A client recently had Decor Grilles (a Toronto company) floor diffusers installed, they're whistling, and I'm trying to decide whether to reuse them (at lower airflows, we're removing 40% of the tonnage presently installed - now 400 SF/Ton doncha know - and there's mold growing).

I downloaded Decor's airflow graphs showing "Flow" and "DP", and I'm trying to estimate velocity (oddly enough, neither Ak nor velocity nor throw are shown). Then I looked at a H&C #421, trying to compare PD's to estimate Ak's. If the units of "Flow" and "PD" are CFM and inches of water column respectively, a 4x12 Decor Grille floor diffuser is roughly equivalent to a 2-1/4x10 #421.

Then it occurred to me Canada might be metric for HVAC (IE: Other than CFM and inches WC). Is that the case, or is it CFM and inches?

Many thanks!

FYI: The PD for a 4x12 @ 120 CFM is about 0.13" WC (if those are the units of measurement), about what one would design an entire supply duct system for.