I have been fabricating sheet metal for the HVAC industry for 11 years. I feel that over that time I have learned a few things about the business. Especially when it comes to residential installations. One thing I tell my customers is to always measure off the actual unit that you are installing. It has become a major pet peeve of mine over the years because it continues to happen.
What happens, you may ask. Well it's simple. An installer will typically give me measurements off the provided secifications sheet of the unit to be installed. Problem is, these sheets can be confusing and measurments are almost always given wrong. Which means that the installer will call me again and need a transition or plenum ASAP. This affects my time and the installers as well. Not to mention the added cost for the contractor and potentially the customer. This can prove very costly for jobs with tight profit margins.
Why do you care? You get double the business.
Although this is true, it isn't the way I want to make my money and you shouldn't either. Mistakes will happen over the course of any job. Will I make money on those mistakes, yes. However, I will make more money over time if my customers are getting jobs done faster and properly. They will be able to provide more word of mouth business for K & E Sheet Metal. The more jobs they are on and complete in a timely fashion, the more work that comes my way. Homeowners and customers can be very loyal. If you show them the respect they deserve, they will do the same for you.
The specefication sheet is helpful for many things when intstalling a furnace or air handler. Plenum sizes are not one of them. Do your fabricator a favor. Take your measurements directly from the unit. You'll save money and time in the long run.