Manual D Calculation - did I do this right?

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Manual D Calculation - did I do this right?

Postby toddmanqa » Mon Aug 01, 2011 7:50 pm

- 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/-

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 -

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 - *.*/FDX9p.jpg
(that's imgur dot com)

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 -

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 -

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 trunnk--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
toddmanqa
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