Soffit Removal and Ductwork Modification

[e015475]

I bought a house from the 70s that has an all metal HVAC duct system that runs through soffits and dropdowns in the house to get air distributed in all the rooms. You can't say the ducts are in conditioned space but they are covered in the attic insulation

The last 10' of trunk has registers in the kitchen, dining room and family room,

I'm going to remodel the kitchen and would like to take out the soffit containing the last ten foot of duct and the kitchen, dining and family room registers and replace them with flexible ducts and ceiling registers. My idea was to use the existing register sizes to determine an equivalent flow in a flexible duct then install collars/connections on the metal square trunk duct and route flex duct to the three rooms.

I'd run round sheet metal 90s off the old trunk and keep the flex duct straight and tight to minimize losses. The flex runs would be two five footers and one ten footer

Is my equivalent area between the old registers and modern flex duct too simplistic? There's a few places around here that will do Manual D calculations - should I do that?

This house has a 4T heat pump where it only calls for about 3T so it probably has a little more static pressure in the ductwork

Thoughts?

 

[chinboys]

You are going to have higher flow resistance when switching out to flex ducts compared to the existing smooth metal duct setup.
The addition of elbows and longer lengths of flex ducts will also add to the flow resistance.
Usually, the the blower fan motor in newer HVAC-R systems is smart. That is it knows to speed up or increase the cubic feet per minute in cooling Summer mode. Conversely, and slow the speed down the or lower the ft3/min in heating or Winter mode as it takes less effort to move less dense hot air.
Nothing to do with your heating or cooling capacity but the cubic feet per minute.
You may be able to increase the air flow to overcome the air flow resistance if you redo the power line setup on a blower motor if so equipped.
But you will hear more noise and may feel more heat drafts too.