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  1. #1
    Join Date
    Sep 2009
    Location
    Massachusetts
    Posts
    4

    Electric Unit Heater in Greenhouse

    I am trying to determine how to size an electric unit heater in a greenhouse, or at least have someone confirm my heater choice. It is in a school on Mass. The room is 17'x25' with a sloped ceiling that averages 16' high. The only glass walls facing the elements are the south side and the roof. The north wall is part of the heated building and so are the east and west walls. I have no idea how to size the heater.

    I have a 20 amp, 208 volt, three phase circuit that is available. I have found a couple of unit heater models that put out 17,000 +/- BTUH, 350 CFM that could be put on the available circuit. Will this size be adequate? If not, what would be a better choice?

    Thank you in advance for any responses. This is my first post.

  2. #2
    Join Date
    Aug 2006
    Posts
    2,603
    Best Bet is to contact the maker of the Greenhouse. Does the school have a set of Prints ? Might be in there. BTW You need something that will handle the Humidity, and not light someone up.

  3. #3
    Join Date
    Sep 2009
    Location
    Massachusetts
    Posts
    4
    Quote Originally Posted by madhat View Post
    Best Bet is to contact the maker of the Greenhouse. Does the school have a set of Prints ? Might be in there. BTW You need something that will handle the Humidity, and not light someone up.
    No prints are available. Good point on humidity. What does light someone up mean?

  4. #4
    Join Date
    Jun 2000
    Location
    B.C. Canada
    Posts
    147
    Quote Originally Posted by mkgrady View Post
    No prints are available. Good point on humidity. What does light someone up mean?
    Electrocute them.

  5. #5
    Join Date
    Sep 2009
    Location
    Massachusetts
    Posts
    4
    I shoud have mentioned, this heater will only be needed when the school heat syatem is not running. So spring and fall only. The greenhouse already has a hot water unit heater from the scool heating system but they don't run the building main boiler except november to march or somewhere theabouts

  6. #6
    Are you planning on "heating" the greenhouse or just keep it from freezing? Most every plant can take the cold, but not frost, and your greenhouse by design prevents frost. Do you get solar gain during the day, so the heater won't need to operate? A 17,000 BTU heater will keep it from freezing, but a glass enclosure that size won't ever be warm on zero degree nights. Circulating fans will help a great deal as well. But it's more of a solarium than a green house, with 3 solid walls, correct? I suspect your plants will be OK.

  7. #7
    Join Date
    Sep 2009
    Location
    Massachusetts
    Posts
    4
    Quote Originally Posted by MissouriBound View Post
    Are you planning on "heating" the greenhouse or just keep it from freezing? Most every plant can take the cold, but not frost, and your greenhouse by design prevents frost. Do you get solar gain during the day, so the heater won't need to operate? A 17,000 BTU heater will keep it from freezing, but a glass enclosure that size won't ever be warm on zero degree nights. Circulating fans will help a great deal as well. But it's more of a solarium than a green house, with 3 solid walls, correct? I suspect your plants will be OK.
    The big glass wall and roof face south so they get sun most of the day. During the cold months this electric heater will not be needed. They have a hot water heating system that runs off the main boiler during the normal heating season

  8. #8
    Join Date
    Aug 2006
    Posts
    2,603
    Here's a neat little calculator I found, http://www.littlegreenhouse.com/heat-calc.shtml I'd still give them a call and let them do the math. What size is the hydronic, that should give you an idea. Can't believe they'd operate a school without a complete set of prints, somewhere. Just make sure you get one designed for a greenhouse, to deal with the moisture. Seen the frames electrified on greenhouses from the heaters.

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