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  1. #1
    Join Date
    Feb 2012
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    Heat-Humidity recapture from Orchidarium

    I have a unique situation that I am looking for ideas on.

    I grow orchids and have a room (bonus room over garage)in the house dedicated to this activity. It is all fluorescent lighting but also has independent humidification that keeps it 60-80% humidity at all times. The lighting and air exchange with the house is enough to keep the room heated throughout the year...the HVAC vents in this room are blocked to not allow HVAC to enter this room.

    There in a 6" inline fan that exhausts air from the room base controlled by a thermostat...when the room gets too warm the exhaust fan kicks on. In summer this exhausts into a very open area of the attic and out through roof vents. In winter the air is redirected into the main portion of the house allowing recapture of both the heat and the humidity from the room. The duct run is 3 feet in summer and adds and additional 7 feet in winter. Both the duct and fan are insulated from the unheated attic space where they reside. Despite this insulation I am experiencing condensation in both the ducts and the fan during cold periods causing both a concern for health (mold) and potential fan problems if it gets wet. No issues of mold/condensation are seen in summer.

    I have been unable to find a solution that would allow me to recapture the heat and humidity from this area and return it to the rest of the house without the electrical and health concerns. I do not want to exhaust it directly outdoors as then I would need to open exterior inlet vents in winter which would be damaging to the orchids from the cold air entering the space. I am hoping someone may have some innovative ideas for either a product or approach to help.

    Thank you,

  2. #2
    Join Date
    Jan 2004
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    Needs to have a damper that closes when the fan isn't running.
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  3. #3
    Join Date
    Jul 2004
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    In order to get informed assistance, we'd need to know whether the condensation is occurring when the fan is in operation or when the fan is not in operation. The relative humidity and the dew point of the air within the duct is the issue. The attic is apparently cold enough that despite the duct being insulated, the air within the duct is cooling sufficiently to reach the dew point of the air, which by your statement is fairly high in relative humidity.

    You need to have someone measure the temperature of the discharge air from the duct into the home as well as the relative humidity and dew point. Then measure the temperature of the attic. If the difference between the DP and actual attic temperature is close, (e.g. attic 35 and DP of air 38) then adding a little more insulation to the duct might be all it needs. But if the attic temperature is 30 and the DP of the air is 62, then the rain begins as soon as the air in the duct is reduced to 62 and even 10-inches of insulation may not be enough to keep the air from cooling to dew point over a 10-foot run. In that case, I'd be looking to dehumidify the air before it's exhausted from the room, thus lowering the DP considerably before it exits to the house.

    If the condensation is only happening when the fan is off, then you could simply use a PVC duct rather than metal and pitch it to collect the condensate somewhere convenient. Maybe right back into the Orchid Room.
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