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Humidity issue with new rooftop units

We have a job with two ten ton Trane rooftop units, installed in 2017. The units are heat pumps with electric back up heat. The units are cooling the space fine with no issue. The problem is there is very little humidity removal, and the space humidity is high, over 80% Both unit’s outside air dampers are closed. I was wondering if there is a field installed accessory that can be addded to the units such as a reheat coil, that could be used to help lower the humidity level. In checking with our Trane salesmen, nothing is offered by Trane that can be added to these units. It would have to be third party manufactured, or using the existing unit controls. Any suggestions would be much appreciated, thanks to all.


    EBEBRATT-Ed Member Posts: 8,467
    You can install any type of air heating coil, hot water, steam or electric in the discharge duct to reheat the space.

    You going to have to design a control system so on a call for high humidity you go into full cooling (over cool the space) and then reheat the air to maintain room temp.

    This is a very expensive energy pig to operate. It is usually only allowed in manufacturing plants and labs that must have tight temp and humidity control and is usually not allowed by energy codes for normal comfort cooling.

    A couple of things to check:

    Seal the space as tight as possible
    Make sure that the duct work return and supply are sealed as well as possible. Leaking or a detached supply or return duct in an unconditioned space will wreck havoc on humidity.

    Outdoor damper closed tight? OA damper can let in a lot of OA especially with restricted or too small return ducts.

    Check you air flow and reduce the air flow to 400 cfm/ton

    Most packaged rtus the coils are sized for 20% latent heat removal (humidity) and 80% sensible (temperature) control

    You may need a special unit designed for the load, a split system or a specialy designed RTU.

    There are some units I think Trane makes them that can use hot gas reheat off the compressor for reheat.

    Good luck. Humidity is a lot more difficult to control than temperature

    And make sure the units are not oversized if they are and they cycle on temperature during high humidity you will loose humidity control
  • ratio
    ratio Member Posts: 2,726
    First thing is to find out where the humidity is coming from. My knee-jerk response to something like this is oversized equipment, check the sizing carefully. You may be able to mitigate sizing issues by disabling the 2nd stage cooling on one or both units (although that may have repercussions wrt heating), or setting one unit to fan-only.

    After that, if you really need dehumidification, the best solution is swapping in a unit with humidity control. For some reason, no one takes that option. Assuming that's off the table, there are several ways to get more latent removal out of a unit. Quickest but least effective is to slow the fan down a little. Hardest (IMHO) is adding a Rawal valve in the lead circuit, which is basically hot gas bypass in a kit.

    Probably greatest dehumidification is electric reheat, & it's what we usually end up with on the hard jobs. You've got heat strips in the units, they may be large enough to use for reheat by themselves, but the best results I've gotten were with SCR-controlled electric duct heaters with a discharge air controller that allowed me to discharge neutral air when dehumidifying.

  • Jamie Hall
    Jamie Hall Member Posts: 15,324
    You don't mention where you are, or if this is residential or commercial or industrial. So let's consider.

    @ratio 's question is fundamental to what you need to do, and how. Where is the humidity coming from? And a basic input there is -- what is the dewpoint and air temperature of your outside environment? If you are bringing in any outside air at all, and you really should be to maintain some sort of indoor air quality, the best air conditioner in the world will have trouble if you are trying to get close to or below the outside air dewpoint.

    If there is any humidity source at all -- outside air, people, cooking, whatever -- your equipment has to be able to cool the air to the desired dewpoint, then reheat it to your comfort level, if that desired dewpoint is below the outside air dewpoint.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • zepfan
    zepfan Member Posts: 309
    Thanks to all. The job is in Maryland, and the space is a medical research lab. We did not install the units originally, they should have been spec’ed with some type of re heat coil. These are just run of the mill rooftop, packaged heat pumps
  • ratio
    ratio Member Posts: 2,726
    We've come behind that, & even done it ourselves once or twice I suspect.

    Something I forgot to mention is the possibility of a stand-alone dehumidifier, either ducted in to the RTUs or separate. That's probably more work than adding reheat but less than a Rawal valve.

    EBEBRATT-Ed Member Posts: 8,467
    Or a Munters unit which is one way but big$$$$$$.

    Somebody screwed on the design

    any type of reheat or dehumidifier is expensive to install and operate.

    tighten the envelope as much as possible
  • unclejohn
    unclejohn Member Posts: 1,700
    You can pull in humidity into a supply duct if the humidity is lower inside the duct then outside. Seal it up tight.
  • hvacfreak2
    hvacfreak2 Member Posts: 500
    Did you have high humidity problems in July ( it was a rather hot July this year in the area ) ?

    I think it was mentioned but RAWAL kits may be the answer if this is an over - capacity issue. If they are multi-stage try disabling the second stages to allow for more runtime or a slower approach to setpoint.

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller

  • bio_guy
    bio_guy Member Posts: 69
    The research building that I work in, they tell me, has no return air in the the lab space. Since this is a high humidity area, I have no idea how they keep it at a reasonable humidity with all that air exchange. When the chillers go out, the floors get slippery and it starts to rain from the supply grates right away. I should has the AC specialist how it is done. I know that he has lots of experience with desiccant wheel (Munters) dehumidifiers. If you can reheat with waste heat from the condensers, you obviously have a more efficient system at the end. Note that the pressures are supposed to be pretty tightly regulated here with the lab space negative to hallways and even some spaces in the labs negative to others. (No, it is not BSL-3 which requires monitoring/alarms on all those differentials.)

    To the problem at hand, does the humidity problem track outdoor humidity in an obvious way? If it gets more humid inside on low demand, like cloudy time in the shoulder seasons or at night changing the run characteristics might help. If it just gets humid on any old humid day, drawing in less outside air is more likely to help, but see above.

    You know, all this talk about more air exchange for COVID-19 seems to ignore this very problem. The people that are suggesting it must live in very temperate areas.
  • motoguy128
    motoguy128 Member Posts: 335
    3 phase power I assume? IF so install a VFD on the motor and have two preprogrammed speeds with a dehumidistat to switch them. Some may even take inputs and allow you to slow way down to maybe 325CFM/ton, but use a temp switch when coil temp or DAT falls under maybe 40F or some value.
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