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Round duct sizing

414 Views 10 Replies 3 Participants Last post by  Gcraay
I need to replace my square ductwork due to a bad leak rusting it out. It’s a crawl space and I’m planning on going all round duct. Down draft furnace. House approximately 1450 sqft.

I’m looking for pointers on sizing. & On when to reduce. This is an old schematic I made. Ignore the side notes.
The red lines are something suggested to me called a pant leg.
I’ve been told to reduce every two take offs.
I’ve been told a 10” extension past the last take off on both sides.
I’ve been told that the box off the furnace needs to be a certain size.
Any help on the specifics. Looking to make this perform the best it can. The 30’ run worries me. The room at that end is always cold.
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It sounds like you got rule of thumb advice. Would probably work, but to be sure you'd have to know how much air each room needs. (especially the room that's uncomfortable.) To know that, you need a room by room heat load calc. You can try to do one yourself if you haven't already had one done. Www.loadcalc.net
 

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Are you looking to replace just the trunk lines, or the entire duct system?

For the latter, you should do a room by room heat loss/gain calculation to see how much heating/cooling each room needs, and airflow is based off of that.
loadcalc.net is the most simple.

To know rough airflow requirements, what is the full model number of the furnace?
Do you have air conditioning and if so, what tonnage?
Post model number of outdoor unit if unsure.
 
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Discussion Starter · #4 ·
Wow! That’s allot to fill out. I’ll have to take some time on that thanks. I’ll get the info together give me a few days.
I am looking for the entire duct system outlet wise. I figure why not. Plus if I try only partial replacement, I’m sure to find more damage as I go.
But, in your opinion, does doing all the calculations make it much more effective or just the right way? I’m all for the right way, but if we’re squabbling over $0.50 cents a month…
 

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Wow! That’s allot to fill out. I’ll have to take some time on that thanks. I’ll get the info together give me a few days.
I am looking for the entire duct system outlet wise. I figure why not. Plus if I try only partial replacement, I’m sure to find more damage as I go.
But, in your opinion, does doing all the calculations make it much more effective or just the right way? I’m all for the right way, but if we’re squabbling over $0.50 cents a month…
It's not a difference of money. It's a difference of guessing in the dark or calculating it right. Sometimes even a blind horse finds his way.
 

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Discussion Starter · #7 ·
Are you looking to replace just the trunk lines, or the entire duct system?

For the latter, you should do a room by room heat loss/gain calculation to see how much heating/cooling each room needs, and airflow is based off of that.
loadcalc.net is the most simple.

To know rough airflow requirements, what is the full model number of the furnace?
Do you have air conditioning and if so, what tonnage?
Post model number of outdoor unit if unsure.
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I think I did this right. I did a block load. I can do the room by room if it’s best. Can you translate?
 

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It is missing most of the inputs - walls, windows, floors, ceilings, doors.

It must be room by room if you are re-doing the duct system from scratch.
 

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The block load is a sanity check when you do a room by room. Same procedure though.

You need to add your exterior walls, roof, floor, windows, etc. It's in square footage of each. Don't negate the window square footage from the wall square footage.
 
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