HVAC Terminology

It is typically canterline radius.

Thank you very much. Is it true the difference between a register and a grille is that the register is adjustable, while the grille is not?

When it comes to grilles and registers, it has always been my understanding that a register is just a grille with an opposed blade damper attached. Diffusers are another issue.

So the question might be, why is it referred to as a register? In what sense is something being registered?

From my 1958 Webster's New Collegiate Dictionary:register (noun) [...from re- + gerere, to carry] (This isn't a complete etymology, but the Latin root is plain enough.)

definition #4: "A device, as in a furnace, for regulating the admission of air; esp. one in a floor, wall, or the like, for heated or fresh air."

So in this instance, a "register" conveys or carries air into a space.

QED

Sounds like they should have called it a "regulator". Oh well, in the meantime, the current flows one way while the electrons flow the opposite way.

I would imagine the flex duct has replaced the galvanized duct in most residential HVAC installations today, because it drastically reduces the installation time. But it seems to me the flex duct might have quite a bit more frictional losses, even with it laid out as straight as possible.

Also, do they make insulated flex duct, or do you just wrap insulation around it, like you would with galvanized duct? TIA.

Galvanized duct is still the main method of air distribution today. Flexible duct is used but has a very high friction loss and is limited to short runs, usually at the end of the distribution system connecting to the diffuser. Insulated flex is readily available.

Galvanized duct is still the main method of air distribution today.

I'm wondering if this doesn't apply mainly to commercial as well as some (say, multifamily and newly-constructed single-family) residential. What I refer to is the fact that flex-duct is an adaptable, essentially pre-fab (manufactured), product; whereas sheet metal ductwork is not and must be custom fabricated in place, requiring higher labor cost both for design and for fabrication and installation.

This information came to me when, while obtaining estimates for a central-air retrofit, I was advised of a necessary additional charge to meet my demand for rigid, sub-ceiling, non-insulated ductwork. (This would be for [and the amount of surcharge would be mitigated by] tube-formed [circular cross section] ducts which, it was said, would be far cheaper as between that and "square-formed" ductwork.

Thx, Icarus. I'm trying to design a small system. The frictional duct loss in the supply side is 0.184 in. w.c., while that in the return is 0.161. Since both numbers are below the 0.30 external static pressure rating of the heat pump, this model would be ok as far as this criteria goes, right? Or is the 0.3 a limit of the total of the supply AND the return figures?

Also, the crude pressure drops I used were from generalized tables and charts in my 50-year old college textbook. Is there a better source, perhaps a free download, or is the $95 Carrier Handbook the only way to go? TIA.

1. "w.c. means "water column", does "wg" mean water column gage?

2. Does Specific Heat Ratio refer to the specific heat of the heat pump's supply air over the specific heat of the return air, during the cooling cycle?