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  1. #1

    Question Rooftop unit - shut down during winter?


    We have a Trane TSC120E rooftop unit. Last fall, we heard some sort of loud noise in the ducts. We have been told by the maintenance company that the unit should never run during winter as it may freeze. They installed a thermostat so it shuts down whenever the temperature goes below 5C (41F), which means it doesn't run from november to april.

    We ended up with severe negative pressure problems (building is very tight, Energy Star compliant). We're now being told by another company that it should run 24/7, 365 days per year.

    Who's right? As it is our primary source of make-up air, I feel it should run throughout the year, but if it's not winter-proof, then I don't know what to do.

    Any clues? Who's right -- is it meant to run throughout the year or should it be shut down during winter?


  2. #2
    Join Date
    Jun 2006
    New Jersey
    That's just a standard 10 ton cooling only package unit. You didn't provide enough info...if it has electric heat.

    So it's your source of make up air? Make up air for what? Severe negative pressures aren't going to be solved by a 10ton RTU running it's with the OA damper at minimum position. Hopefully it wasn't installed as a want to be MAU with a manual damper wide open.

  3. #3

    Sorry, here is more info, although the contractor still has to provide more. It is a "12.5 ton TSC120E-THC120E rooftop unit". According to the plan, it should bring 3100 CFM inside (approx 500 CFM/floor x 6 floors).

    I went to see if there was an electrical heater, and the duct goes through something called "Thermolec Electric Duct Heater SC-CTPBEX 100.0 KW".

    It is used as make up air for 82 residential dryers exhausts, powered by 13 Penn Barry fans (538 PC, 0.71" PS, 1725 RPM, 1/5 HP, 115/1/60, Model #DX-11Q). The plan says 450 CFM for each of them.

    The negative pressure problem is probably not "severe", but its consequences are: we have 82 residential apartments in which air is being pulled through the range hood ducts -- so the floors and walls get very cold in the rooms adjacent to the range hood exhausts.

    Not sure if you have enough info, I'm far from being a HVAC expert


  4. #4
    Join Date
    May 2000
    Indianapolis, IN, USA
    The maintenance company is tripping you up. It may not be set up to cool, but if the idea is to bring in outside air and reheat it, it should be running. The last thing I want to smell is a nursing home with negative pressure

  5. #5
    That's what I thought. But what was the noise we heard last fall? And is the unit meant to be used no matter the temperature?

  6. #6
    Join Date
    May 2000
    Indianapolis, IN, USA
    Cooling systems need to have "low ambient" controls if the compressor is going to run for cooling in cold weather. Sometimes that is standard, other units it is field installed. But sounds like this unit will just be sucking in air to be heated by the electric heaters for makeup so it won't be cooling. If there are noises when it runs, get a tech in there, not the maintenance people.

  7. #7
    Thanks a lot! We'll get the unit re-powered... and find a good HVAC tech.


  8. #8
    Join Date
    Jan 2009
    Keokuk, IA
    Units that pressurize buildings (have economizers) are meant to run 24/7. Whether you use a low ambient kit, depends on you buildign cooling load and whether not you have economizers controls, or just ventilation/static pressure control. IF you have a regualr economizer control, you shouldn't need a low ambient kits, but it's not a bad idea and not that expensive.

    Commercial building codes require fresh air ventilation... ans do most heath departments.

    FYI - we have 4 similar units and a old hospital coverted to offices. They run 24/7 and economizers stay open at least 5% for fresh air.

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