How To Replace and Install a New Gas Furnace

By | August 28, 2014

installing a Gas furnace
Installing a new furnace is a great way to save money for a do-it-yourself type of person. Some HVAC / construction skills will be needed. But if you feel comfortable connecting a gas line, hooking up a vent , can use tin snips and a hammer then you will have minimal if any problems.
If you do not feel comfortable with a gas line, ventilation, or modifying existing frame and duct work then it is best to hire someone to do it for you. Mainly for safety concerns since gas leaks can lead to explosions and wrongly ventilated piping can lead to asphyxia.
If you do feel comfortable handling the things listed above then replacing your furnace is a great way to save money. Not to mention looking back at a job well done and that feeling of accomplishment.
The first thing you will need to do is examine your furnace’s current setup and locate were the air goes into the unit and were it pushes out. This is called the Return Air and Supply Air. The return air is where the air goes into a unit usually at the bottom and the supply is where the air is forced out and vented throughout a home.
Here is a unit were the Return Air is pulled into the left side bottom of the unit from metal duct work that the goes up (this is a basement). The supply air on the top of the unit pushes air up into the vents of the home.
Gas furnace install
The duct work for the supply and return air are usually custom made for each unit out of tin that is easily cut. Some homes will have return air’s built out of a wood frame that a furnace sits on top of. All of this needs to be examined to try and buy a replacement furnace that will match as close as possible to the existing duct work. This will help minimize the amount of work that needs to be done.
Some type of modification will no doubt have to be done but the closer the old unit is to the new unit in dimensions the less work that will have to be needed. For example if the height of a old unit is 5 feet while the new unit is 4 feet duct work will have to be extended to the new unit for the new unit to work properly.
There is no way to escape any modification and some type of retro fitting will have to be done. Most duct work is done with sheet metal that is easy to cut and bend with HVAC tin snips and hammered into the correct shape or form. There are three different types of HVAC tin snips used Right Cut Left Cut and Straight Cut. You most likely will at least need some straight cut tin snips but they are cheap if you don’t already have some usually leas then $20 dollars.
After being fitted into place metal tape is used to seal joints and any place the air will escape.
For any large modifications hiring a company to make the duct work for you is a common practice. Most HVAC companies hire out their duct work to HVAC companies that can fabricate metal to specific measurements. Of course this will ad to cost so modifying duct work your self is the best option when possible.
Most Furnaces have the return air located at the bottom of the unit where the air after circulating through the house comes back into the furnace to be reheated. This can be at the very bottom or bottom side of a furnace.
furnace all

When the air comes in from the very bottom of a unit it will sit on the top of a boxed in frame and simply pull the air in from the box. Some units will have the air come in from the side and have the return air feed in through metal duct work. Air fed into from the side is usually done when a home has a basement and needs to go up, but it can vary and depends on the people who did the original installation.
The gas line will also need to be looked at for the distance it is from the unit. Most of the time it will be plenty far enough away that a new unit will slide past it no problem. But sometimes installers can put a gas line close to a unit and when a new unit is installed the gas line has to be extended away.
gas line 2
Next thing to examine would be the ventilation. Check the diameter for size 4 inch 6 inch and so on. Also check the length to the roof and whether more will need to be added or taken out. this will depend on the height of the new unit and the location of the ventilation shaft between the two units.
ventting 1

Buying a new unit for home owners used to be a problem but thanks to the internet that is a thing of the past. There are many companies that sell furnaces on the internet but for me trust is always a factor. But this is also easily solved since Amazon now carries a huge selection of gas furnaces. This is a personal option and everyone will most likely have a different opinion but for me is a easy one. I have been buying from Amazon for years with minimal problems.
Furnaces can be heavy and awkward to handle; although depending on how strong you are they can be installed alone but I don’t recommend it. Find a friend or pay someone to help you lift the old unit out and new unit in. Having a moving dolly will come in handy when moving the units in or out of a home.
After you have the old unit out and the new unit in all the connections will have to be made. The return air will have to be fitted properly along with the supply air. The supply and return air are secured using screws. Many times metal will have to be bent and hammered into place. This is more of an art then anything and some are worse then others. This is why metal tape is used to seal the joints and holes that will let the air escape.
cutting 3
The gas line will need to be hooked up and is usually a straight forward process. Hopefully the line didn’t need to be moved or extended; which is not hard to do. If the line does need to be moved or extended then measure the gas pipe for the correct size usually it is 1/2 inch then order what length, and elbows you need to move it out of the way. Yellow Teflon tape is used for gas and should be used at each joint. Joints should always be checked for leaks. A mild soapy water mixture can be used at each joint to spot any leaks.
The exhaust ventilation pipe needs to be secured firmly on the unit and extend to the roof letting out the toxic gasses. The exhaust pipe will have a inner liner and a outer liner that is separated by insulation to keep it from getting hot. The exhaust pipe simply side together and turn to lock into place. Different size pipe can be bought from width to length depending on your needs.
vent thru roof
The final things to hook up would be to plug the unit into a wall socket and to hook up the thermostat. You should have no problem using the old wiring although sometimes new thermostat wiring is need when upgrading from very old units. A new thermostat may also be needed in the same way. For example a old mercury thermostat may not work with your new unit.
As always be safe when when handling the unit and hooking it up. The tin that is used to make the return airs and supply air can be sharp and cut you if you are not careful.

2 thoughts on “How To Replace and Install a New Gas Furnace

  1. John Salla

    Can you use the old return and the old supply air if it fits, or modify it to make it fit?


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