OK, But How Do I Actually “Do” a Manual T?

A few weeks back, I wrote an article called “5 Takeaways from Manual T.” That was a nice article, but I sense there was some disappointment. Some of you still want to know how to “do” a Manual T. Well, this article is for you! We are going to go through the process of selecting supply registers for a home. Selecting return registers is covered here.

Selecting Supply Registers

Section 10 of Manual T deals with supply register selection. Here’s the process:

  • You need to know the design CFM for the room. This room below has two 8-inch ducts running to it. So each 12×6 vent will have about 200 CFM coming out.

  • Now, I need to look up some performance data for some floor supply registers. I’ll start with a Hart and Cooley basic floor register.

  • Check the noise requirements. No noise criteria are published in this data, so Manual T says to stay below 700 FPM face or neck velocity.
  • Check the spread and throw. Now, on this part, you need to look at where your registers are going to be placed. Are they going in the ceiling or the floor? This will make a difference. Manual T has sections that dive into the best practices for registers in various locations. 
  • Check the pressure drop. Manual D recommends 0.03”w.c. as the calculated pressure drop of a register. If I was trying to put 285 CFM through this register, my pressure drop would go up to 0.06”w.c., and I would be way past the noise recommendation.

That’s it! I mean, obviously, there is a lot more detail in the following sections, but those are the basics. Know your CFM for a room, and know how to read the engineering data for the registers. Let’s finish out the grille selection for the house I’m working on

As noted earlier, my application is floor registers because this house has no attic. So, all the ducts are in the crawlspace. Section 10-8 covers floor outlets. For floor outlets, Manual T says, “Just considering performance, the models with longer throws, wider spreads, and lower face velocities are preferred.” So, pick registers that maximize throw and spread, while keeping noise to a minimum.

For floor registers, it’s recommended to have at least 4-6 feet of throw for heating and 6-8 for cooling. 

For ceiling registers an acceptable throw is between 75% and 120% of the distance to the nearest wall. So, if you have a 20-foot-long room, you need a register that has a throw of at least 15 feet, and the maximum allowable throw would be 22 feet. Here is another Hart and Cooley register chart, this time for ceiling registers. 

Manual T also recommends selecting registers specifically for variable speed systems, as they operate over a wide range of airflows. The floor grille I picked with 10 feet of throw at 200 CFM is not going to throw nearly as far when the system is ramped down and it only has 100 CFM coming out of it. Two smaller registers would be a better option, and it is recommended to put your registers in the ceiling for variable speed units because the airflow can take advantage of the surface effect (or Coanda effect) and increase your throw.

Selecting proper registers is not that hard. You have to do a little digging, but once you look at a chart or two, you will have a solid idea of what works and what won’t. Now you know how to “do” a Manual T!

—Matt Bruner



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