tableclocks Posts: 404, Reputation: 2 Full Member #1 Jul 1, 2020, 08:07 PM
hvac duct size
i know my duct is to small 20 x 8, it runs one way from the furnace 23 feet, is 20 x 12 ok for my 100,000 btu, 12000 cfm furnace
if it was running two ways, 12 foot each way, what size should it be ??? thank you for your help and time, richard
 talaniman Posts: 54,297, Reputation: 10854 Expert #2 Jul 2, 2020, 04:39 AM
https://www.hunker.com/13418400/how-...20duct%20size.

A duct sizing calculator may then be used to determine appropriate duct size.
A lot of measurements to take and plug in.
 tableclocks Posts: 404, Reputation: 2 Full Member #3 Jul 2, 2020, 07:12 AM
will the 12 x 20 be better then the undersize 8 x 20
trunk line runs one way from the furnace, 23 foot to heat one floot house is 35 x 35,
i have a 8 x 10 to the main LR, 6 rd, 6" to the other RMs, one 8" rd
soooo, is the 12 x 20 ok???
 talaniman Posts: 54,297, Reputation: 10854 Expert #4 Jul 2, 2020, 07:29 AM
What are your total square feet so the CFM can be calculated?
 tableclocks Posts: 404, Reputation: 2 Full Member #5 Jul 2, 2020, 10:25 AM
1300, one floor
 talaniman Posts: 54,297, Reputation: 10854 Expert #6 Jul 2, 2020, 11:16 AM
Sorry I should have asked for cubic feet which includes the height on the one floor.

Maybe this chart is easier to use

https://hvacdirect.com/hvac/pdf/Fiel...zing-chart.pdf

My guestimate is 22x12 or 20x12 depending on the height we are talking about.
 tableclocks Posts: 404, Reputation: 2 Full Member #7 Jul 4, 2020, 10:21 AM
about 35 ft x 34 ft x 8 ft ceiling
lr is 12 x 16,one 8 x 12 reck,,, 2 BR one 8 " rd, other is 6 " rd
kit and din rm is 11 x 35,with 0ne 8" and 2-6" rd, Bath rm, one 6" rd
looks like i have about 1150 cfm at the registers, main trunk is 20 x 8,
getting ready to make ie 20 x 12
i will consider and of you ideals and thoughts
line is insulated with the silver back 1 " stuff
THANK YOU, THANK YOU
the 8 x 14 reck is at the end of my 23 ft run
 tableclocks Posts: 404, Reputation: 2 Full Member #8 Jul 5, 2020, 07:29 AM
about 35 ft x 34 ft x 8 ft ceiling
lr is 12 x 16,one 8 x 12 reck,,, 2 BR one 8 " rd, other is 6 " rd
kit and din rm is 11 x 35,with 0ne 8" and 2-6" rd, Bath rm, one 6" rd
looks like i have about 1150 cfm at the registers, main trunk is 20 x 8,
getting ready to make ie 20 x 12
i will consider and of you ideals and thoughts
line is insulated with the silver back 1 " stuff
THANK YOU, THANK YOU
the 8 x 14 reck is at the end of my 23 ft run
 talaniman Posts: 54,297, Reputation: 10854 Expert #9 Jul 5, 2020, 11:22 AM
Before you go buying stuff can I please ask what you are trying to accomplish here as you sure have a big furnace to begin with.
 tableclocks Posts: 404, Reputation: 2 Full Member #10 Jul 5, 2020, 02:23 PM
i think i need more air flow from the furnace, my main trunk line goes one way 23 foot and it is only 20 x8, think i need about 20 x 12
rheem 100 btu, 80%
i have all the stuff i need at very little cost
my return is 24 x 16,, 3 ton air, thats where i think i need more air movement
 talaniman Posts: 54,297, Reputation: 10854 Expert #11 Jul 5, 2020, 04:20 PM
Let me know how it works out. Good Luck.
 tableclocks Posts: 404, Reputation: 2 Full Member #12 Jul 5, 2020, 06:21 PM
thanks, after all the reading, do you have a opinion?
do you think 20 x 12 is better then 20 x 8 ??, thank you
 talaniman Posts: 54,297, Reputation: 10854 Expert #13 Jul 6, 2020, 01:25 AM
What works best with your system is what you want, and as far as duct work goes too small is as bad as too big, and location and conditions play as big a role in matching ductwork to furnace efficiency as insulation, as maintenance. They all work together and you do have a rather large furnace for the house you have. LOL, I suspect that you replaced a smaller one with this one you have now, which makes the existing duct work way to small. There must have been some noticeable changes in performance and efficiency or COSTS that prompted this project, and I was curious if that was the case. How old is the house?

Plus I tend to be nosy and all up in peoples business concerning such things. 8D
 tableclocks Posts: 404, Reputation: 2 Full Member #14 Jul 7, 2020, 04:22 PM
can we get to the bottom lie, is 20 x 12 better then 20 x 8 ??
for my 100, 80% btu furnace, 3 ton air with the registers i have listed, about 1300 cfms coming out, one floor
i thank you for your input !!!
 talaniman Posts: 54,297, Reputation: 10854 Expert #15 Jul 8, 2020, 05:03 AM
20x12 should be fine for your furnace, 22x12 and 24x12 works as well for such a huge furnace, if you have the room for it. To be honest the furnace is oversized for the house.
 tableclocks Posts: 404, Reputation: 2 Full Member #16 Jul 8, 2020, 07:49 AM
thank you, my wife is either hot or cold, so .....thank you
 talaniman Posts: 54,297, Reputation: 10854 Expert #17 Jul 8, 2020, 08:54 AM
Originally Posted by tableclocks
thank you, my wife is either hot or cold, so .....thank you
LOL, know exactly how that works, but I open and close vents and doors.

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