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Furnace Installation Process From Start to Finish

We talk through a typical furnace installation from start to finish, including best practices for HVAC companies installing a furnace unit.

Furnace Installation Process From Start to Finish

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Joshua Rodriguez


October 29th, 2020

Furnace installation is complicated. Don’t let anyone tell you otherwise. One of the biggest misconceptions we encounter is with those who think HVAC equipment is akin to a refrigerator or microwave: plug it in, and it’s good to go.

Each HVAC system requires specific design requirements to match it to the home it’s being installed into. This requires specialized training and tools to do the job right.

You don’t have to become a furnace installer to know the process behind it, though. That’s where we come in. This article is designed to get you up to speed on the process. It will get technical at times, but the goal is to give you a better understanding of how your furnace works and why good installation practices are so important.

It also helps to know what to expect, so that you can hold your HVAC contractor accountable. Unfortunately, many of the important steps we’re about to outline get skipped a lot in furnace installation. You deserve better.

Alright then. Ready to become an expert on furnace installation? Let’s get started.

Before the Furnace Installation

We’re going to start the day you meet with a sales representative. Why? Because a lot that matters for the installation should be happening on this day.

The Bad Way to Do Things:

  1. Take the square footage of your home and make a quick recommendation for a furnace.

The Good Way to Do Things:

  1. Do a proper load calculation of the home, taking into account things like insulation, number, size, and quality of the windows, and other factors that affect how powerful your furnace should be.
  2. Check to make sure the ductwork is properly sized to facilitate the airflow of a new system. This includes the supply from the furnace to air ducts and the return air supply that feeds back into the furnace’s blower motor.
  3. Measure the physical area that the new furnace will be installed, to make sure it can fit (not all furnaces are the same size).
  4. Check gas lines and electrical wiring to ensure that they’re set up properly for the installation. For example, if galvanized piping is there from a previous installation, it can be illegal to leave them, but most homeowners won’t know that.
  5. Depending on the furnace chosen, a new chimney flue liner or PVC drain line may need to be installed. Measurements will be taken to account for these possibilities.
  6. Depending on the air conditioner that’s already installed, it may necessitate a certain type of equipment so that the two are compatible. This is primarily because the blower motor in the furnace moves air for both heating and cooling, and needs to be calibrated for each.

All of these are important because they can all lead to huge problems if they’re not checked.

Say you have improperly sized air ducts. This happens more than you might expect, and the result will be 10-30% higher utility bills for the life of your system, and equipment that needs to work harder and will break down sooner as a result. That’s thousands of dollars in expenses, just because someone didn’t take the time to do the proper checks when they visited your home.

This information should be passed along to the installer, who will come prepared for any necessary alterations.

Many problems that homeowners experience with furnaces aren’t from inevitable wear & tear but are from improper installation practices. Starting off on the right foot is the best way to ensure that you avoid these long-term problems.

Day of the Installation

So it’s time to get out your hammer and wrench and get to work…

Ok, not really. It’s actually time to sit back and relax while a trusted HVAC partner installs your new furnace. But that doesn’t mean your involvement will be entirely passive.

On the day of the installation, several things should happen before the actual installing process begins:

  • The lead installer should discuss the details of the equipment with you, to make sure you’re getting the correct furnace and (if applicable) correct accessories. If you’re ever in doubt, ask to see the model number of the furnace unit being installed.
  • The install team should put floor coverings down where they’ll be traveling, to protect your home.
  • The area around the furnace should be “prepared” to make sure it’s safe for both the home and the installers who will be working in the area.

Safety Checks

Any install team worth its salt will have a series of safety checks that they run through both before and during installation.

Before an installation, this generally includes things like shutting off any electrical breakers that run to the furnace, shutting off gas or fuel lines, and checking the area for other hazardous materials or items that could fall during installation.

Lastly, it’s important to note that while a single person can install a furnace, it will both take longer and be less safe. It’s not that a single installer can’t go through the same safety checks, but there’s always at least a small chance of an accident. Sharp metals, heavy objects, and power tools are used throughout the process. Even well-trained installers should ideally have a partner to work with.

At Fire & Ice, we only use individual installers for simple jobs that carry far less risk. It also allows us to perform thorough installations without rushing.

Removing an old furnace

Removing Old Equipment

By now, the electrical power and gas supply (if applicable) has been turned off.

In removing the old furnace, the first things that need to go are items that are connected to the main furnace unit.

So the install team will be disconnecting venting, any electrical wiring, gas lines, ductwork, and anything else that’s tied physically to the main unit.

It’s during this stage that preparation will also occur to the area to make it ready for the new furnace unit. What does this mean? It depends, but may include the following:

  • Cleaning under the unit, which will often have dirt or dust.
  • Cleaning the entry points to the ductwork. Often, dust and dirt have accumulated here over years or even decades. While it’s not a full ductwork cleaning, it can do a lot for the health of your system.
  • The evaporator coil (for the air conditioning side) is often being maneuvered, and now is a great time to ensure that it is clean. This can be a quick but important step to keep the entire HVAC system running smoothly after installation.

Lastly, the area itself may need some alterations depending on the size and orientation of the new equipment. For example, if the dimensions of the new furnace are different from the old one, alterations to the ductwork may be necessary to ensure smooth airflow once it’s installed.

