioning systems will realize significant savings in their energy bills and greatly reduce their energy use," said Secretary of Energy, Samuel W. Bodman.  

Energy Savings Calculator You can compare efficiency of different air conditioning systems as you research by checking the SEER rating. Use the calculator to see exactly how much you could save.

By upgrading to a high-efficiency system, you can cut your energy costs. According to EIA, heating and cooling account for almost half of a home's energy use, while water heating accounts for 18%, making these some of the largest energy expenses in any home.

Did you know? On average, a modern furnace will last 15 - 20 years; a central air conditioning system will last 12 - 15 years, depending on the quality and efficiency of the equipment.

Purchasing a New Air Conditioner? Consider SEER What is SEER? What SEER should you buy?

Smarter House: Heating Systems A combination of conservation efforts and a new high-efficiency heating system can often cut your pollution output and fuel bills by one-third, and in some homes by half.

Understand Your BTU and AFUE Rating

BTU (British Thermal Unit) rating is an indication of the capacity or ability of a furnace system to heat the necessary space. Most furnace manufacturers make several sizes of their models. Common sizes are 40,000-, 50,000-, 60,000-, 80,000-, and 100,000- BTU units.

British thermal unit or BTU rating is the measure of heat output. A larger capacity furnace will have a larger BTU rating. A furnace that has a higher capacity than is appropriate for your home may cause rapid-cycling. This occurs when the set temperature is reached too quickly and the furnace shuts off. This operation repeats causing excessive wear on system components.

Here is an excellent guide, What You Need to Know British Thermal Units (BTU)

Your contractor will also advise you on the AFUE rating of the recommended furnace unit. AFUE or Annual Fuel Utilization Efficiency refers to how efficiently the unit turns fuel into heat for the home.

Modern furnaces have an AFUE rating of 90% and higher, meaning these high-efficiency units are the best at putting your energy dollars to work and can also lower energy bills.


Chapter 5

Old and New Technology Not Always a Match

Actually mixing the two can decrease your system performance.

Matched HVAC Systems Complement Each Other Having a matched system with an efficient handler will allow both systems to operate properly, giving you the home comfort you are looking for.

You'll also save money. When components are "matched," they are designed to work together for maximum efficiency. By replacing both systems together, you'll save money if repairs are necessary because both systems will be under warranty.

Overall, new systems will require fewer repairs. You also may save money on installation when you purchase both systems together, because the process will be less complicated and involved.

Down the road, you'll have to pay for a second installation when you decide to replace the older unit.

More Potential Issues with an Unmatched System

The results of not replacing both parts with new, efficient equipment can lead to discomfort, frustration and unnecessary costs. Matching a new system with an old, inefficient system will potentially decrease the lifespan of the new system you just invested in.

In terms of warranties, manufacturers are not obligated to honor warranties of new systems that are combined with older systems if they break down during the warranty period.

Replacing both systems at the same time will ensure you have the protection of your warranty if you should need it.


Chapter 6

Know the Pitfalls to Avoid When Purchasing and Be Prepared About Cost

Make an educated decision based on the cost benefits. Crunch some numbers. You'll come to the right decision.

Heating & Cooling Cost Guides 2019 HVAC costs: Average heating and air conditioning prices

Make sure you understand all the details involved in this project before you proceed. Homeowners have many choices, and brands of HVAC's to choose from.

How Much Would it Cost to Replace my Furnace and AC at the Same Time?

Plumbline agrees there are factors that effect the cost of an AC and Furnace Installation Together.

The 4 factors are...

1. The efficiency level of each unit

Bottom line: "The higher the SEER and AFUE of the units, the more the installation will cost."

2. Size does matter. The size of each unit will reflect the cost.

Bottom line: "The higher the tonnage and BTU output, the more the installation will cost."

AC's are sized in tonnage (the amount of cooling the unit will provide in one hour), furnaces are sized by BTU output.

3. The features / technology of choice

Bottom line: "The more “comfort” features you choose for your furnace and/or AC, the higher the installation cost."

4. The quality HVAC professional of choice

Bottom line: "The quality of your contractor matters. They will install it correctly the first time which means less repairs, and lower energy bills.

HVAC Installation Cost 2019 - What's a Fair Price for a New Heating and Air Conditioning Installation? "The fair price of an HVAC installation will vary greatly based on the type of heating and air conditioning system installed, as well as the options needed or desired." Do your research - it will cost you less in the long-run.

Six Pitfalls to Avoid When Buying a New A/C or Furnace "A low price does not always equal the best value. A higher efficiency system might cost an extra $800 up front, but could save you $300 per year in energy costs."

What Factors Affect Cost?

  • Climate conditions
  • Ductwork
  • Home age
  • HVAC features/technology you choose
  • Size of the units

It doesn't hurt to get a few estimates. Buying a new system is a big decision.

Here's another cost calculator. Let's estimate the cost of your heating and cooling project:

HVAC Installation Costs Calculator


Chapter 7

Advantages of Replacing Your Furnace and AC Together

Reducing strain on your furnace and air conditioner equals a longer lifespan overall. One of the ways to ensure your air conditioner performs at its rated efficiency is by replacing your furnace at the same time.

Having a properly matched system installed means...

It will help increase the longevity and efficiency of the system as well as the comfort level in your home.

"By combining components that are made to work seamlessly together, you can create a more efficient ideal system for you and your family."

Service Champions agrees, and notes ...

4 Top Reasons to Replace at the Same Time:

  • If your furnace is over 15 years old
  • If you're purchasing a high-efficiency air conditioner with SEER above 16
  • If the HVAC installation is complex
  • If you don’t want constant HVAC repairs and replacements

Main advantages to replace both units at the same time:

1. Matched systems complement each other.

Having a matched system with an efficient handler will allow both systems to operate properly, giving you the home comfort you want.  

2. Mixing old and new technology can decrease system performance.

By replacing just one part of your overall system, you effectively lower the performance of both systems. When you couple a new highly-efficient system with your older system, the components don't "match," and you won't be utilizing the new technology to its full potential.


They go on to say there is one time to consider NOT to replace both units at the same time...

"There’s really only one instance when you should consider waiting to replace your furnace at the same time as your air conditioner — if the furnace is less than 10 years old."

Should I Replace My Air Conditioner and Furnace at the Same Time? (in order to install a "matched system")

According to Service Champions,

"If you choose a high-efficiency air conditioner with a SEER rating above 16 and your furnace has an old blower motor, you are using a mismatched system which will sacrifice some efficiency. Although it’s possible to mix and match the systems, it’s not recommended, especially if the furnace is over 15 years old."

Longevity - Should I Replace My Furnace and Air Conditioner at the Same Time? Less strain means less wear and tear. There are benefits that over time, really pay off. The main advantages are performance and energy efficiency.

Bottomline: If both your furnace and air conditioner are 15-20 years old, you might as well invest in replacing both at the same time to double your energy-savings.

Trane Furnace and Air Conditioner Installation [video] Easy installation of units at the same time. One installation fee equals more savings in your wallet.


If your air conditioner died once and for all, but your furnace still works, (or vice versa) the daunting question is to repair, replace or upgrade the entire HVAC system or one unit at a time.

For maximum efficiency and cost savings, replacing both at the same time is the right course of action. Now is the time to think about your heating and air conditioning system.

Use our guide to decide which path is the way to go for you.

We'd love to hear what you think about our new guide.


Have you recently replaced your system?

Let us know your experience in the comments below to help others in their decision.

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