# Manual D with slim ducted minisplit indoor unit?

| 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. GBA Editor
| | #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. | | #2

Martin,