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HVAC Duct System

I have a Laundromat with a 30 year old hvac duct system. The indoor air handler and electric coil (for heat)  is located in the basement. The compressor is outside in the yard.

My question pertains to the return duct. There are (2) separate fresh air ducts that tee off the return duct and terminate outside. They both have electric dampers. One fresh air duct is connected on the first floor return and the second is connected to the return directly before it enters the air handler.

Can anyone explain why this is necessary? Can I eliminate it and install indoor return grills? (I do not see any indoor return grills in the Laundromat but they might be inside the drop ceiling) The system is about 30 years old and it was originally installed for a Chase bank.


  • Techman
    Techman Member Posts: 2,144
    Laundromat AC

    Those dampered return ducts might be an "economizer" of some sort, using cool/cold air to cool the building . You have a little bit of understanding to do here. Your using the building ,the "use" of the bldg is using "MASSIVE" amounts of outdoor air,unconditioned outdoor air Each cubic foot of gas burned requires upto 20 cu.ft. of air for  the boilers, driers, and exhaust air. If you live in hi humidity areas the AC will not have any affect on the humidity. You will need a lot of AC
  • Spence
    Spence Member Posts: 316

    If there are no other returns, you may have all outdoor air as a return with the negative issues as explained above. You will need some OA to satisfy the code requirement for ventilation. Ask your local HVAC professional to help with OA amounts, load calculations (commercial) and duct sizing based on your loads and the capacity of your blower.
  • Positive Solution
    Positive Solution Member Posts: 20
    Indoor Return

    On this same system, I found the indoor return. It is located above the drop ceiling in the Laundromat (1st floor). There is a blower fan right after the return louver (on the 1st floor) as well as a blower inside the air handler (in the basement). Does it make sense to have 2 blowers in series? Also, is it okay to locate the return louver between the ceiling joists and the drop ceiling tiles? 
  • Techman
    Techman Member Posts: 2,144

    Two blowers in series? Usually, no! Return duct above a hung ceiling? Usually, no! Are you sure the blowers are not in parallel? How does the air return from the room ,thru the ceiling and into the ducted return above the hung ceiling? Usually ,above drop ceilings there are lots and lots of wirers, those wires are the "no" in ,"usually no"! Then there usually is some sort of "fire shutdown" w/ any system over 5tons,2000CFM of air.
  • Spence
    Spence Member Posts: 316
    Laundry Return

    Since you haven't mentioned ducts, you may be describing a ceiling return plenum. This brings up a host of code issues if not installed correctly. This I assume, a you mention a second blower at the grille. Enclosing a joist space for a return will only give you 150 CFM if you're lucky, and it will throw your building pressures out of sync.
  • Eugene Silberstein_2
    Eugene Silberstein_2 Member Posts: 349
    Ventilation requirements

    I do not have my copy of ASHRAE Standard 62 handy, but I am relatively confident that the second outdoor damper is to provide the extra outside air needed to satisfy the requirements for ventilation.

    You mentioned that the space was previously a bank and is now a laundromat. This makes perfect sense since the ventilation rates are higher for a laundromat than for a bank.

    These dampers are likely set to a minimum position to provide the required ventilation for the structure and one was likely not sufficient to meet the ventilation requirements.