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Thread: low pressure at vents

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  1. #1
    DIY Junior Member toohott7718's Avatar
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    Question 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"

    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 nd 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

  2. #2
    Retired prof. engr. gator37's Avatar
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    toohott
    A few questions. Do you have an electric heater section (30KW) or gas furnace?
    If you have a 4 ton, you should have a nominal 1600 CFM. Your supply opening off the unit should transition to something on the order of 20"x14" or 20"x12" main trunk line, for straight cool w/ electric heat and probably something on the order of 20"x16" for a true gas furnace. It really depends on what pressure your unit can work against.
    What size unit did you replace..... 2.5 ton?
    Sounds like you have a duct size problem.
    Stepping down duct sizes for your existing duct system only add external static, more pressure drop for your system to overcome. Even your return duct of 8"x20" is to small if I understand you right.
    Can you do a sketch of your existing duct system system with sizes ands lengths and post it? Include the manufacture and model number of your new unit.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The air handler can only move so much air through a specific duct before the back pressure, noise, and resulting friction get to the point where you've maxed things out. Making the duct(s) smaller won't help.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member toohott7718's Avatar
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    I believe the ac unit before was a 2.5 or 3 ton and the furnace was a 20kw (electric) the supply is 48' long and the return is 28' long. all supplys ran off main trunk line is 6" pipe to a 4x10 register. I made a sketch and scanned it but I dont see an upload file butto. thanks

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    DIY Junior Member toohott7718's Avatar
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    I think I figured out how to upload
    Attached Images Attached Images  

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    DIY Junior Member toohott7718's Avatar
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    I couldnt draw it exactly how it is but the supply and the return are runningside by side like most homes. thanks again!!!

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    DIY Junior Member toohott7718's Avatar
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    whoops sorry I forgot to post that part the ac is an arcoaire model #nac048aka1 and the furnace is suppose to be a goodman ( thats what guy told us he was putting in) model# nfcp4800d1 there was also another similar one right below that #abf4800d1

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    Retired prof. engr. gator37's Avatar
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    How many 6" take offs do you have? I need to see how to send a message also. Not sure if I can post an my email address or not but will check.
    Do you have the model and manufacturer of the new system and is it a package unit or split system. Sounds like a split (indoor unit and outdoor unit)

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