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  1. #1
    Join Date
    Sep 2004
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    654
    So I still got flooding of my air handler (it's got a catch pan under it so no damage below). Latest theory yields excessive static pressure (vacuum) in the air handler due to too small a return plenum and filter grill. Even though I have the condensate drain line trapped, it cannot drain fast enough with all that neg. pressure in the air handler. Filter grill is a single 24"x24" grill but the return plenum only fits a 20"x20"x1" air filter. I figured this was OK since the 3 ton air handler came shipped with a 16"20"x1" filter. I removed the 16" filter and installed the 20" size in the return plenum box instead. I even lowered the fan speed to low since i wanted the best humidity removal possible...and the unit does not freeze up at that fan speed either.

    So how much neg. pressure can a condensate line accomodate before it just can't drain? The trap I built has the downrun length almost 3 times the upslope length. Any ideas?


  2. #2
    Join Date
    Apr 2004
    Posts
    7,680
    Your trap (as you describe it) should be sufficient for a properly operating system, lowering the fan probably didnt help because now you make more water. Your return static probably should not be any more than 0.1" or 0.2" The coil may be dirty which increases your pressure drop and makes water management difficult.

  3. #3
    Join Date
    Sep 2004
    Posts
    654
    Originally posted by docholiday
    Your trap (as you describe it) should be sufficient for a properly operating system, lowering the fan probably didnt help because now you make more water. Your return static probably should not be any more than 0.1" or 0.2" The coil may be dirty which increases your pressure drop and makes water management difficult.
    Yup, no doubt the lower fan speed must really 'rinse' the evaporator, but apparently the system designer over did it on my new addition and spec'd out 3 tons. I suspect it really only needs 2 tons by the way the 3 ton system cycles now. Cycling did reduce with it on low fortunately. Humidity control is real good actually, typically less than 55% in the house, but there is another air handler that handles the bedrooms and the other side of the house.

    The air handler and complete A/C only system is brand new as of April 2005, the coil is spotless no doubt. I'll see if I can measure the static in the air handler and take it from there.

    Thanks!

  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Model numbers would help.
    Sounds like horizontial,is it?

    What's the return duct size.

    Doc says .1 or .2,I'd say keep it below .4 or better .3.The filter is likely .1,what type filter,if it's 3M ,1" pleated ,change to a standard and the problem will likely ,go away.

  5. #5
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    4,474
    Originally posted by casturbo
    So I still got flooding of my air handler (it's got a catch pan under it so no damage below). Latest theory yields excessive static pressure (vacuum) in the air handler due to too small a return plenum and filter grill. Even though I have the condensate drain line trapped, it cannot drain fast enough with all that neg. pressure in the air handler. Filter grill is a single 24"x24" grill but the return plenum only fits a 20"x20"x1" air filter. I figured this was OK since the 3 ton air handler came shipped with a 16"20"x1" filter. I removed the 16" filter and installed the 20" size in the return plenum box instead. I even lowered the fan speed to low since i wanted the best humidity removal possible...and the unit does not freeze up at that fan speed either.

    So how much neg. pressure can a condensate line accomodate before it just can't drain? The trap I built has the downrun length almost 3 times the upslope length. Any ideas?

    The ideal gross sizing of Return Air (RA) filter grilles: 200 sq. ins. per ton. or 3-ton = a 600 sq. ins filter grille, very few meet the level.

    Another major factor that would increase the negative SP would be under sized supply ducts and/or undersized diffusers, because the return suction can not be satisfied even by outdoor air infiltration. It is better to have a positive pressure on the suppl air side to reduce air infiltration.

    Perhaps the supply ducting and diffusers were sized for a 2-ton application. If the airhandler and blower are oversized in relationship to the SA duct and diffusers you would probably end up with a lot of Negative Static!

