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How to Install a Furnace

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Neal JVerified

Former HVAC Service Technician

Residential HVAC

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Types of furnaces

There are three main types of furnaces: gas, electric, and oil. Cost and size of home will dictate which you use.

Installing a new heating and cooling unit in your home can become very expensive, especially if you need to hire an HVAC contractor. However, if you are inclined to projects around the home, you can try out DIY furnace installation and save some money. This step-by-step guide will provide you with the general information you need for installation, although installation should probably proceed with the help of an experienced contractor unless you’ve done it before. Most furnaces have the same basic installation procedures, whether it is electric or gas. Once you have decided on what type fits your needs, you will need to make a supply list.

Find the load calculation to determine furnace size

Whether this is a brand new furnace installation or you are getting ready to replace an older heating or cooling system, it’s very important for you to get a load calculation done by a contractor.

This load calculation is an estimate of what size furnace and air conditioner you need for your home. This calculation includes:

  • Inspecting your home from top to bottom, including walls, floors and ceilings
  • Identifying windows, doorways and room sizes
  • Identifying drafts
  • Recording insulation
  • Meeting building codes and industry standards

If your local codes do not require you to have a load calculation done to obtain a permit, and you have not added significant space to your home through remodeling, it is best to replace your furnace with one that has the same BTU rating as your original furnace. (This assumes that the original was sized accurately and a load calculation was performed by the contractor who installed it.) If you have made changes to your home, it is best to have a load calculation done by an HVAC contractor before you purchase the furnace to ensure it will heat your home correctly.

Basic supplies needed to install a furnace

Make a supply list, whether you’re installing a new furnace or simply replacing an old heating system. Furnace replacement might only require a few items on the list, but installation will require almost all.

Basic furnace installation supplies

  • Tools: The instruction manual for furnaces usually comes with a list of tools. Generally, you will need tin snips, sheet metal, ¼” sheet metal screws, a wire stripper, wire connectors, cable ties, a cordless screwdriver with ¼” hex bit, pipe dope, and pipe wrenches.
  • Adhesives and sealants: Metal HVAC tape, electrical tape, and silicone RTV.
  • Energy efficient air filters: Make sure the air filters are the correct size and not restrictive.
  • Thermostats and controls: According to your preference. Thermostats range from simple digital models to programmable models that include smart WiFi models.
  • Ductwork: Where your furnace will be installed will determine how much ductwork and what exact supplies you will need. Make sure that you have enough ductwork to run to at least one supply and one return vent in each room, depending on the size of the room.
  • Registers, grilles and diffusers: Floor registers, grilles, ceiling diffusers, or louvered wall vents will depend on personal preference as well as how you intend on setting the ductwork.
  • Supply lines and fittings: These include pipes, metal fittings, flow control valves, protection devices and flexible connectors. Read the instructions in the furnace manual before purchasing these supplies as many of them may be included in the kit.
  • Return air box: This is a metal box that the furnace sits on that allows you to hook the ductwork to the furnace. It also has a place to install an air filter.
  • Supply air plenum: You will need to cut holes in this sheet so that the air ducts can supply heated air to the rest of the house.

It is a good idea to take the diagram that came with your furnace or heating and cooling unit to the hardware store to help you determine what exactly you will need.

A diagram of a furnace should help you install a furnace

* Not all furnaces require a return plenum as pictured above, and instead connect the return air ductwork directly to the side of the furnace, which requires the side of the blower compartment to be cut out. See your furnace manufacturer’s installation manual for details and attach accordingly.

Installing a furnace efficiently and safely

Finding proper placement

The furnace will need to sit on a pad to reduce any unwanted noise. However, it should be at least 2” inches off the floor to allow air under it and keep the bottom from rusting out. If this is a brand-new install, before placing the furnace, keep in mind the extra noise a furnace can make. Basements, closets, and attics are all good places to keep a furnace.

Connecting ductwork

Before setting your HVAC unit in the cabinet, make sure you know where the return air duct is going to be connected on the unit. If the hole is not precut to match the cabinet, then you can do this part yourself. The unit will need to drain any condensate; therefore, the furnace should have a slight slope to allow for drainage. Place the HVAC unit in the cabinet area and connect the ductwork system. The ductwork should be connected using a metal foil tape or duct sealant. Do not use duct tape as it will melt or fray and eventually disintegrate.

Venting pipes for drainage

Next, connect the vent pipes. There should be PVC pipes for an inlet and exhaust. Make sure these pipes are clean so the glue will stick to the pipes and stay connected. Ensure the pipes have a 1/4” tilt per four feet of so it will drain.

Connecting drain pipes

Using a ¾” PVC pipe or hose, connect the condensate drain from the furnace to the floor drain or drain pan.

Preparing the gas supply

To install a gas furnace, the next step is to connect the gas supply. You will need a shutoff valve and a drip leg, or T valve, right outside the furnace on the gas line. Spray a gas leak detection solution to check for gas leaks. Do not light if there are any bubbles. If you are not experienced with gas heater installations, it is recommended to get professional help as this can be dangerous.

Finalizing electrical connections

Now you can connect the electrical connections. First, make sure you know which is low voltage and which is line voltage. The line voltage should run from the furnace to the disconnect switch, located three feet from the unit. Your furnace unit is polar sensitive so ensure you install the line correctly to prevent shock or damage to the unit. The low voltage line is what runs the thermostat. It runs from the control box in the furnace to the thermostat. Make sure the red wire is connected to the R terminal on both the furnace and thermostat, the green wire to the G terminal and the white wire to the W terminal. Do not confuse the white wire with the C terminal as it will have a blue wire for the thermostat and a white wire that goes to the air conditioner. The Y terminal will have a yellow wire for the thermostat and a red wire for the air conditioner unit. If any wires differ from this, check the manufacturer’s instructions on how to install a furnace.

Checking the unit

Now, start the furnace and turn the HEAT on and let it cycle to make sure it is working. Check the outlet air temperature and the return air temperature and make sure it matches the manufacturer’s specifications. Check the drain and make sure it is not leaking onto the floor or around the furnace. If it is a gas heater, check for gas leaks that may have been missed. This is a crucial step in installing a furnace.

If you still have questions after reading this guide, or need further help with your furnace installation, feel free to ask a certified HVAC Expert. They have years of experience and will answer any questions you have.

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