MANUAL D Duct Design Questions

[Eweneek1]

Have a question on my Manual D Duct design concerning the 10" branch line on the left side supplying the Kitchen, Mud Room, Dining room, and Bath. The area requires 362 CFM, but the 10" supply only can handle 325 CFM. What am I missing? Also the 8" supply after the elbow would only supply 180 CFM maximum, but needs 310 CFM. Should I increase the 10" to 12",, and the 8" to 10" up to the kitchen and then drop down to 8". Or, just install as indicated in the duct design?

My furnace replacement will be a two stage 60,000 BTU Rudd U96V with an output of 1250CFM. Duct design system uses 1000 CFM. What does the furnace do with the extra 250 CFM?

This is the first floor of an 1888 Victorian with about 1850 square feet.

 

[Dana]

If you did the math for the ducts, leave it as-is or it'll unbalance the distribution ratios. If it needs more cfm, you'll need to fatten the ducts &/or reduce the number of turns. The difference between 325 amd 396cfm is almost "in the noise" of error, probably not worth changing but the difference between 180 & 310 cfm is real. Are you using a software package to tweak this design or just a lipstick on the mirror/crayon on wall approach?

There are no "...extra CFM...".. The furnace blower's cfm rating is at a specified static pressure. In your case that pressure is 0.2" water column, and it's 1025cfm @ 2"w.c., not 1250, probably it's low blower speed. It's not an absolute number. At high speed (for cooling) it's rated 1050cfm @ 0.5" w.c. . If the duct impedances are lower (fatter ducts, fewer turns & restrictions) the static pressure will drop a bit, the cfm will rise a bit.

 

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[Eweneek1]

If you did the math for the ducts, leave it as-is or it'll unbalance the distribution ratios. If it needs more cfm, you'll need to fatten the ducts &/or reduce the number of turns. The difference between 325 amd 396cfm is almost "in the noise" of error, probably not worth changing but the difference between 180 & 310 cfm is real. Are you using a software package to tweak this design or just a lipstick on the mirror/crayon on wall approach?

There are no "...extra CFM...".. The furnace blower's cfm rating is at a specified static pressure. In your case that pressure is 0.2" water column, and it's 1025cfm @ 2"w.c., not 1250, probably it's low blower speed. It's not an absolute number. At high speed (for cooling) it's rated 1050cfm @ 0.5" w.c. . If the duct impedances are lower (fatter ducts, fewer turns & restrictions) the static pressure will drop a bit, the cfm will rise a bit.

Dana, thanks for clarifying the blower CFM. Will call Savoy Engineering who did the Manual J and D. Maybe they made a mistake and the first wye should be a 10 x10 x6 instead of a 10 x8 x6

 

[Eweneek1]

Forgot to ask about the three way split on branch run on the right side going to the Office. Used a 10x10x5 wye and then a 10x6x6 wye at the end. Does the distance between the two wyes matter?

 

[Eweneek1]

Forgot to ask about the three way split on branch run on the right side going to the Office. Used a 10x10x5 wye and then a 10x6x6 wye at the end. Does the distance between the two wyes matter?

Got an answeer back from Savoy who did the Manual D. Here is what they said, "Manual D looks at the velocity of the duct, not the CFM. The 10” duct you speak of has a velocity of 603 fpm. Manual D requires the velocity in a sheet metal Supply rigid duct to be < 900 fpm so it passes Manual D and does not need be to increased in size.
The 8” duct you speak of has a velocity of 887 fpm. Manual D requires the velocity in a sheet metal Supply rigid duct to be < 900 fpm so it technically passes Manual D. However we agree maintaining the 10” until the 1st kitchen 8” take-off would be a better design.

The distance between the wyes should not matter".