    You "add" the negative Static reading to the positive SP reading to get the total External Static Pressure (ESP) of the system.


    http://www.udarrell.com/proper_cfm_b...g_systems.html

    When the equipment application properly meets the heat-load calc, --reducing the CFM is normally not necessary because adequate run-time will efficiently and effectively take care of both the latent and sensible heat-loads. - Darrell


    [Edited by udarrell on 09-01-2005 at 05:07 PM]

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by udarrell
    Originally posted by casturbo
    So I still got flooding of my air handler (it's got a catch pan under it so no damage below). Latest theory yields excessive static pressure (vacuum) in the air handler due to too small a return plenum and filter grill. Even though I have the condensate drain line trapped, it cannot drain fast enough with all that neg. pressure in the air handler. Filter grill is a single 24"x24" grill but the return plenum only fits a 20"x20"x1" air filter. I figured this was OK since the 3 ton air handler came shipped with a 16"20"x1" filter. I removed the 16" filter and installed the 20" size in the return plenum box instead. I even lowered the fan speed to low since i wanted the best humidity removal possible...and the unit does not freeze up at that fan speed either.

    So how much neg. pressure can a condensate line accomodate before it just can't drain? The trap I built has the downrun length almost 3 times the upslope length. Any ideas?

    The ideal gross sizing of Return Air (RA) filter grilles: 200 sq. ins. per ton. or 3-ton = a 600 sq. ins filter grille, very few meet the level.

    Another major factor that would increase the negative SP would be under sized supply ducts and/or undersized diffusers,


    Undersized suppy duct and grilles,don't increase the negative(return) static.They can reduce cfms ,which will make the return static lower.







    because the return suction can not be satisfied even by outdoor air infiltration. It is better to have a positive pressure on the suppl air side to reduce air infiltration.

    Perhaps the supply ducting and diffusers were sized for a 2-ton application. If the airhandler and blower are oversized in relationship to the SA duct and diffusers you would probably end up with a lot of Negative Static!

    You "add" the negative Static reading to the positive SP reading to get the total External Static Pressure (ESP) of the system.


    http://www.udarrell.com/proper_cfm_b...g_systems.html

    When the equipment application properly meets the heat-load calc, --reducing the CFM is normally not necessary because adequate run-time will efficiently and effectively take care of both the latent and sensible heat-loads. - Darrell



    Depends on the Latent load,here in humid Florida,350 per ton,does much better than 400 per ton.


    [Edited by udarrell on 09-01-2005 at 05:07 PM]

  7. #7
    Join Date
    May 2004
    Location
    Rapid City, SD
    Posts
    7,403
    I might have missed the rest of the story... But if you have a high static (higher than normal anyway) in the return, the trap might need to be deeper. This only applies for a negitive pressure coil.

    If it's a positive pressure coil and 3/4" pvc can't drain the water fast enough... turn it into 2" and hook it up to the spinkler in the yard lol.

  8. #8
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,545
    maybe the drain pan is cracked!
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  9. #9
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    4,474
    Originally posted by dash
    Originally posted by udarrell
    Originally posted by casturbo

    So how much neg. pressure can a condensate line accomodate before it just can't drain? The trap I built has the downrun length almost 3 times the upslope length. Any ideas?

    The ideal gross sizing of Return Air (RA) filter grilles: 200 sq. ins. per ton. or 3-ton = a 600 sq. ins filter grille, very few meet the level.

    Another major factor that would increase the negative SP would be undersized supply ducts and/or undersized diffusers, (Darrell)


    (Dash) Undersized suppy duct and grilles,don't increase the negative(return) static. They can reduce cfms, which will make the return static lower.

    (I do not disagree when it comes to lower SP capacity blowers.) It is true that a lot of blower wheels will unload when the supply static increases too much and that would not increase the negative static. However, the newer higher static capacity blowers might increase both the SA and RA static levels. Dash, - am I wrong?

    (Darrell) - because the return suction cannot be satisfied even by outdoor air infiltration (using High static capability Blowers). It is better to have a positive pressure on the supply-air side to reduce air infiltration.