Another consideration is access. Depending on where things like the furnace’s filter are housed, the orientation of the furnace may need to be altered, or slight alterations to walls may need to be made.

None of this should be a surprise to you on the day of installation. If it is, the installing company didn’t do its job. Ideally, your sales representative prepped you days or weeks ahead of time to let you know what steps would be involved.

Installing a New Furnace

We’re blending the steps together a little bit here. The preparation of things like plenums, ductwork, gas lines, and wiring mentioned in the last section is very much part of the full installation process. However, when most people think of the installation, they’re thinking of the primary furnace unit.

Airflow is the key. If that isn’t properly managed in the installation, the entire project will be sub-par. So as the new unit comes in, adjustments to the plenum, the flue lining, electrical wiring, and other elements need to be structured in such a way that everything flows together.

You want a direct shot from into the plenum, and from there into the ductwork system. In some cases, a “transition” will need to be built that runs to the plenum, to ensure that you don’t have excessive air leakage.

It’s here that the installers will be working with electrical tools, potentially woodworking tools, and certainly sheet metal tools. They need to be able to cut, bend, or fold the metal properly to create the correct airflow. This can be a meticulous process.

Once the main unit is in place, it’s adjusted to make sure it’s plumb and level. There should also be something separating it from the floor. Often this is a rubber pad, but other solutions are just as common. The important part is that the metal of the furnace isn’t touching the ground. If it is, the metal will immediately suck the moisture out of the ground and begin to rust.

RELATED: How Much Does a Furnace Cost? A Comprehensive Breakdown

Reconnecting and Sealing

Once the main unit is in place, the process of sealing the unit and reconnecting all lines takes place.

A caulk gun is used, with specialized HVAC caulk that is designed to withstand pressures inherent in the furnace system.

Electrical and fuel lines are also reconnected at this time. Depending on the technological sophistication of the furnace, additional wiring may need to be run. This is rare but can be a consideration when upgrading to a modern system.

Even more rare is needing to install a new breaker. A licensed electrician will be needed for this. However, if this step isn’t needed, the HVAC technician can handle all electrical work. Usually, the furnace is on an isolated breaker, and the wiring remains identical or similar to the older system.

Once this is complete, it’s tempting to think the install process is “done.” We’re not quite there yet, but we’re getting close.

Commissioning HVAC Equipment - The Forgotten Step

Commissioning a furnace is a fancy way of saying that we test it. But the point isn’t just to test whether or not it turns on and runs. Thorough commissioning - the kind that will ensure that your system is running as efficiently as possible - can take up to an hour and involves dozens of calculations.

What is being calculated? Quite a bit. Static pressure, temperature rise rates at various stages of furnace power, CFM (cubic feet per minute) of airflow, and many other pressures and parameters.

We’ve written a longer article all about commissioning (linked below) so I won’t go into a ton of detail here. But I want to make two very important points about commissioning:

  1. All of the tests are to ensure the equipment is operating to manufacturer and safety codes.
  2. A lot of HVAC contractors don’t do this step at all.

Failing to make adjustments from commissioning can (and often will)  result in hot and cold spots in your home, improper airflow, noisier equipment, higher utility bills, and lowered lifespan of your furnace.

Even more surprising, it can make things worse on the air conditioning side too. The reason for this is that the blower motor (which moves the air) is tied to both systems. If it’s improperly calibrated, you’ll have inefficient airflow for cooling as well as heating.

RELATED: The Importance of Commissioning in HVAC

Cleanup and Review

Once commissioning is completed, it’s time to review everything with the homeowner. Part of this will be rehashing what was installed, but it will also include a walkthrough of the new equipment.

Among other items, the installer should show you how to replace the furnace filter, so that you’re able to do this yourself as needed. They should also walk you through any new functionality that you might have in your thermostat and heat settings.

Lastly, you should be able to hear the new unit turned on and running, and be given a chance to ask any remaining questions of the installation team.

After Your Furnace Installation

The job is completed, and you’re enjoying your new furnace and comfortable, warm home!

While the installation itself is complete, there are some steps you should take to guarantee that you’re getting the most out of your investment:

  • Based on the schedule for replacing your filter, set reminders so that the filter doesn’t become clogged (which can be very bad for the furnace).
  • Schedule a reminder to speak with your HVAC company to schedule maintenance on the unit one year after installation.
  • Even better is if they offer maintenance plans that will allow you to “set it and forget it” with maintenance for your entire HVAC system.

The difference between a well-maintained system and one that’s neglected can be 30-50% higher energy bills, and years off of the life of your equipment. These steps are necessary to avoid that pain.

The Importance of a Good Installation

If you’ve gotten anything out of this article, we hope it’s that thorough installation practices are vitally important for the health, safety, and comfort of your new furnace.

Even more important is that you choose an HVAC partner who will go out of their way to make sure they’re doing each step, and not cutting corners to save themselves time or money.

Want to know what questions to ask a company before you hire them, to make sure they’ll do a good job? Click the graphic below to download our questionnaire checklist.

HVAC Contractor Checklist






And if you’re ready for your free in-home estimate to replace your existing furnace and upgrade to a modern system, we hope you’ll consider Fire & Ice. Check our service area below to get started!

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