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Thread: low pressure at vents

  1. #16
    Retired prof. engr. gator37's Avatar
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    Doesn't look like there is much you can do without a major renovation of the duct system. Anything made bigger would help but I doubt if it would ever achieve optimum. Next time you up grade consider two systems. Sorry Shawn

  2. #17
    Nuclear Engineer nukeman's Avatar
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    You know, it might be something as simple as the fan speed not being properly setup. Is it a model that automatically adjusts the fan speed or is it a fixed speed setup (one speed for AC, another speed for heat)?

    Did they do a manual J calc to figure out the heat load before changing the size of the unit? There are a lot of factors, but the 4 ton seems like it might be too big. I live in a similar climate, larger house (~3100 sqft), and probably older house (1968). Anyway, I have a 3 ton AC and 24 kW electric furnace. It is a 2-storey house w/ finished basement. There are two zones (controlled by motorized dampers, only single air handler). Upper floor on one zone, the main floor and basement on the other zone. Like I said, there are a lot of other factors too, so the only way to know what size of unit to use is to do a manual J.

    If they did do a manual J and figured that you did need 4 tons, then they should have done an additional calc to make sure that your ducts were sized to support a 4 ton unit.

    In terms of the electric furnace, the larger (or smaller) unit won't save you any money. With electric, heat is heat, so a larger unit will run less but will also use more power when it does run. You might save a tiny amount with the blower running less, but the power that the blower uses is nothing compared to what the actual heaters pull. I am surprised that you didn't go with a heat pump or gas furnace. The only reason I am still on the electric furnace is that the AC unit is still good. Once it dies, I'll go for the heat pump. We live in pretty mild climates, so a heat pump does pretty well most of the time. As you know, the electric furnace is very expensive to run.

    Some other things to look at:

    - check your filter(s). Even if clean, some types cause a large pressure drop. You might want to try a cheapie filter (the ones that are under $1) to see if that changes the flow
    - besides the supply size, the return also has to be large enough to support the flow. Look at all of the returns (grilles) and check their size. Make sure that none are blocked. Sometimes, they will also install filters at large returns. Maybe there is a clogged filter in the system that you never knew about?

    - get a tech to check the blower speed, static pressure, etc. This will tell you if the air handler is setup properly and if the ducts are sized correctly.
    Last edited by nukeman; 09-24-2010 at 07:27 AM.

  3. #18
    DIY Junior Member toohott7718's Avatar
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    the blower has 3 different speeds but it is set on high

  4. #19
    DIY Junior Member toohott7718's Avatar
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    the guys who installed it was my brothers friend and his dad, which the dad has been doing for like 20 years. he basically just told us what we needed and no calculations were ever made. I know now these guys dont have a clue what their doing especially since they didnt changed any ductwork.

  5. #20
    DIY Junior Member jaycanter's Avatar
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    Pull the blower panel off & run the blower see if that makes a difference most of the time i find the returns are undersized that will tell you if it's supply or return problem unless you can get somebody with a flow hood to actually test how much air is moving.

  6. #21
    DIY Junior Member toohott7718's Avatar
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    I removed the cover from the blower and it doesnt seem that anymore air pressure is at any vents

  7. #22
    DIY Junior Member jaycanter's Avatar
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    That says the problem is in the supply an 8x20 duct at 1600 cfm for 4 tons of cooling is .25 static which is o.k but leads me to believe it's the taps off of the supply. how many runs come off & what size are they?

  8. #23
    DIY Junior Member toohott7718's Avatar
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    there is 25 total runs and there all 6"

  9. #24
    Retired prof. engr. gator37's Avatar
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    Based on what you said 1600 CFM and 25 six inch vents the six inch ducts are fine.
    (~64CFM thru a 6" duct is sufficient, assuming you unit is OK, it is your supply and return size.)
    Note: the mention of .25 SP is .25" per hundred feet of 20"x8" straight duct. This does not count the grill pressure drop, filter loss, elbows,transitions, 6" duct, plus the similar losses for the return system. I doubt if your unit puts out 1600CFM at a .75" external SP but something less.

  10. #25
    DIY Junior Member toohott7718's Avatar
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    how exactly would I know what cfm my furnace puts out, the guy who installed it said it should be 2000cfm, but Im not sure and like I said before these guys didnt know what they were doing as I know now. but as far as for the sp how would I know what my system is at. is there a meter that would test that. Im not sure if the meter I plan to buy will tell me that but Im going to get a meter that tells me how much cfm each vent is putting out. thanks again

  11. #26
    Retired prof. engr. gator37's Avatar
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    Try searching for "measuring external static pressure" on the web. One page that gives you an idea is http://contractingbusiness.com/enews.../cb_imp_70061/

    To give you an example for air volume of air look at the following: http://eetd.lbl.gov/ie/pdf/LBNL-5155...omeowner%22%22

    The contractor should have paperwork similar to that for the Trane equipment below, giving you an idea of what your unit is capable of. As an example look on pg 8 of this web page for a Trane unit http://www.trane.com/CPS/Uploads/Use...7_10012009.pdf (static pressure vs cfm, note the heating section if external to the AHU also has a pressure drop assciated with it that has to be overcome by the AHU).

  12. #27
    DIY Junior Member toohott7718's Avatar
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    so what size duct would you reccomend for the supply and the return? or do you need a drawing of what my setup looks like

  13. #28
    Retired prof. engr. gator37's Avatar
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    Need a plan view rough sketch of the trunk lines and the approximate location of each of the 6" duct in reference to each other and approximate length in feet between the 6" ducts totaling the 48' of trunk line. (Some of the 6" ducts may be acrossed from each other or off set.)
    You still have my email if it would be easier. I can accept pdf older versions of autocad or smartsketch.

  14. #29
    DIY Junior Member toohott7718's Avatar
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    I emailed the sketch to you and I will also post it here, it might be too big a file to upload but I'll try. thanks again!!
    Attached Images Attached Images  

  15. #30
    DIY Senior Member Hube's Avatar
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    25 supplys at 6" apiece adds up to (28" x 25= 700 inches of air. But the main is only 8x20= 160 inches. That means the MAIN DUCT is much TOO SMALL to handle this amount of supply takeoffs.
    You need a pro to come in and size the supply AND return properly.
    Last edited by Hube; 10-02-2010 at 06:33 AM.

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