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Thread: Static Pressure

  1. #1
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    Static Pressure

    I recently had a 17 SEER 3 ton Gas/AC 2 stage system installed. I upgraded a 3 ton system. I have a 1700 square foot floor and I was told that the static pressure was high. I was told should be around .50. Believe it is around .8. The service guy cut a return in my basement, which is not part of this system.
    My questions are...
    1. Is cutting a return duct in the basedment ok (pros/cons).
    2. What is the tolerence for static pressure, as I want to get my 17 SEER but I don't want to overkill? Do I need .5 exactly, or is .6 OK, etc.?

  2. #2
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    DO you have natural draft appliance like a gas water heater in the basement? Do you have any supplies in the basement?

    The return and supply needs to be balanced on all levels, and static pressure is impacted by both resistance in the supply ducts as well as return. Its' also affected by the air filter size and type. 0.5" is ideal. 0.8" isn't terrible, but could be improved. As the air filter gets dirty, it will likely increase.

    At 0.8" you're no longer getting 17 SEER. Actualy matched with a furnace a "17 SEER" condenser is usually 16 SEER. Goign form 0.5" to 0.8" static probably increase pwoer consumption by about 200 Watts or so, so you probably lose around 5-7% total system efficiency, or almost 1 SEER.


    for example I replaced a 3.5 ton unit with undersized ductwork with a 2 ton unit in my own home and my static pressure is running around 0.35" in high stage so I'm probably getting a "bonus" 0.5 SEER above the nominal rating.

  3. #3
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    Jul 2012
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    Thanks for the response. Yes, I have a brand new Gas Hot Water heater right beside this Furnace. The return they placed in the trunk line is proably 5' away from the gas source.
    I have one of those fancy 4" filters on the unit. When we opened the door/removed the filter, the pressure went to @ .6.
    I have a Bryant Evolution variable speed Furnace (315AAV), a 127 AC, a CNPVP4217 coil, which told me 17 seer. So you think I'm down to 16 now?
    I'm trying to determine am I loosing minimally, or is their a significant payback by having additional returns added or other solution? (of note, I'm not sure where we would install a return)

  4. #4
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    May 2012
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    Your blower fan can give you higher airflow when you have lower static pressures. If your blower fan is designed to see 1200 CFM at 0.5 external static pressure, then at 0.8 you might only be able to see 1000 CFM. Less airflow will mean some rooms won't be as comfortable. Depends on the size of your house and ceiling heights.

    I don't know the layout of the system serving the basement... but your return and supply should be balanced, a little positive pressure is fine but too much and your coils might be starved of airflow and freeze up.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  5. #5
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    The longer your duct run the more static pressure you need. Additional returns will add static pressure to your system. But you also want to balance your supply and return air.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  6. #6
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    Thanks Sam, but I'm not sure I understand all your reply. The tech that was at my home did use the term that the system was starving for return air.
    I live in Atlanta, 1/2 the 1700 sq ft, are two story. I have a couple far runs that the air doesn't come out very much, nor the new return I added to this area feels like it pulls very hard.
    Would the resolution be to add returns and if I took this direction would that return on investment be worth the chase, or what I'm loosing fairly insignificant?

  7. #7
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    The Return should have been put in the conditioned area of the home where the supply air is, not in an unconditioned basement where vented appliances could back-draft.

    Both Returns ought to be large filter racks; U can't get too much filter area. ACCA says, initial velocity through a clean media filter should be 300-fpm or less, at over 400-fpm low pressure drop 'media' filters loose their effectiveness; a statement by Hart & Cooley in their Engineering Data.
    Last edited by udarrell; 07-17-2012 at 05:16 PM. Reason: Clarity...

  8. #8
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    Quote Originally Posted by Gurney View Post
    Thanks Sam, but I'm not sure I understand all your reply. The tech that was at my home did use the term that the system was starving for return air.
    I live in Atlanta, 1/2 the 1700 sq ft, are two story. I have a couple far runs that the air doesn't come out very much, nor the new return I added to this area feels like it pulls very hard.
    Would the resolution be to add returns and if I took this direction would that return on investment be worth the chase, or what I'm loosing fairly insignificant?
    Basically if you need 0.8 static pressure and your fan can only product 0.5 then you will get less air flow as seen in your far runs.

    I would get a measurement of all the airflow at your supply and return registers. Then see if your supply airflow levels match your return airflow levels. Most houses (at least in California) have a single return... so instead of adding more returns, you might be able to just enlarge your return that is 5' from your furnace. The fancy 4" filter is great for air quality, but seems to add 0.2 in static pressure loss. Maybe the easiest fix is to just replace your 4" filter with a 1" standard filter, but I don't know how your allergies are or it you like the extra filtration.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  9. #9
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    yes, allergies are an issue in my household. My 1st floor has 4 returns presently, plus a 5th in the basement now. But the idea of enlarging the return in the basement makes me think. The guy that installed put on a supply register that closes here, as he said I might like to close in winter and leave open in summer. Thoughts.
    Also yesterday was very humid here in ATL, all my windows had dew on the outside. Is this just due to humitity or is this part of my issue as well?

  10. #10
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    udarrell,
    I'm not sure what you mean by "large filter racks"? My old hot water heater was on a stand, the new one was put on the floor, as the guy told me that code no longer required them on stands, as this back draft issue has gone away with technology improvements. Is your take that this isn't the case?

  11. #11
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    Quote Originally Posted by Gurney View Post
    udarrell,
    I'm not sure what you mean by "large filter racks"? My old hot water heater was on a stand, the new one was put on the floor, as the guy told me that code no longer required them on stands, as this back draft issue has gone away with technology improvements. Is your take that this isn't the case?
    If you have a sealed combustion water heater your criteria will change , but if it is atmospheric back drafting is an issue. There should be no return within ten feet of a atmospheric combustion appliance.

    Is your equipment in the garage requiring the HWH to be on a stand 18 inches above the floor?

    A larger filter rack Darrell was referencing would be using instead of using a 16X20 on one side of the furnace you could use one on each side or increase it to a 20X25 etc.

  12. #12
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    Jul 2012
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    No, the stand was done away with, when I purchased the new appliance (water heater).
    The deal was replaced by a licensed and reputable plumber, so I don't think he took any short cuts.

    I'll need to check on the filter this evening. I have 2 identicle systems now, but there is one difference in the two. One system has a 16x25x4 filter and the other has a 20x25x4 filter. I'm not certain if the larger is in my basement or in the attic. But the HVAC guy did say my upstairs was right at the .5, but we attributed that to less curves and shorter duct runs.

    I thank everyone for thier aid here, I feel I'm learning alot, though I'm not quite sure what my resolution is going to be as of yet.

  13. #13
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    Quote Originally Posted by Gurney View Post
    No, the stand was done away with, when I purchased the new appliance (water heater).
    The deal was replaced by a licensed and reputable plumber, so I don't think he took any short cuts.

    I'll need to check on the filter this evening. I have 2 identicle systems now, but there is one difference in the two. One system has a 16x25x4 filter and the other has a 20x25x4 filter. I'm not certain if the larger is in my basement or in the attic. But the HVAC guy did say my upstairs was right at the .5, but we attributed that to less curves and shorter duct runs.

    I thank everyone for thier aid here, I feel I'm learning alot, though I'm not quite sure what my resolution is going to be as of yet.
    Why was it on a stand to begin with? The only requirement for it to be on a stand would be if it was located in a garage and was required to be elevated 18 inches from source of combustion.

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