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Rooftop unit question

I think Rick is on to it. Look for duct leakage. If you can take a traverse near the unit then the sum of outlets, the difference would be duct leakage.

You would not lose airflow to static pressure losses (beyond de-rating a fixed AHU operating point of course), for what goes into a duct, comes out of a duct. Question is, "where?".

The fact that you had two closed fire dampers tells me that you may have opened up a duct when they slammed shut, assuming they did so when the fan was in operation. Even a small static system (say 0.50-1.0 inches external SP) may well have 3.0 to 3.5 inches total pressure available to handle coils, filters and dampers. When the FD's close, all of that pressure becomes instantly available as sudden static pressure. If you have 2.0 inch class ductwork and no SP high limit, there goes your ductwork, usually at the longitudinal seams.

The other things I would look for is the duct layout, sizes and airflows, even a simple line diagram with duct sizes and associated airflows and SP rate per 100 feet for each section. Quick SP calculation to see if you are even close.

My $0.02



  • Michal
    Michal Member Posts: 213
    Question about a rooftop

    OK, I have a good one here thats driving me nuts. At work I have a roof top unit, its a trane unit, I do not have the tonnage info at hand right now, but will get it. The unit serves the lower part of the building maybe 12 supply registers total. Recently the unit seems not to have enough push behind it as if there is not enough static. I did find 2 fire dampers closed, but had them fixed. The unit has been checked by a local well known westchester company and says its fine, but the Registers do not blow near the quantity that they should at the velocity they should. So if anyone has any insight please feel free to help me out, since my service company thinks the unit is fine. I will try to get more info tommorrow, Thanks all, stay cool
  • rick_45
    rick_45 Member Posts: 13

    is it not cooling ,when unit is on try pulling off cover to fan compartmentshould be resistance, measure supplt and return duct work let us know size,also what kind of business,.do you have a draft hood to measure cfms .have found blocked returns ,crushed ductwork .broken linkages what position is econnomizer .my question is why are you indicating airflow.was establishment rehabbed ,now unit is undersized.

  • larry_15
    larry_15 Member Posts: 55
    Air flow


    Here are a few things you can find out Model number of unit, Static pressure at the discharge of the blower motor, The size of the pulley and the sheave along with the cfm's required for the area. Then we can tell you if it is doing what it is suppose to be doing.

    One thing to check if you think it is air flow is the pullyey to see if it is slipping.

  • Empire_2
    Empire_2 Member Posts: 2,343

    I wonder if you think the air flow was greater when in actuality one of the fire dampers were closed??? Just a thought. If in A/C mode then any restriction should be seen on the gauges. Also, when you fixed the Fire Dampers, did you install the link or just prop them open???

    Mike T.
  • Michal
    Michal Member Posts: 213
    here is the info you guys requested

    Ok, here is the Info, as for me I am a assistant village engineer. Prior I was a mechanical engineer and focused on plumbing and fireprotection as well as mechanical systems. I do not have a draft hood, and I cannot open the unit as if I get caught I will get into a grievence situation with the DPW dept and myself. (as much as I would love to open the cover and get you all the info)

    The unit is on Village hall and serves the Police department. There have been no upgrades or renovations since it has been built.

    The unit is YORK model: D1EG090N16525ECD
    1.20" STATIC

    SUPPLY AND RETURN ducts are 36X14 going up in a shaft. 2 point returns(36x14 flared to 40x8) in the plenum with screens.

    The unit runs, compressors run, the air is cooled, just seems like the quantity is not there.

    The fire dampers that dropped shut were inline, so the first shut then the second on the same branch duct. The FD was a 24x10. The linkages were replaced.

    The HVAC company who has the service contract checked the pully and belts and said its fine
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Out on a limb here

    without all data one responsibly would need, but one has to punt sometimes.

    From the duct sizes you describe, I can see the airflow rate being about 3,000 CFM. Maybe a 7.5 to 10 ton unit depending on amount of outside air. The 090 in the York model number may indicate a 7.5 ton unit (90 MBH).


    The fact that the fire dampers were in series and tripped (close to the unit I assume) reinforces my thinking about duct bursting. Double seal right there at the highest pressure point. The 1.20 inches of static pressure I take as available external including the return. (7.5 ton units may not have return fans; few do and instead rely on the supply fan as prime mover.)

    From this I would think that the total static would be about 2.5 inches, allowing for cooling coil, furnace, dampers, filters as stated. A suddenly closing damper has the full dynamic force of air under motion come to a stop. Something has to give.

    I would check above the ceiling near and above the fire dampers, see if it feels cool and breezy...

    I would also check the temperatures going into the unit and then leaving the unit. If being cooled to the 52-56 degree range when in full cooling, the unit could be said to be running properly at the unit level. If colder than that, say in the 40's or less, you may have a frosted coil (too much outside air) and from that comes less airflow. Often the cycle of frosting starts with reduced airflow so there is some circular logic at work here.

    Check air temperatures first then poke your head above the ceiling. And stay the heck away from the grievance-prone.
  • Michal
    Michal Member Posts: 213
    dampers were not close to unit

    So far I would like to say thanks to all that are helping me. The dampers were far away from the unit and on a secondary branch line, but I will take all this info you all are giving me and i will call the service company back and i will go through all these steps with them
  • rick_45
    rick_45 Member Posts: 13

    just wondering I have commercial buildings and the airflow at ceiling diffusers does not feel like it comes out .check all supplies .when you shut unit off do you hear the duct work move .Is unit cooling areas and did it work good last year and if it is largely oversized and poor duct work could run continously or run long enough to satisfy space temp and not remove humidity.have company come in and do actual airflow and balnce. that will tell you if your cooling atic ,my 2 cents . is duct work installed in unconditioned space ,is it wrapped
  • larry_28
    larry_28 Member Posts: 1
    Air flow

    Michal/ Brad :

    Brad is on the right tract it is a 7-1/2 unit. At 1.20 static pressure you have a maxium of 3000 cfm's. With out the pulley and sheave sizes we can not figue out your rpm's which would tell us your true cfm's. More than likely you have 2200 to 2500 cfm's avaibile to your defusers. Now you have to figure out how many cfm's are required for your system to opperate within the design parameters. One way is to take a heat high raise over the coil and times it by 1.08 then divide it by your BTU rating this will give you the avaibile cfm's. With the required cfm's from a design print you can tell if it is duct leakage our a balancing problem or it is under sized. With out this information it is a guess at best. Unlesss your do a heat loss cal.

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