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Old 03-17-2010, 03:36 PM   #106
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Also. You don't want to use anything smaller then a 4". And only for a bathroom or mudroom when you do.

Sometimes you give up velocity, to keep static pressure low.
Duct velocity, and register velocity aren't the same thing. You can have 200 FPM in a duct, and 700FPM from the register.

You may want to get a copy of Manual D. It will have all the equivalent lengths of your duct fittings, and go over how to figure FR.

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Old 03-17-2010, 03:42 PM   #107
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What you need is a duct calculator like this.http://www.datalizer.com/

Last edited by richiemoe; 03-20-2010 at 09:17 AM.
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Old 03-17-2010, 03:47 PM   #108
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Jim-

Thanks! Did something get cut off from your first post? It ends with "Then..."

Do you have any samples of a multifloor diagram? I am not sure how to draw that up without it being totally confusing?

Thanks!
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Old 03-17-2010, 04:02 PM   #109
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Not sure how that "then" got typed there .
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Old 03-17-2010, 04:06 PM   #110
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Don't have any handy.

But, what I usually do if the second floor is just branch supplies(just oval or round pipe runs).
Is just draw a line for it, put a supply reg symbol at its end. And mark how long it is.

Don't need to be 3D, fortunately for me. Or I'd be in trouble with some of my drawings.
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Old 03-17-2010, 08:51 PM   #111
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Excellent. I am working on that diagram now. I will check to see if I can find a copy of the Manual D somewhere around here too.

Have you ever seen or used this? It seems pretty neat, but maybe too easy to be true. What do you think?

How do you go about getting higher velocity out of the register than there is in the duct? Is it a matter of closing the dampers? Or using a special register? So, for example if I only need 40 cfm out of the duct in the dining room, I should use a 6", even though that is better sized for 80 cfm, right? And then I will just use the right register to make sure I have the right velocity and throw?

So, am I crazy for thinking about keeping my current condenser and coil? They are brand new and so I feel bad just ripping them out, also AC is a different beast for DIY because of the refrigerant AND the need to braze the lines. Our home is actually more comfortable in the winter than in the summer and the AC has seemed to work well enough. But past performance is a poor indicator, I think because the return duct is so minimal (12x8 for the whole 1600 cfm system) AND because the addition with are adding will make the house much more efficient as it will replace a lot of the un-insulated exterior walls. DC has extremely high humidity... we used to be a swamp! I know it is oversized, but can I go with it for now and replace it later, if necessary, or when it dies? Or is it just foolish?

What do you think of ordering HVAC equipment online? Are there any good companies? Are the prices better? I think it can be hard to get a lot of this stuff locally, as they prefer to sell to contractors/pros.

Thanks!!

Last edited by stubits; 03-17-2010 at 08:55 PM.
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Old 03-18-2010, 04:32 AM   #112
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Velocity is a function of free area to volume of air.

A 6" pipe has an area of 28.26 sq in, or .2 sq ft.

A 4X10 register may have the same area, or you can get one with more or less, depending if you want a higher or lower velocity.

EG: A Hart&Cooley 4X10 #210 has a free area of .2 sq ft. So at 60CFM it has a velocity of 300FPM the same as a 6" pipe.
A 4X10 #411 has a free area of .17 sq ft, so it has a velocity of 352FPM moving 60 CFM.

Both registers are 4X10, but the vanes are different, making the area the air passes through smaller on the 411.

A #411 2X10 has a free area of .09 sq ft, and would have a velocity of 700FPM at 60 CFM. Even though it was being feed by a 6" at only 300 FPM.

Velocity in your supplies is important. But, maintaining CFM is more important.

I would probably use 5" instead of 6" on some runs that a 4" could handle. Only because once you drop below 300 FPM in a supply. You are at risk of loosing or gaining too much heat from the are that the pipe runs through. And not supply or removing enough BTUs to that area.

