That is technically a commercial supply outlet. No, a duct ring is not mandatory but I'd be very careful with the design of the system when using those. The throw from the outlet is very small compared to other shapes and designs. If your HVAC contractor is well versed in using the engineering data, he can work it out but you may end up with substantially more outlets in the room than you would have with something like the A500 series. Also, since they're only omnidirectional, they'll always be installed in central parts of the room. The A505 can be installed close to an inside corner and the A 503 can be put near to an outside wall. This can be important when your electrician plans on putting a light fixture dead center in the room and your HVAC contractor also needs to be there because of throw issues.

The #16 round supplies have somewhat better performance, albeit at the expense of having to use a duct ring and having a visible handle in the center of the diffuser.

Careful design will allow for a comfortable system but if your HVAC contractor isn't well versed with the ACCA manuals J, D, and T, you may be disappointed in the system operation. Aesthetics are only one part of the equation an life is full of compromises. So shop carefully, ask all the right questions and you'll have a good result.

 

Thanks for the tips, skippedover. We planned them for 2/3 toward the center of the room. The lower throw of the round style is all the more reason to use the smaller one which will have higher throw, right?

Too bad the 16 model has a hole in the middle. That takes a lot away from the aesthetics. Also the 16 model is smaller overall diameter for a given duct size. It is interesting that the 20 model has so much larger rings for a given duct size.

 

Here's a picture comparing the Hart & Cooley 20 vs. the Krueger RM2. I think these are the two nicest recessed mount round diffusers available..