Bldg pressure/airflow problems/magnehelic
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  1. #1
    Join Date
    Jul 2008
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    420

    Bldg pressure/airflow problems/magnehelic

    Can you use a magnehelic to verify room/bldg pressure? I find that if i light a match and open the door there is a slight draft to the outside so that tells me that im slightly positive- good. However a have a compuund magnehelic and if i zero it outside, cover/plug the low side port the pressure always drops slightly below 0" ? Ill spare the long history but the reason im checking such things is because the bldg I take care off uses 2 AHUs serving vavs that run off of about 1.8" static. There is hardly any flow to a few of the boxes and inlets at the boxes that are open to the ceiling plenum return where the filters go the air actually blows back at you rather then pulling in. FAQ- rotation is correct on everything, the dampers modulate as they should, there are no restrictions in the duct to push the air back- Ive actually taking runs off to verify the latter, all diffusers are open? Very strange.
    "It's just like it doesn't work. I mean it seems to be ok now, but it usually like never works"

    "Never an always and always a maybe"

  2. #2
    Join Date
    May 2011
    Location
    Fort Worth, Texas
    Posts
    49

    Static PSI

    Can you use a magnehelic to verify room/bldg pressure?
    YES

    However a have a compuund magnehelic and if i zero it outside, cover/plug the low side port the pressure always drops slightly below 0" ?
    You should be correct here, however a dual port manometer might be more useful in your situation, if you have one. (It'll be more accurate)

    Ill spare the long history but the reason im checking such things is because the bldg I take care off uses 2 AHUs serving vavs that run off of about 1.8" static.
    How far open is your OA damper?
    Also, are the 2 AHU's sharing the same SA ductwork? If so, does it have backdraft dampers and are they completely closing?

    There is hardly any flow to a few of the boxes and inlets at the boxes that are open to the ceiling plenum return where the filters go the air actually blows back at you rather then pulling in.
    Check your SA ducting connections. You might have a separated joint in your mainline or multiple breaks in your branches. Also, do all your SA registers have a weak output? - that would confirm whether it's the mainline or branches!

    FAQ- rotation is correct on everything, the dampers modulate as they should, there are no restrictions in the duct to push the air back- Ive actually taking runs off to verify the latter, all diffusers are open? Very strange.
    Your diffusers are fully open and do all of them have good air flow velocity?
    It sould like you have a higher static psi within the RA plenum (ceiling plenum), than you do in the conditioned space. If so, given the motor's not running backwards, as you've stated, you have a break in your SA ventilation. The only way to troubleshoot that is to get a big ol' step ladder and someone to hold it for you, then get up in there maybe with anemometer and something to mark it with and locate your leaks.

    Last note:
    If you're AHU's are tied into the same SA ducting and both are running, measure your square ducting and verify its capacity. It should be large enough to handle the amount of CFM's of both units.

  3. #3
    Join Date
    Feb 2010
    Location
    Cal
    Posts
    1,596
    Quote Originally Posted by dave1234 View Post
    Can you use a magnehelic to verify room/bldg pressure? I find that if i light a match and open the door there is a slight draft to the outside so that tells me that im slightly positive- good. However a have a compuund magnehelic and if i zero it outside, cover/plug the low side port the pressure always drops slightly below 0" ?
    Not sure about the cover/plug deal~High too room or duct, low to outside reference seems to be what I would think
    Ill spare the long history but the reason im checking such things is because the bldg I take care off uses 2 AHUs serving vavs that run off of about 1.8" static(@the box inlet?). There is hardly any flow to a few of the boxes and inlets at the boxes that are open to the ceiling plenum return where the filters go the air actually blows back at you rather then pulling in. Air flow takes the path of least resistance, seems like adjoining spaces are transferring rather than returning.

    FAQ- rotation is correct on everything, the dampers modulate as they should, there are no restrictions in the duct to push the air back- Ive actually taking runs off to verify the latter, all diffusers are open? Very strange.
    Yes very strange. I wonder, do your observations change as doors open & close or various boxes serving other areas open & close?

  4. #4
    Join Date
    Jul 2008
    Posts
    420
    thanks for the great responses. Ill try to answer things to clarify- Im using a -5" to +5" mag gage with a high and low port- i assume thats what you mean by dual port. I zerod it outside as a reference point and covered the low port before going inside with the idea then that only then would I be comparing the bldg pressure (high port) to that of the atmosphere (low port). Not much experiance with magnehelics. The issue of air blowing out of teh filter inlets and not going through the box never changes regardless if the building is using 100%OA or in the middle of summer and using none. Exhaust dampers all work as they should. Also seems to make no difference what the other boxes are doing and as apoint of interest on the other end of the floor (space in question) you could fly a kite there is so much airflow. Ive inspected every inch a supply ducting leading to the problem areas and there are no breaks and if there where just curious as to how that would cause the air to blow out of the filter rack. This has actaully been going on for years. One more thing using the method I spoke of to measure the room static and assuming it was the correct way- how when I did the match test does it clearly draft outside if my bldg pressure tests out at less the atmospheric???
    "It's just like it doesn't work. I mean it seems to be ok now, but it usually like never works"

    "Never an always and always a maybe"

