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Old 09-23-2010, 10:06 PM   #1
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low pressure at vents


we have a 2400 sq ft house in missouri, when we first bought it new it had a builder grade furnace and ac. The furnace always ran and our bill was rediculously high. So we replaced with a bigger furnace and a/c furnace is a 30 kw and I believe the a/c is a 4 ton unit. We have never had alot of pressure at any of our vents before or after our new system was put in. Our main trunk line is 48' long and its 8"x20" the whole length. the plenum is 16.5"x22
the return air is (the box the furnace sits on) 28"x36"x10" then connects to a 10"x25" that goes up to the return trunk line which is 8"x20"

All the runs from the trunk line is 6" round pipe to 4x10 registers, 14 vents upstairs and 10 vents downstairs

I wanted to know if I stepped down to 8"x16" or 18" for a section or two on the main trunk line for my supply, would that up my cfm to all my vents? I have asked several hvac guys and noone will give me an answer or is willing to come out.
thank you for your help!! sorry if Im not using the right terms. thanks again, shawn

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Old 09-24-2010, 05:07 AM   #2
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low pressure at vents


Your 8X20 trunk line is more in line with what a 2 ton system would be attached to.

If you want better air flow for that probably now over sized 4 ton unit. Your duct work should be more in the range of a 8X32 at the furnace. And then transition down a step every 4 to 6 supplies.

Your old unit was starved for air, and so is your new unit

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Old 09-24-2010, 02:41 PM   #3
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low pressure at vents


I have about 14' exposed trunk line can I change that to the 8x32 trunk line? and leave the other remaining 8x20 trunk line in place because it is covered by drywall? will that work? thank you for all your help
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Old 09-24-2010, 04:52 PM   #4
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low pressure at vents


Is that 14' of exposed trunk line. The first 14' starting at the furnace.

You also need to increase your return. Can't blow out what the furnace can't draw in.
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Old 09-24-2010, 05:22 PM   #5
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low pressure at vents


the furnace is in an unfinished area, that is what im calling exposed. that is about 14' and the return is about 10' what would I need to make the return? 8x32 also? thank you everyone for all your help
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Old 09-24-2010, 05:32 PM   #6
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low pressure at vents


Yep. Same size atleast.

The return being large is better. But the same size will still be a big improvement over what you already have.
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Old 09-24-2010, 06:00 PM   #7
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low pressure at vents


so if for now I just change the ductwork in the unfinished area (supply and return) I should still get alot more airflow then what Im getting now though. Right? thanks again
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Old 09-24-2010, 08:13 PM   #8
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low pressure at vents


Yes, it will increase it. How much, depends on how many supplies are in that first 14'. And how many returns.
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Old 09-24-2010, 09:34 PM   #9
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you said it should have an 8x32 supply and then step down every 4-6 supplies, what would it step down to each time. thank you
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Old 09-25-2010, 04:25 AM   #10
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low pressure at vents


That would depend on how many CFM those 4 to 6 supplies remove from it.

Since your only able to get to 14'. Probably not going to want to reduce it much, so that the other section still gets more air then it currently is. After 6 supplies, if none of them are a bathroom, might drop to 8X28.

Then reduce from that 8X28 to the size of the duct that is in the drywalled ceiling.
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Old 09-25-2010, 03:19 PM   #11
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low pressure at vents


I have only found 1 company online that sells duct work and the biggest they sell is 8x30 . do you know of any companies online that sell duct work bigger than 8x30? I drew up sketch of the layout of my furnace supply and return ducts and all the supplies so you can get a better idea what I have. I will upload it when I get back to my house in a couple hours. thank you again for all your help and knowledge

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Old 09-25-2010, 05:01 PM   #12
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You may not find any online places that sell 32X8. Might need to use a local fabrication shop.

I'll look at your drawing when you post it.
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Old 09-27-2010, 03:06 AM   #13
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low pressure at vents


here is a layout of my duct work. thank you
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low pressure at vents-ductwork.jpg  
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Old 09-27-2010, 08:42 PM   #14
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low pressure at vents


I finally found a site that sells 32x8 duct work and their really helpful
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Old 09-28-2010, 04:27 AM   #15
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I'll post more info tonight.

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