How to Measure and Make Transitions between New Furnace and Old Plenum 

In the pictures below you can see how different HVAC Companies changed-out the furnaces and connect old ductwork to new equipment:

The fronts of the plenum and cold air drops in this case are wider than their sides; it’s why an installer made these complicated transitions. You can purchase a tutorial on how to make them below.

Plenum extension luckily was the same size and in order to adapt air cleaner casing instead of cold air boot the installer used a piece of 24 x 8 duct.

The plenum and cold air drops in this case were pretty big it’s why an installer used these complicated transitions.

An installer in this case got out with a short and simple transition.

The installer in this change-out could propose the entire ductwork replacement, but usually homeowners put down such offers – too expensive! I hope now someone would come up with a different idea.

More Change-outs:

The air filter casing swung the cold air drop a bit aside. And installer saved a couple of bucks by installing case less A/C coil, but made a short transition at the bottom.

The transitions in the next installation, just like in the previous ones were design to wrap duct around the air filter casing. 

The same ideas in these installations, but why they added that sleeve in the second installation no one would explain… And again the installer used larger than necessary A/C coil it’s why he built those transitions.

For this replacement an installer completely rebuilt the plenum and supply ductwork parts, but saved the opening in the return duct.

This huge system of ductwork around the air handler require a lot of space, but make one think that that was done for a reason…

Two more examples of the air cleaner installations.

And More:

Two pictures before and after installations.

And another two.

The cold air boot and riser in this case made by someone who doesn’t know what he is doing, but surprisingly the air is going to find its way through this system anyways!

The short and simple transition for the next installation anyone can do – just purchase my tutorial below.

In the before and after pictures you can see that this is already at least second change-out. Lazy idiots probable saved some money on the first one, but the eventually the second installation was done right.

And again the homeowner decided to save on replacement and have all these huge round ducts exposed.

If you would like to learn how to replace a furnace, install an air cleaner casing and make all necessary transitions please purchase a chapter from my Ductwork Installation Guide and paid pages below.

59. Furnace Changeout* – the chapter is dedicated to another part of the trade and describes everything that an
installer is suppose to know to perform a furnace replacement.
The chapter has 72 pictures; 27 pages. $11.98 (Instant Access)

*Link to:
Furnace Replasement in Pictures.
On 04/18/2015 text of this chapter was significantly modified and improved.
PDF files:
A Guide to Residential Wood Heating.
Proper Subcooling Charging Techniques.

How to Make Transition from Your New Furnace to Old Plenum with Hand Tools

How to Make Transitions from Your New Furnace to Old Plenum and from Cold Air Boot with Hand Tools 

In the example above I took a hypothetical furnace and plenum and made a transition.

In the picture on the left you can see a real furnace, plenum and cold air drop. A transition between a furnace and plenum you can purchase below. However you can order any other transition that represents your installation and I’ll make outlines for all its parts for you. How would you do that?

Let’s say your old furnace is 46” tall and the plenum is 30” x 25”. Your new furnace most likely will be 34” tall and its flange would be good for a 16” x 20” or 20” x 20” plenum. So, in order to make a transition for you I need to know all these numbers plus if you’re able to line up your new furnace with the plenum’s side on the right. If not, I need to know how much you’re planning to move your furnace one way of the other.

Also I need to know how you are planning to set your furnace in another dimension. In the example above you can set it in the middle (25”) or line up with one side.

The price for making your transition will be only $10.00 and after you pay, I'll make a tutorial for you and you’ll be able to gain an access to the paid page where I’ll set the pictures in step-by-step sequence with brief explanations or I may email them to you. 

  How to Make 20 x 20 to 30 x 25, One Inch Offset Transition with Hand Tools

You can buy a tutorial for the transition presented on the picture above. For the price of only $3.00 you can gain an instant access to it.

How to Make 24 x 10 to 10 x 24 Duct Transition with Hand Tools

You can buy a tutorial for the transition presented on the picture above. For the price of only $3.00 you can gain an instant access to it.