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  1. #1
    Join Date
    Apr 2008
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    Princeton, NJ
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    581

    Is 0.5 max static pressure a realistic target?

    I ask the question because I am in the process of having my ductwork enlarged. The 0.5 max water column static pressure is what is listed on my nameplate and from what I can tell is the same max for many systems.

    I have a new Trane 4-ton XL15i/ 100K btu-XV95 system in a 2800 sq. ft. 2-story home in NJ. With some key dampers closed the system works great but the static pressure is sitting at 1.1 with a new filter (positive =0.8 and negative = 0.3). I decided to have some ductwork done to get the SP down to nameplate or below to save some energy cost and not shorten the life of the motor.

    My Duct guy said it is unrealistic for people in most homes to think that they can get the 0.5 SP or below. He said some want to get to 0.1 so they will only be using the "75 watt light bulb" amount of energy that the salesman sold them on. He said when you consider ~0.25 for the coil, ~0.2 for a good filter that only leaves you 0.05 for everthing else. He said it is no way that with grills, elbows, turns, etc. no matter how gigantic you make your ducts you will ever get the 0.5 or below value.

    Do you guys agree with that? Or is it that my guy just does not know how to get there? Why would the manufacturers make 0.5 the max if it is unacheivable in most homes?

    Key1

  2. #2
    Join Date
    Sep 2006
    Posts
    1,253
    I have a number of transitions that are creating higher static and branch dampers on all the branches. My 3 ton system has the following:

    1,202 CFM (measured)
    Supply .14
    Return .08
    Filter .125 (20x30 media at filter grill)
    Total .345

    I think it can be done.

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,790
    .5" is unrealistic for many existing homes. And higher tonnage coils/systems.

    But, your can be lowered from tha 1.1", so you can get more use out of a media air filter.

    1.1" is TOO high.
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  4. #4
    Join Date
    Apr 2008
    Location
    Princeton, NJ
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    581
    Quote Originally Posted by mchild View Post
    I have a number of transitions that are creating higher static and branch dampers on all the branches. My 3 ton system has the following:

    1,202 CFM (measured)
    Supply .14
    Return .08
    Filter .125 (20x30 media at filter grill)
    Total .345

    I think it can be done.
    What size are your ducts?

    Do you have an AC coil?
    Key1

  5. #5
    Join Date
    Sep 2006
    Posts
    1,253
    As all women supposedly say - size doesn't matter.

    Your duct system will have to be designed for your situation and no two (or at least very few) are alike. Having said that, the trunk (in attic) is 20x8, it splits into two 8x10 to go around an obstacle and includes several tight 90s (the split and these 90s are the biggest static creators). That part is all metal ducting. Branch runs are six inch flex stretched tight and straight. Any turns required on the branches where they come off the truck or into the boot are done with metal adjustable ells. Return is 22 feet of 18" flex with two 90s and a somewhat funky return plenum (a bit of a static creator).

    System is a heat pump - no separate coil.

  6. #6
    Join Date
    Jan 2004
    Location
    PA
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    mchild.
    Is yours a furnace, or an air handler?
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  7. #7
    Join Date
    Sep 2006
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    Quote Originally Posted by beenthere View Post
    mchild.
    Is yours a furnace, or an air handler?
    Heat pump air handler.

  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    If space is available,and ducts are not buried inbetween floors or walls,.5 should be doable.

    Tell us more about your duct system.

    Pics of the indoor unit and atteched ducts would be a good start.Often improving the fittings near the unit makes all the difference you need.

  9. #9
    Join Date
    Apr 2008
    Location
    Princeton, NJ
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    581
    Quote Originally Posted by dash View Post
    If space is available,and ducts are not buried inbetween floors or walls,.5 should be doable.

    Tell us more about your duct system.

    Pics of the indoor unit and atteched ducts would be a good start.Often improving the fittings near the unit makes all the difference you need.
    I'm sure it is possible but I don't think it is realistic (when a coil is involved). If the othe poster "Mchild" had a coil contibuting ~0.25, he/she also would be over the 0.5 max.

    My duct work is already underway but when it started, the duct guy cut a 14x14 hole in the front of the plenum for a 14x14 additional trunk. We checked the static pressure at that point with no ductwork attached coming out of the front of the plenum (just a big hole) and a 12x9 trunk coming out the top (unchanged) and a 8x9.5 trunk coming out the back of the plenum (also unchanged). The static pressure was 0.79 (versus 1.1 when the 14x14 hole was not there). Next, we opened the filter door and the SP dropped to 0.57. So with a big 14x14" hole in the front and the filter door wide open only 0.57 was acheived. That is why I beleive the 0.5 max is unrealistic.
    Key1

  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Quote Originally Posted by key1cc View Post
    I'm sure it is possible but I don't think it is realistic (when a coil is involved). If the othe poster "Mchild" had a coil contibuting ~0.25, he/she also would be over the 0.5 max.

    My duct work is already underway but when it started, the duct guy cut a 14x14 hole in the front of the plenum for a 14x14 additional trunk. We checked the static pressure at that point with no ductwork attached coming out of the front of the plenum (just a big hole) and a 12x9 trunk coming out the top (unchanged) and a 8x9.5 trunk coming out the back of the plenum (also unchanged). The static pressure was 0.79 (versus 1.1 when the 14x14 hole was not there). Next, we opened the filter door and the SP dropped to 0.57. So with a big 14x14" hole in the front and the filter door wide open only 0.57 was acheived. That is why I beleive the 0.5 max is unrealistic.
    Key1
    14X14 hole, 12X9, and 8X9.5 trunk isn't much in they way of supply duct,if that's what you have,for 4 tons.

    What size system did you have before?


    At what "points" on the equip./duct is the static being tested?

    What's the supply and the return static reading?You are giving us the total of both,I assume.

  11. #11
    Join Date
    Jan 2004
    Location
    PA
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    68,790
    On 4 ton A/C systems installed on furnaces.
    .8" static is usually not too hard to attain. Some times when a 5 ton evap coil is used, .9" is doing good on a swap out, with duct alterations.

    It depends on what you have to work with. And how much money you are willing to spend.
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  12. #12
    Join Date
    Apr 2008
    Location
    Princeton, NJ
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    Positive static is being checked with a magnehelic gauge after blower before coil. Neg static is beging checked in blower bay after filter. Before any duct work was started static was 0.8 pos and .3 neg. for 1.1 total. I do not know what is was on my 30 year old GE system because I and apparently neither did my installer know anything about static pressure.

    My return duct is 24 x 10 entering the blower bay

    Key1

  13. #13
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    Apr 2008
    Location
    Princeton, NJ
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    Quote Originally Posted by dash View Post
    14X14 hole, 12X9, and 8X9.5 trunk isn't much in they way of supply duct,if that's what you have,for 4 tons.
    I have room to also expand the 8x9.5 to 16x8. Do you think that will be enough or is more still required?

    Key1

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