Replacing furnace - wondering about flex duct sizing

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JimLS

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Have an attic furnace/AC that has a maze of 6" R6 flex duct. 100k standing pilot furnace and 3T (I need to check this to be sure) AC. Planning to replace the furnact and questioning the ductwork. The existing system has just a box on the inlet and outlet and immediately transitions to many 6" flex ducts. 7 on the inlet side and 14 on the outlet. 3 of the returns are only about 4' long and go to the main return but the others are fairly long. This looks to be undersized by quite a bit.

What got me thinking about this is that I have had some recent trouble with furnace filters clogging in about 3 months enough to seriously reduce the airflow. This is in a rental house and haven't noticed this with prior tenants. These have a humidifier and I am using 3M filters that are a bit finer than the cheap ones. Both these things add to the restriction. I am thinking that the filter restriction along with the undersized ducts is just too much for the blower. The coil is clean so no obvious issue with restriction there.

Wondering what the best approach to improving this is. I am going to check the size of the return grills and think they may need to be increased also

The furnace is just above the floor in the attic. I could go with larger round rigid or flex duct or make the duct out of duct board to extend a trunk the length of the house and just bring the 6" lines off of that, increasing the number/size of returns.

And plan to go with R8 on the ducts, at least the new ones. Since the furnace will be more efficient and the duct losses will be less I figure an 80k BTU, 80+ furnace will be as effective as the current unit which has been ok for heat output. AC was recently redone so not planning to change that.

Looking for suggestions, mainly on the ducts...
 

Tom Sawyer

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To do it right, you will need a heat loss/gain done and only then can you properly figure the room by room requirements.
 

DonL

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Why not change the filters more often ?

Changing the duct work is not likely to keep the filter from plugging up, Could even make maters worse.

The more air you move, The more dust you collect.

Good Luck on your project.
 
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