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Manual D with slim ducted minisplit indoor unit?

Jerry Liebler | Posted in Plans Review on

In trying to use manual “D” to size the ducts for my ducted mini splits I’ve generated some questions.

To start out with what is the effective length where the air handler’s output goes straight into the main trunk duct, there is no such example, the examples all show a change of direction, and I assume, with reservations, that the duct starts at the boundary between the air handler and the trunk duct?

Next question, which may be unusual but I have the situation, If the branches all occur in pairs on opposite sides of the trunk, is it fair, proper and correct to mentally divide the system into two parallel systems where each has branches from one side only ( thus the number of “downstream” branches in section 2 is halved.)?

FWIW I have a 25′ trunk that is 8″x16″ and 4 ea, 18′ branches of 3 1/4″x12″ each ending in a 12″x6″ ceiling register with un-duct ed return. I believe I squeak under a 0.2″ static bar. operating at 360 CFM

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Replies

  1. User avater GBA Editor
    Martin Holladay | | #1

    To the best of my knowledge:

    Q. "What is the effective length where the air handler's output goes straight into the main trunk duct?"

    A. If the register is at the end of the straight run, and there are no takeoffs or elbows, the effective length is the length of the duct.

    If there is a takeoff or an elbow between the appliance and the register, the effective length is the length of the duct(s) plus the effective length of each takeoff and the effective length of each elbow.

    Q. "If the branches all occur in pairs on opposite sides of the trunk, is it fair, proper and correct to mentally divide the system into two parallel systems where each has branches from one side only?"

    A. No.

  2. Jerry Liebler | | #2

    Martin,
    Thank you. I've been using :https://www.acca.org/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=7fc1b1b0-d18d-8eb1-7949-3fa6ed32d50f
    I'm confused a bit by your statement. It is my reading that each register has an effective length that is the sum of trunk length, branch length and fittings from groups 1-- 12 and the longest of these is used as the system effective length. With no change of direction from the blower to the "trunk duct" the group 1 fitting has zero length.. I'm using a very simple branch take off, a readily available rectangular duct start collar so my group 2 fitting is type 2d and by using a trunk duct size reduction it 'll either be 50'EL or 40' EL. My register " boot May be either 4n or 4A0 with EL of 45' or 40' So my longest supply "run" is 30' trunk + 20' branch + 40 group 2 + 45 group 4 =135'. With 0.1" available static blower the friction rate is0.074 and I could use 3 1/2"x10" run outs but the cost saving is tiny so I'll use 3 1/2"x12". I can use the low static fan coil units!
    Edit: Here is my "jump duct" return path. https://www.bakerdist.com/search/result/?q=thru-wall
    Plus I'll have a duct ed ERV with inlets from 3 bathroom areas, the laundry room and kitchen. I'm thinking that I shouldn't have jump ducts for the rooms served by the ERV so if all the bathroom and laundry doors are closed it's input will almost all come from the kitchen.

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