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Old 08-01-2011, 09:08 PM   #1
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Manual D Calculation - did I do this right?


Background: I recently bought a Fantech 3005R Heat Recovery Ventilator (HRV). I also purchased the ACCA Manual D book, so I could perform the duct sizing myself. The following calculations are just for the fresh air supply into the house.

The Fantech has EBM fans(which I believe are ECM), and the specificattions list the following Supply Ventilation Performance.

in Wg - CFM
0.1 - 277
0.2 - 270
0.3 - 253
0.4 - 238
0.5 - 226
0.6 - 211
0.7 - 202
0.8 - 194
0.9 - 181
1.0 - 168
1.2 - 151
1.4 - 121

I am also installing a Fantech Filter box that has the following pressure drop/performance:

CFM - in Wg
120 - 0.19
151 - 0.22
168 - 0.24
181 - 0.25
194 - 0.26
202 - 0.27
211 - 0.29
226 - 0.30
238 - 0.33
253 - 0.36

I selected a fan speed of 181 CFM, which corresponds to a static fan pressure of 0.9 in Wg. The fan reduces that by 0.25, giving me a Static Pressure of 0.65 in wg.

My total effective lenghth is 356 feet:

Here are my calculations for TEL (all round metal ducts):
From the HRV, I move into a 6" to 7" 12A transition (20' TEL) (the trunk)
I then move into a 7", 3-piece 8A 45 fitting (10' TEL)
I then have a two foot section of 7" pipe (2'TEL)
I then move into a 9I supply trunk junction fitting/reducer that is 7" on the incoming end, 6" on one end, and 4" on the other (TEL = 85' branch
& 5' main)
From the 4" end, I move to a straight section of round duct that is 6" wide and 5' long (5' TEL). I will call this S1.
Then, from the 4" round, I move into a Stackhead 4AG (60' TEL)
From the 6" end of the supply trunk junction fitting/reducer, the trunk continues, and I move into a 5' section of 6" round (5' TEL)
Then, I have a Wye, that is 6" on the incoming end, then 6" on one outgoing end, and 4" on the other outgoing ends (TEL = 30).
From the 4" end, I transition to 4' section of 4" duct (10' TEL). I will call this S2.
Then, from the the 4" duct, I move into a Stackhead 4AG (60' TEL)
Back at the Wye, and from the 6" end, I move into a 6' section of 6" duct (4' TEL). I will call this S3 (but I could consider it a continuation
of the trunk).
Then, from the 6" round, I move into a Stackhead 4AG (60' TEL).
TEL = 356 feet.

I calculated my friction rate as (0.65 * 100) / 356
FR = 0.182

Here is a link to a schematic of the duct system: http://i.imgur.com/FDX9p.jpg

I used my ductulator and came up with a duct size of 6.4 inches for a round duct. However, that size duct produces a FPM of 836, and I read that the maximum is 700 for the trunk/600 for the branches. When I move up to a 7" duct, FPM falls to 674.

I calculated the sizes of the supply trunk/branches as follows:

Section - CFM - FR - Size - Used - FPM

Fan to S1 - 180 - 0.182 - 6.41 - 7 - 674
S1 to S2 - 135 - 0.182 - 5.8 - 6 - 688
S2 to S3 - 100 - 0.182 - 5.2 - 6 - 509

s1 - 45 - 0.182 - 3.7 - 4 - 516
s2 - 45 - 0.182 - 3.7 - 4 - 516
S3 - 100 - 0.182 - 5.2 - 6 - 509

I have a few questions:

1. Did I do the calculations correctly?

2. The HRV has 6" ducts. I immediately transitioned from a 6" to 7" round duct for the trunk--does that even make sense to do?

3. I'm a bit confused on how I get to a fan speed of 181 CFM. The unit is advertised as having three speeds (65, 121, and 231). When I put it on high speed, based on my design, it shoudld yield 181 CFM? I guess I confused about picking a CFM and then using the static pressure that corresponds to it (from the manufacturer).

4. My calculations were for 6.4", but I moved up to 7". When I look at the ductulator, I can take my 0.182 friction, and move it up to ~220 CFM before I hit the 7" duct size. Does that mean I should get 220 CFM vs 180 CFM?

5. Do I need to add dampers on each supply line and on the end of the trunk/S3 for balancing purposes?

Any other advice would be appreciated.

Thanks,
toddmanqa
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Old 08-01-2011, 10:25 PM   #2
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Manual D Calculation - did I do this right?


You don't add all the branches together. You do each of them separately. then use the longest one for TEL. So figure out the TEL of S1, S2, and S3 separately. And use the one that comes out the longest.

As you said, Subtracting the filter box's PD from the 3005R gives you a ASP(Available Static pressure) of .65". After you redo you TEL calcs, I think you'll find you can use a lower FR.

Using 7" for the trunk should help to prevent noise issues. 8" would probably be better though. And will allow you to use a lower speed setting.

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Old 08-02-2011, 11:20 AM   #3
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Manual D Calculation - did I do this right?


beenthere,

Thanks for that information!

I redid the calculations:

S3/Trunk was 191 Length
S1 was 187
S2 was 121

using 191 at TEL, my FR fell to 0.34 at 181 CFM.


In the ACCA Manual D, I've read that:

Rigid supply trunk ducts should have 700-900 FPM.
Rigid branch ducts should have 600-900 FPM.

I have the following options for my initial Trunk (180 CFM):
7" at 695 FPM, or 8" at 560 FPM

For the trunk between S1 and S2, I have the following options (135 CFM):
6" at 695 FPM, or 8" at 400 FPM

For the trunk between S2 and S3, I have the following options (100 CFM):
5" at 740FPM, or 6" at 520 FPM

For S1 and S2, I have the following options (40 CFM):
3" at 820, or 4" at 460

For S3, I have the following options (100 CFM):
5" at 740, or 6" at 520

What are the better options in terms of velocity and noise vs throw?

Thanks.

Last edited by toddmanqa; 08-02-2011 at 11:25 AM. Reason: error
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Old 08-02-2011, 07:29 PM   #4
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Manual D Calculation - did I do this right?


Go with the lower velocity. Less noise. Throw is determined by the register you use. The velocity in the duct has no bearing on the registers throw.
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