Ducting Systems and Air Ventilation - Properly sizing supply/return regsiters?

I am installing a new furnace. Changing from hot water hydronic to forced air. Did the manual-j calcs, did the manual-d calcs, used a ductulator and set the desired friction at .07 All the duct works seems simple enough and installing has been a breeze thus far.

My question is what size register boots should I be using where the duct work actually enters the room? I know how to calculate the square inch surface area and would have no problem sizing them IF I didn't have to take into account the actual register grate that will be installed at the end. Should I size my boots 50% larger to account for all the actual surface area lost when the register is installed? Does it not make a difference on the supply lines since they are under positive pressure? What about the returns?

I have been able to find a ton of useful information on the internet along the way during this project and everything makes a lot of sense to me but I cannot find any info on accounting for the friction of the actual grate. I even called my local salesman at an HVAC supply house and he wasn't sure either.

Normally residential :
4X10
4X12

:coffee: But check out this link for more data :coffee:


Hart & Cooley - Products - Registers, Grilles & Diffusers (http://www.hartandcooley.com/grd/all_grd.htm)


Under , Residential , Floor registers % grills , 210 Floor Register ,Engineering Data (PDF file)

Ok so do I need to size the free area of the actual grille/diffuser to meet the area of the supplying duct? For instance a 6" duct has an area of 28.27 sq inches. Should I then size the boot so that I can get a register/diffuser which achieves a free area of that same 28.27 sq inches?

Does the free area of the grille/diffuser really make much difference when you figure it only restricts the flow of air for such a short period of of the air run?

If I CAN use a grille/diffuser with a smaller free area than the branch that it's hooked to, what % smaller can you go before affecting the CFM delivered to the room?

If you are installing a different air , handler as in removing a strait ac air handler , and new eqiu. is same size tonage , and duct is existing ,what you have should be fine , just .

Their is really no big deal with floor reg. the hart n cooley is common to be used .

In the original post I stated I am replacing a hydronic hot water system so all the duct work will be brand new. I want to make sure the right size stuff is installed and I have found plenty of info everything but the supply registers and how important the free air space of the diffuser/grille is. I'm hoping someone has a formula for figuring free space needed at a given cfm and friction rate. I'm guessing systems designed to operate at .07 friction would need a different free air space than those designed to operate at .1 friction. Or maybe it doesn't matter because the actual restriction is only at the very end of the run and isn't over a long enough length?

Example: reducing a 100 cfm branch down to 4" diameter pipe would be a problem UNLESS the 4 reducer was only for the last 6" of the run. Then I'm guessing it wouldn't really matter because it's not like the air is having to travel through 5 feet of 4"

I'm not actually reducing my 6" branches to 4" but using the example to make my point. If I have a 6" branch supplying 100 CFM to a room and install a grille/diffuser on the end of it that has the actual free space of a 4" diameter pipe would it be a problem?

If this is in floor you need to , go from 6'' pipe to a 6''4x10 or 6''4x12 boot and use a standerd floor grill dont put so much emphasis on the friction.

Rule of thumb ;

2ton unit 400 cfm a ton = 800cfm

You would have approx. 8 - 6'' runs or 5 - 6'' runs and 2 - 7'' runs