Duct function?

Discussion in 'Shop Talk' started by agmantoo, Aug 1, 2005.

  1. agmantoo

    agmantoo agmantoo Supporter

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    I worked on the AC on 2 mobile homes recently. Both had electric furnaces with A coil AC evaporators added. Coming into the top of the unit in the area where the evaporator is located is a duct about the size of a dryer vent. This duct is connected to a vent on the roof of the mobile home. This duct allows outside air to be blended with the recirculated inside air which is then drawn through the A coil and conditioned to be dispersed throughout the home. Obviously this outside air is hot and that heat increases the load to the AC. What is the main function of this small duct on an electric furnace?
     
  2. beowoulf90

    beowoulf90 Well-Known Member Supporter

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    The small duct allows freash air to be brought into the system. Otherwise you would be breathing stale air all the time and this is not a healthy idea. There should be a volume control on so you can regulate the amount of air coming into the return duct. It should be about 10% of the air needed with the rest coming from the returns. Hope that helps.
     

  3. agmantoo

    agmantoo agmantoo Supporter

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    My observation of most older mobile homes seems that you could throw a cat through the cracks in the fitup of the doors, plumbing, windows if you had them totaled.. To me the designer was overly optimistic when including the need for an outside vent to the electric furnace to provide fresh replacement air. What is the opinion of deleting this vent?
    PS....I see no method to throttle the amount of air coming through the referenced duct. There is a damper that closed when the blower is not running.
     
  4. Nature_Lover

    Nature_Lover Well-Known Member

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    I would not block off that air intake without doing more research on Positive Pressure Ventilation Systems (PPV).

    It draws in a small percentage of new air to mix with stale indoor air, and creates a flow of air out any cat-sized-holes/cracks/pipe holes/unsealed vent hoods etc.
    The air is dehumidified and cooled before it ever gets into the mobile home, if you cut that flow, hot humid air will seep in any cracks, and cause problems with temperature and humidity level maintenence.

    Does this home have an air return duct system, or a vent/grate on the door of the air handler closet?
    It makes a difference:

    If it has a return duct system, humidity will condense and mold will grow in there without a positive air pressure preventing hot humid air from coming in through gaps.

    If it has a simple grate in the closet door, then without PPV, any airtight rooms with the door closed don't get air pumped into them, period. The air pressure in that room remains the same as the rest of the home, and the air is just moved through the rooms with the least resistance. The rest of the home gets all of the air.
    In this case, PPV would actually force a higher air pressure in the sealed room, and force air out to the rest of the home, instead of it being equalized.

    I have simplified this as much as I can, but my main point is, humidity grows dangerous mold, rots organic matter (wood,) and rusts ductwork where you can't see it.
    Don't mess with PPV unless you know all of the consequences.
     
  5. agmantoo

    agmantoo agmantoo Supporter

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    Both of the units have vented closet type doors. The individual rooms have the doors about 2 inches off the floors so the rooms are not nearly air tight. The amount of air allowed to enter thru the discussed duct certainly seems to me to be excessive and that air is unfiltered also. I have seen positive pressure systems on large buildings that prevented major air flow in when large doors were opened often. Frequent door opening is not a problem with these homes as they have single occupants. I cannot in my minds eye see any difference in closing this vent while using the existing AC and using a number of window units set on recirc. The cook stove and the baths have fan vents. I do not want to create a mildew problem but I would like to improve the efficiency of the AC while reduction the utility bill.
     
  6. Nature_Lover

    Nature_Lover Well-Known Member

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    The amount of air drawn in is regulated by the amount of loss via leaks, weatherize the home.

    The PPV system in a mobile home is not intended to prevent hot air from coming in through an open door, it prevents air from entering through leaks, primarily under cabinets, through walls (outlets and window gaps,) and vents where louvers don't seal.

    The energy bill will be higher with the air leaking in, instead of it being cooled and dehumidified upon intake.

    IMHO weatherization is the resolution, not modifying the air handler.

    It's your call, and your liability insurance.
     
  7. tyusclan

    tyusclan Well-Known Member

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    Agman, you're absolutely right that it would increase the efficiency of the unit to close it off, because the unit is having to condition the hot humid outside air instead of just maintaining the recirculated air. If it were your personal home, I would recommend closing it off. If it's in a rental then you have to weigh the possibility of having a tenant who is sensitive to mold. If you should have a complaint of a sick building and the fresh air were closed off, it could cause you a problem.