HVAC For Beginners

Furnace Installation

A Guide To Installing A Forced Air Furnace

With this furnace installation guide, HvacRepairGuy will show you the steps to replace an existing gas furnace with a new high efficiency gas furnace. The typical HVAC company will charge hundreds of dollars for the installation as well as charging a higher price for the furnace itself but, with this guide, you can save that money.


Everyone should begin with finding the proper size of unit and this can only be done with a load calculation. Since you are replacing an existing furnace without replacing the duct system, a simple block load calculation is sufficient. HvacRepairGuy will do a free calculation if you submit this form.

Replacing a gas furnace

We will look at the basic procedure for gas furnace replacement but these principles can be easily adapted for the other types of units. The first step is to shut off the power to the furnace at the breaker box or fuse panel and shut off the gas to the furnace at the shutoff valve that is usually next to the furnace. After you have verified that the power is off, you can disconnect the main power wires to the furnace as well as the thermostat wires. If you are not familiar with the thermostat terminal designations, then, it is a good idea to write down the terminal letter and the color of the thermostat wire that is attached to it. The gas connection to the furnace can be disconnected usually at a union fitting and the vent pipes can be disconnected as they are usually screwed to the furnace vent connection for metal vent. If you are replacing a high efficiency furnace, then the PVC vent pipes can be cut at a convenient place with a reciprocating saw. Next, you want to disconnect the return and supply ducts and, if you have a central ac unit, the coil that is above the furnace will need to be supported. Once the metal ducts are disconnected, the old furnace can be moved out of the way.

The next step of the furnace installation is to prepare the unit for installation. This includes removing all the loose parts and instructions from the unit. Then, you need to decide which side of the unit the return air duct will connect to or whether it will be attached to the bottom of the unit. The cabinets on most units are marked with the appropriate size opening, it is a simple matter of cutting the required opening in the appropriate side or in the bottom using tin snips. Next, you can put the unit in the desired place. The unit should be on rubber isolation pads to minimize noise during operation. If the unit is to be located in a basement, it should be supported by blocks at least four inches above the floor with the rubber pads between the blocks and the furnace. If the unit is to be located in the attic, some special precautions should be taken. A secondary drain pan should be placed under the unit and the air conditioner evaporator coil. This is a plastic pan that is placed under the unit to prevent damage to the home in case of blockage of the air conditioner primary condensate drain. Special attention should be given as to the location in the attic for maintenance and to minimize noise to bedrooms. **NOTE** A condensing furnace should not be installed in an attic in climates that are subject to temperatures below freezing. You need to decide which side of the unit the condensate drain will exit. When you set the unit in place, it should be close to level but sloped slightly towards the condensate drain exit. Once you have the unit in place, you can continue your furnace installation by connecting the duct system. The connections to the unit should be sealed with a metal foil tape or a duct sealant.


Now, you can connect the vent pipes. On a high efficiency furnace, there should be an inlet and exhaust made of pvc. The key thing to these is to properly clean and glue the pvc joints. The pipes should be sloped back to the furnace at 1/4 inch per 4 feet. This is to allow proper condensate drainage. The next step in furnace installation is to connect the gas supply to the unit. There should be a shutoff valve installed outside the unit. A drip leg should be installed prior to the line entering the unit. This is usually accomplished by using a tee with a capped nipple sticking out the bottom. The gas supply comes in the top and the center of the tee goes to the furnace. After installing the gas piping, ensure there are no gas leaks. This can be accomplished by spraying a gas leak detector solution on the connections. Watch for bubbles to form which is an indication of a leak. Now it is time to make the electrical connections. Your new furnace has low voltage (24v) and line voltage (120v) connections. The line voltage connections should run from the furnace junction box to a disconnect switch within three feet of the unit. It is important that you pay attention to the polarity because most of the new units are polarity sensitive. The low voltage wires run from the furnace control board to the air conditioner and/or the thermostat. The standard connections are as follows: R terminal on furnace board connected to R terminal on thermostat usually with a red wire. W terminal on furnace board connected to W terminal on thermostat usually with a white wire. G terminal on furnace board connected to G terminal on thermostat usually with a green wire. C terminal on furnace board connected to C terminal on thermostat usually with a blue wire. There is usually a white wire also connected to the C terminal on the furnace that goes to the outdoor part of the ac unit. Y terminal on furnace board connected to the Y terminal on the thermostat usually with a yellow wire. There is usually a red wire also connected to the Y terminal on the furnace that goes to the outdoor part of the ac unit. You should verify all wiring with the manufacturer's instructions prior to turning on the power. Now, you should connect the condensate drain and run a pipe to a drain location (usually a floor drain). This should be a 3/4 inch pvc pipe.

Furnace Installation, Final Checks

The final step in your furnace installation is to start the unit and run it through a heating cycle. You should check the outlet air temperature and the temperature rise from the return to supply and compare to manufacturers recommendations. Check for proper condensate drainage. Also perform another check for gas leaks.

**HOT TIP** It is normal for there to be a smell similar to oil burning when a new furnace is started. This is residual oil from the manufacturing process and it will go away pretty quickly