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Thread: low pressure at vents

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    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

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    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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    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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    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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    DIY Junior Member toohott7718's Avatar
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    its a split unit, 14 vents upstairs and 10 vents down stairs

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    Retired prof. engr. gator37's Avatar
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    Looks like for what you are telling me that the 6" vents are OK. It appears that the main supply and return are undersized.
    Note: you indicated a townhouse which with one unit you are always going to have a problem satisfying both upstairs and down stairs at the same time without always adjusting the grills. My email is fdraa@yahoo.com.
    Are you able to increase the supply and return duct sizes?
    30KW is a whopper is that what the requirement calculated out to.

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    DIY Junior Member toohott7718's Avatar
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    we have a ranch house, I guess thats the same as a townhouse, normal house with a basement. one half of the supply and the return is in an unfinished area while the other has drywall around it. it was suppose to be wired where the furnace would run at 25kw and on colder days 25 degrees and under it would use the remaining 5kw to warm the home faster. but they never did it. in fact for the first 4 years the furnace was only running 20kw because they didnt run extra power when the new furnace was installed. the 30kw does heat our house well but still were back to the low vent pressure. I know the ac is still too small because it will never cool the house to 74 until the sun goes down

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    DIY Junior Member toohott7718's Avatar
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    when they put in the new furnace and ac they didnt change any ductwork at all either, which didnt make any sense to me either. so now im trying to find some answers. thanks again

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    Retired prof. engr. gator37's Avatar
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    toohott
    From what you have told me so far I believe you will see an improvement in cooling and heating.
    One way to look at it is the unit is trying to satisfy the T-stat but you don't have enough air movement to satisfy the T-stat by bringing the temp to it set point therefor the unit runs and runs. If you could increase the duct size Looks like the trunk line, you would get more air to the rooms during heating and cooling season. Do you have the room and access to the duct system to enlarge the main supply and return? I am surprised you have not had the electric furnace cutoff with the internal safety switch (clixon we used to call them)

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    DIY Junior Member toohott7718's Avatar
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    there is maybe 15-20 feet of supply and 10-15 feet of return that could be changed the other portion is under drywall. but if it comes down to it I might be able to tear that down to hang new duct, I just didnt want to have to go through all that work and mess if something else could be done.
    would it help if I made the plenum bigger and the supply trunk line in the unfinished area. and make the return bigger also.

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    DIY Junior Member toohott7718's Avatar
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    I just measured 14' of supply duct and 5' of return that I have access to, also the plenum looks like it cant go any bigger, it almost covers up the whole top of the furnace
    Last edited by toohott7718; 09-23-2010 at 07:57 PM.

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