  5. #5
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,330
    Quote Originally Posted by dave1234 View Post
    thanks for the great responses. Ill try to answer things to clarify- Im using a -5" to +5" mag gage with a high and low port- i assume thats what you mean by dual port. I zerod it outside as a reference point and covered the low port before going inside with the idea then that only then would I be comparing the bldg pressure (high port) to that of the atmosphere (low port). Not much experiance with magnehelics. The issue of air blowing out of teh filter inlets and not going through the box never changes regardless if the building is using 100%OA or in the middle of summer and using none. Exhaust dampers all work as they should. Also seems to make no difference what the other boxes are doing and as apoint of interest on the other end of the floor (space in question) you could fly a kite there is so much airflow. Ive inspected every inch a supply ducting leading to the problem areas and there are no breaks and if there where just curious as to how that would cause the air to blow out of the filter rack. This has actaully been going on for years. One more thing using the method I spoke of to measure the room static and assuming it was the correct way- how when I did the match test does it clearly draft outside if my bldg pressure tests out at less the atmospheric???
    First off....I would check what the AHU is seeing in reguards to building pressure. Not knowing what exactly is controlling it, there should be a transducer(if newer) or some pressure switches(if older) at or near the AHU's. Check there with your mag.

    As far as the VAV boxes have air flow out of the filter areas......it sounds like the fans in the vav's aren't running or not moving enough airflow. Common scenario......not enough airflow out of diffusers, someone decides to increase primary airflow to box, mean while it's really a problem with the fan in the vav, and then I show up to find the filters blown off the box.

    I going to guess these are series boxes. You don't happen to have a model number......so can start checking the boxes for proper airflow and operation.

  6. #6
    Join Date
    May 2011
    Location
    Fort Worth, Texas
    Posts
    49
    Quote Originally Posted by dave1234 View Post
    thanks for the great responses. Ill try to answer things to clarify- Im using a -5" to +5" mag gage with a high and low port- i assume thats what you mean by dual port. I zerod it outside as a reference point and covered the low port before going inside with the idea then that only then would I be comparing the bldg pressure (high port) to that of the atmosphere (low port). Not much experiance with magnehelics. The issue of air blowing out of teh filter inlets and not going through the box never changes regardless if the building is using 100%OA or in the middle of summer and using none. Exhaust dampers all work as they should. Also seems to make no difference what the other boxes are doing and as apoint of interest on the other end of the floor (space in question) you could fly a kite there is so much airflow. Ive inspected every inch a supply ducting leading to the problem areas and there are no breaks and if there where just curious as to how that would cause the air to blow out of the filter rack. This has actaully been going on for years. One more thing using the method I spoke of to measure the room static and assuming it was the correct way- how when I did the match test does it clearly draft outside if my bldg pressure tests out at less the atmospheric???
    Im using a -5" to +5" mag gage with a high and low port- i assume thats what you mean by dual port. I zerod it outside as a reference point and covered the low port before going inside with the idea then that only then would I be comparing the bldg pressure (high port) to that of the atmosphere (low port). Not much experiance with magnehelics.
    That is a dual port or compound analog gauge. I'm referring to a digital dual port manometer with hoses. This way you can insert one hose inside and the other outside. Not a big deal, though! You could simply get some 1/4" plastic tubing and do the same thing with your analog gauge to do the test I was recommending.

    The issue of air blowing out of the filter inlets and not going through the box never changes regardless if the building is using 100%OA
    I'm assuming you're saying the air is blowing through the wrong plenums or is that VAV's?

    Also seems to make no difference what the other boxes are doing
    VAV's? If so, using your newly purchased tubing ;-) , measure the DP across the VAV's to see if the dampers are opening or fans are moving any air. You'll have to punch a hole on the in/outlet of them, of course.

    A point of interest on the other end of the floor (space in question), you could fly a kite, there is so much airflow.
    Is this from the RA or SA diffusers?
    I'm assuming it's the SA! If so, you apparently don't have a problem with moving air throughout the SA ducting (no leaks in the ducting).
    What about the RTU, though? Is there any way the SA discharge is getting into the RA plenum either by breaks between them at the unit or possibly by a bypass-controlled damper box?

    Ive inspected every inch a supply ducting leading to the problem areas and there are no breaks and if there where just curious as to how that would cause the air to blow out of the filter rack.
    You said there are no leaks in the ducting leading to the problem areas, but do some of your SA diffusers blow at high velocity or none at all?

    This has actaully been going on for years. One more thing, using the method I spoke of to measure the room static psi - using the match test, does it clearly draft outside if my bldg pressure tests out at less the atmospheric???
    I would forget about the match test! Your match is gonna blow out either way, unless you have equal psi in/outside, although highly unlikely! Quick check, although not highly accurate, when you open the door to the outside does is give resistance or does it blow open with minimal effort?
    If neither, then you're going to need some 1/4" plastic tubing (or whatever size your barbs call for) and stick one hose outside and keep one inside. This will give you an exact measurement between in/outside static psi. Also, make sure to take this measurement at all your exits (at different zones) and write em down.
    Also, I really need to know if your two (problematic) zones are sharing the same ducting.
    Given you've already checked for SA leaks throughout, they should either be segmented or both feed into one mainline - that feeds to the branches.

  7. #7
    Join Date
    Mar 2010
    Location
    Hot Desert SW
    Posts
    27
    What is controlling the exhaust dampers? Is there a return fan in the AHU?

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