Supplies that are run inside of outside walls. Are supplies that you want to keep the velocity up on.
An inside wall is not as critical.
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Old 03-18-2010, 08:36 AM   #113
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OK, I think this is making sense now. It is kind of like partially covering the end of a hose with your finger, you get the same amount of water, same pressure, you just change the flow, right? The idea behind CFMs is that you need to pump in and remove a certain amount of air from each room to make it comfortable right? It is about changing over the air in the room. Velocity effects how far and how fast the air travels, but you still end up with X CFM, regardless of velocity, right? I am not a stupid person, but I cannot believe it has taken me this long to come to this "a ha" moment! Hopefully I am right.

So, what is the optimal amount of FPM that you want? Some of what I have read says you don't want it to go above 600-800 depending on supply or return, and you say that less than 300 is bad, but is anything in between good or is there a best case?

So, what do you think about my AC condenser and coil? Am I stupid for keeping them?

Thanks!
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Old 03-18-2010, 01:02 PM   #114
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You got a fairly good handle on velocity now.

What confuses most people is that the term "velocity" is applied to both ducts and registers. Instead of the terms "duct velocity" and "register velocity"

Velocity and static pressure MUST be a compromise, with static pressure getting the bigger half of the deal(kinda like that Jerry Reed song goes. They split it down the middle, and gave her the bigger half).

300FPM would be something used for a closet, or small bathroom/powder room.

800FPM in a 6" pipe is gonna be loud. And have a high static pressure(that would be moving 160CFM). At a FR of .195".
Drop that FPM to 407, and you have a quiet supply moving 80CFM. At FR of .052".

800FPM in a 8X10 trunk line is semi ok, at 450 CFM and a FR of .11".

So, larger ducts, can have a higher FPM and be quiet, and have a low FR. Smaller ducts, need a lower FPM to have a low FR and be quiet.

You want your main trunk line to be 800FPM and less as it reduces. And your supplies maxed at about 600 FPM for short runs, 400 FPM for long runs, such as second floor runs.

Often. Installers will try to stay at those "recommended" velocities. And thats when you start getting second floors that won't cool right. Because the static pressure becomes too high to force the required CFM through the long pipe to the second floor, instead of the easy short path to the first floor.


Long way of saying there is no ideal velocity.

Give more credence to FR to keep static low, and you'll have lots of air flow(CFMs).
Look at velocity too much. And you'll lack air flow, and kick yourself in the butt.
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Old 03-18-2010, 03:59 PM   #115
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I am glad I am finally getting it!

I also just got my ananometer today, so I will get a feel for what I am getting out of my system currently... I am sure it is not enough!

Also, I hope to draw up some diagrams of my proposed duct system tonight which should help me figure out the friction rate. Just to confirm, I need to figure out the length of the longest duct run, so, starting from the air handler, through the trunk, through the branch and onto the register for both the supply AND the return, right? And then add to that distance the equivalent distance of all the fittings, take offs, angles, register boots, etc? Do I have it?

I am also going to start looking at new furnaces... I will definitely go with a dual stage, variable speed, 95% efficient unit. Do you have a preferred brand, or even a favorite unit? 60-70,000 BTUs is the proper size, right? Most of these seem well paired with 3 ton AC, so, what do you think about my AC condenser and coil? Am I stupid for keeping them?

Thanks!
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Old 03-18-2010, 04:12 PM   #116
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Yes, you have the length calc right.
Minor exception. i think I typed it a bit misleading.

You add up the pressure drops for the registers, grilles, air filter, evap coil. And then subtract them from your furnaces ESP rating(can probably use .06 for the supply and return regs/grilles).

Most manufacturers will have the rating for their registers and grilles. But sometimes its not easy to find, or tell exactly which model you have.

But other then that, you got it right.

Don't forget to do your CFM checks by averaging the air flow from the registers and grilles.
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Old 03-18-2010, 11:31 PM   #117
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Ok, so I just calculated the friction rate and I ended up with a rate of .08. Here's how I got there, does this seem reasonable?

