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Earnie 12-31-2009 07:14 AM

Determining Boot/Register Size
How do you determine the size of floor boots and registers?

I am working through some heat pump and duct work installation issues. I want to see if the floor boots and registers are the correct size for the installed flex duct diameter.

It seems to me that the register is the restricting factor since most have some sort of moveable deflector blades and fancy grill work. I'm sure these restrict the air flow to some degree, so selection a size to overcome this air restriction would be important.

As an example, I have a floor register that is 4" x 12" and is supplied by an 8" flex duct. The boot is 4" x 12" x 8". Is this the correct size register ffor an 8" flex duct?

beenthere 12-31-2009 07:27 AM

Size of the supply run(duct), boot and register is determined by how much air you need. And what kind of throw the register is to give.

Depending on model of floor register. It could allow an air delivery of 75 to 250CFM.
Or 65 to 210 for a different type.

Another style is only good for between 40 to 160CFM.

So how much air does the room need?

Earnie 12-31-2009 07:34 AM

The information I have states the room "needs" two supplys (which it does), each providing 115 cfm at around 600 fpm through a 6" duct.

What is actually installed is an 8" duct with a 4" x 12" x 8" boot. I have no info on the actual cfm as installed.

From looking at various sites online, I have yet to see a register that states how much cfm it can handle.

beenthere 12-31-2009 08:03 AM

Hart&Cooley, Lima, Titus, and all others have listings for how many CFM their supply and return registers and grilles move at what FPM and pressure drop across them.On their websites.

The CFM's in my other post is from Hart&Cooley's engineer catalog. Downloaded from H&C's website.

Don't take this wrong, but you didn't look too hard.

PS: not often will you get 230 CFM from an 8" flex duct.
And a ductulator for sheet metal duct can't be used on flex. Flex has a high resistance to air flow then sheet meatal.

Earnie 12-31-2009 08:13 AM

Thanks for the info. I'll check at those sites.

Actually I just looked at Atlanta Supply and they do provide a spec sheet on the US*Aire registers they sell.

Sorry, I miss stated on the duct. The room actually has two 8" flex ducts installed which branch off a 12" rigid trunk.

beenthere 12-31-2009 08:58 AM


Now. How did you determine a velocity of 600FPM for the register.?

Earnie 12-31-2009 09:28 AM

With this software I did a complete residential heat loss and heat gain calculation. From there it gave me a duct sizing report.

My house has a heat pump with R4.2 ductwork in unconditioned crawlspace and attic. Boots are either in-floor from the crawlspace or in-ceiling from the attic.

I'm looking into upgrading the ductwork R value so I want to make sure the duct sizes and boots are the correct size.

At this point I'm not sure if this will be a DIY or contractor job. Either way I need to know what the house needs vs a guess which is what I believe happened with the original installation.

PS. I did read a past post from you about determining F.R. using PD, ASP, TPD and TEL. It took a few times through the concept to understand the correct way to size a duct. I'm learning. This has helped me also.

beenthere 12-31-2009 11:16 AM


So you realize that you will only have about .06 to .08"wc to work with in your supply duct. If you want to stay within the .5" of it rating.

That will put 8" flex at about 180 to 200CFM.
6" flex at 60 to 75CFM.
Sheet metal will be about 20% more.

Don't confuse duct velocity with register face velocity. They are 2 entirely different things.

A duct can have only 300FPM velocity, and the register have 800FPM face velocity.

Earnie 12-31-2009 12:10 PM

A bit clearer but I don't completely understand all of it just yet.

This Friction Rate Calculator makes the math a bit easier.

My sticking point right now is determining External Static Pressure of the air handler. The way I understand it is you need a static pressure tester to read both the air handler's supply and return. Those values are added together to get ESP. How many HVAC techs actually do this?

I have the air handler performance data per ton and blower air flow setting. The table lists external static pressure form 0.1 to 0.9.

If I understand this correctly, once I know the ESP I can find the units CFM per the provided performance data table.

More than this DIYer can presently handle.

Thanks for the info and Happy new Year.

beenthere 12-31-2009 12:20 PM

You can make your own water manometer. A little hard to read, but easy to make.

A small piece of wood or cardboard. A few feet of clear vinyl hose.

Fasten the hose in a u shape to the board/cardboard, then mark on the tubing or board 1/10 inch marks.
Start at the middle of one side of the tubing. Mark 0. then make marks in both direction.

Tape it to the furnace, or hold in your hand and fill with water to the 0 mark.

Insert on end in your supply plenum and take reading(if the water moves .2 inches. Double it, because if it moved up .2, the other end moved down .2).

Then insert in return at air handler intake and record reading(same double of reading).
Then add both readings together, ignoring the - of the return.

If your aur handler has rating to .9"wc. You still don't want your ESP at that. Since you don't have enough air flow left for the air filter to get dirty.

Happy New Year.

Earnie 12-31-2009 12:35 PM

Neat homemade tool. I might try it.

The air handler data table (with filter) for the 3 ton unit I have shows:

ESP of 0.1 = 1200 cfm
ESP of 0.9 = 1145 cfm

That is with a Normal airflow setting. Without opening the unit I don't exactly know what the DIP switches are set at.

Doesn't appear to much of a change though.

There are a few notes that say at continuous fan setting, airflow values are 50% of listed values. And for variable speed, low speed is 30% of listed value.

beenthere 12-31-2009 12:38 PM

Post brand and full model number.

Earnie 12-31-2009 12:59 PM

Trane TWE040E13FB

Pub Number 22-1655-05-0301 (EN)

beenthere 12-31-2009 02:48 PM


Look at the data where you got your 1200 and 1145CFM listing from.

See how at .1 and 1200 CFM, the wattage is only 235.
But at .9 and 1145 CFM, the wattage is 475.
While a 240 watt difference may not sound like much. When you consider that its also moving less air. It adds up in increased run time for the heat pump in both heating and cooling also.

The lower the static your duct system has. The less it cost to run the unit.
So if you can get your static down to .5 or .6"wc. It will save you money in both heating and cooling mode.

So check that static this weekend if you can.

Earnie 12-31-2009 04:53 PM

Now I see it! Thank you for the great explanation.

Part of my duct upgrade plan was to use hard pipe as much as possible and then transition to flex just before the boot. Right now I have 15 - 20 feet flex runs off the trunk.

Also, I wanted to change the 90 degree start collars to 45 degree off the main trunk. Now I can see why this is a good change, less air restriction in the pipe.

I'll post back when I have some good ESP numbers.

Be safe tonight.

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