    Perhaps the supply ducting and diffusers were sized for a 2-ton application. If the airhandler and (a high static) blower are oversized in relationship to the SA duct and diffusers you would probably end up with a lot of Negative Static!

    You "add" the negative Static reading to the positive SP reading to get the total External Static Pressure (ESP) of the system "on the blower."


    http://www.udarrell.com/proper_cfm_b...g_systems.html

    When the equipment application properly meets the heat-load calc, --reducing the CFM is normally not necessary because adequate run-time will efficiently and effectively take care of both the latent and sensible heat-loads. - Darrell

    (Dash) Depends on the Latent load, here in humid Florida,350 per ton, does much better than 400 per ton.

    (Reply) (I do not disagree.) However, equipment sizing to the heatload is the critical factor toward achieving adequate run-time. When the latent load is higher, that increases the coil's latent-load capacity and somewhat reduces the sensible capacity which increases run-time, -- even at 400-cfm.

    It does depend on how low you want to get the %RH, -- at the lower %RH levels it will take a lot more run-time and slowing the blower speed would help improve several factors, including lowering the FPM air velocity through the coil.

    [Edited by udarrell on 09-01-2005 at 05:07 PM]

  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by udarrell
    Originally posted by dash
    Originally posted by udarrell
    Originally posted by casturbo

    So how much neg. pressure can a condensate line accomodate before it just can't drain? The trap I built has the downrun length almost 3 times the upslope length. Any ideas?

    The ideal gross sizing of Return Air (RA) filter grilles: 200 sq. ins. per ton. or 3-ton = a 600 sq. ins filter grille, very few meet the level.

    Another major factor that would increase the negative SP would be undersized supply ducts and/or undersized diffusers, (Darrell)


    (Dash) Undersized suppy duct and grilles,don't increase the negative(return) static. They can reduce cfms, which will make the return static lower.

    (I do not disagree when it comes to lower SP capacity blowers.) It is true that a lot of blower wheels will unload when the supply static increases too much and that would not increase the negative static. However, the newer higher static capacity blowers might increase both the SA and RA static levels. Dash, - am I wrong?

    (dash reply)The VS fan will often be able to continue to deliver full cfms,so no decrase in return ESP,but no increase caused by high suppy ESP either.

    (Darrell) - because the return suction cannot be satisfied even by outdoor air infiltration (using High static capability Blowers). It is better to have a positive pressure on the supply-air side to reduce air infiltration.

    Perhaps the supply ducting and diffusers were sized for a 2-ton application. If the airhandler and (a high static) blower are oversized in relationship to the SA duct and diffusers you would probably end up with a lot of Negative Static!

    You "add" the negative Static reading to the positive SP reading to get the total External Static Pressure (ESP) of the system "on the blower."


    http://www.udarrell.com/proper_cfm_b...g_systems.html

    When the equipment application properly meets the heat-load calc, --reducing the CFM is normally not necessary because adequate run-time will efficiently and effectively take care of both the latent and sensible heat-loads. - Darrell

    (Dash) Depends on the Latent load, here in humid Florida,350 per ton, does much better than 400 per ton.

    (Reply) (I do not disagree.) However, equipment sizing to the heatload is the critical factor toward achieving adequate run-time. When the latent load is higher, that increases the coil's latent-load capacity and somewhat reduces the sensible capacity which increases run-time, -- even at 400-cfm.

    (Dash reply)Lowering the cfms is what increases the systems latent capacity and reduces sensible,not increasing the latent load.With lower sensible and higher latent capacity,the run time increases,which helps dehumidify as well.

    It does depend on how low you want to get the %RH, -- at the lower %RH levels it will take a lot more run-time and slowing the blower speed would help improve several factors, including lowering the FPM air velocity through the coil.

    [Edited by udarrell on 09-01-2005 at 05:07 PM]

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