First, total effective length:

Supply
Take off - 15 ft.
Vertical Duct - 30 ft.
90 - 10 ft.
Horizontal Duct - 10 ft.
90 - 10 ft
Horizontal Duct - 22 ft.
Register Boot - 15 ft.
TEL SUPPLY - 112

Return
Take off - 15 ft.
Vertical Duct - 30 ft.
90 - 10 ft.
Horizontal Duct - 10 ft.
90 - 10 ft
Horizontal Duct - 14 ft.
Register Boot - 15 ft.
TEL Return- 104

Total Effective Length (TEL) - 216 ft.

Blower ESP: .5
AC Coil: .22 ( this is at 1,200 CFM, the minimum for my current coil)
Filter: .05 (cheap pleated filter)
Supply Register: .03
Return Grill: .03

Total device pressure losses (DPL) - 0.33
Available Static Pressure (ASP) - 0.17

End result is a friction rate of .08.

So, does this make sense? If so, where do I go from here as far as figuring out the duct design? How do I figure out what to do now?

Thanks!
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Old 03-19-2010, 04:48 AM   #118
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On your return drop. Does it ell into the furnace with a 90 that has radius corners, or are the corners sharp 90 right angles?
If sharp 90 angles you need to add a min of 65' for those angles.

Add 65' to supply if the trunk goes into/attaches to the side of the supply plenum.

That will bring your TEL to 346'. Not an unusually long system.
And at an ASP of .17 will reduce your FR to .05".

Its not uncommon to end up with a FR that makes your duct system larger then what you have room to install, or ability to change to.

Often. The furnace fan chart has to be looked at to see if using a .6, or .7" ESP to get an ASP of .27 or .37". So that an more acceptable duct size can be used.

The fan chart will have CFM listings for different fan speeds at different ESP's.

With the FR you use a ductolator to check your duct work sizes to all rooms. Since you have the CFM each room needs. You can now see if the size run the rooms have is sized right for the CFM it needs.

You also do that for your trunk line.

EG: If your trunk line needs to carry 1200 CFM. The first piece of duct should be 8X26 or equivalent. For a system that is using .08" FR. Or 8X32(reason why some systems need to run at a high then standard ESP rating) for a system using a .05" FR.

If it is sized right. You then check the size of the supply runs coming off of that section, to see if they are sized right by the ductolator.

Then you subtract the CFM of those supplies from 1200, and get your new CFM for your next piece of duct work. And check if it is sized right. Along with the supply runs to the rooms. And continue until you have checked each trunk piece and supply run.

This now tells you what duct alterations you need to make.

From there though. You need to check to see if you can make those duct alterations. As in can you install the larger ducts you need both trunk line and supply runs.

Often the trunk line isn't a problem to increase size. But, some of the supply runs are, such as runs to the second floor.

In the case of second floor supplies. Instead of ripping open walls. Generally, you add additional supply runs to the rooms that need it. And then chaise the new run, so the wife doesn't shoot you.
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Old 03-19-2010, 07:13 AM   #119
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Excellent, thanks, I really feel like I can see the end from here. I knew this was going to be more complicated than I could comprehend, thanks for walking me through it.

So, I currently have a return air ell that looks like this:

But I have been considering something more like this if it saves me some room:

Is one better to use than the other? I need to replace my current one because my current drop is only 20x8, so too small, even for the smaller furnace. You say I need to add 65' for a right angle boot, what about for the more curved one? Are there other options with even less friction?

As for the supply run, the run I used for the TEL calculation, the physically longest run, comes right up out of the top of the supply plenum. Do I need to add anything in this case? There are at least 2 trunks that will come out of the sides. Does that matter?
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Old 03-19-2010, 11:19 AM   #120
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The shoe ell is better, if it doesn't have an air filter in it.
The filter box is better if the other also has a filter in it.

A radioused ed ell is the best thing to use to keep TEL down.

The run that is off the plenum may be the longest physical run. But, it may not be the longest TEL run.

The supply that you used, would have a TEL of 30, tapping into the plenum.
But your ducts that tap into the side each have 65' of TEL. You need to check each supply for its TEL to determine which is the longest.

Its like determining who's coffee taste best. You must sample each one, to know. Color and aroma don't tell the who story.

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