Help with a wire resitance heating project

Discussion in 'The Projects Forum' started by SeanO, Jan 14, 2015.

  1. SeanO

    Thread Starter New Member

    Jan 14, 2015
    Hello, Newbie here trying to do a DIY seed starter heating bed project. I'm looking for advice, thoughts, leads, etc.

    I have a four 96" long by 5" wide shelves I am looking to run a heating element under to bring up the temperature of a bunch of seed trays in my cellar. Two shelves run 14" high, one 16" and one 18" high.

    Target temperatures would be around 75, 75, 80 and 85 for the fours shelves. I would run four systems, as seeds go in and out of each shelf on different schedules.

    The unit is insulated front (door panels) and back (fixed), and the flourescent lighting throws off some heat, so the goal is to raise the ambient cellar temperature (50's) up the small amount needed to hit those numbers above.

    The preliminary design is to loop a wire run up and back inside sealed pvc (project involves watering of seeds above the heating element). The run would be around 17 feet total, and into a controller on the outside of the shelves. We would run the system off 110v, which is already on the unit for our lighting switches.

    The first round of research indicated that NiChrome might be overkill, and that something like Kanthal wire might be a better choice for this project? I also found some digital thermostat controllers (link: ) which should keep the units at the desired temperature.

    As stated above, I'm a Newb with all this and looking for advice.
    What I "think" I am grasping of this is that:

    1. When I apply 110v current to the wire, it will heat up depending on the gauge and run of the wire.
    2. The controller can read the temperature of the space (or the soil) and send the 110v into the system as needed.
    3. I will need to figure out the gauge and type of wire needed for the length of run and target temperature.
    4. I will need to correctly wire the run into the controller.

    1. Am I on the right track here?
    2. Any recommendations on wire?
    3. Will the PVC shielding be enough to isolate the wires from coming in contact with the water?
    4. Does it matter? (the water issue)
    5. Would the controller have something in it in case of a short?

    Any thoughts, concerns, or ideas would be most welcome!
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    You will kill yourself if you try this with 110Vac.

    Run black 1/2" irrigation tubing under the soil bed. Circulate heated water through the tubing. Much easier to transfer heat to water than from a resistance wire direct to soil.
  3. MaxHeadRoom


    Jul 18, 2013
    Look for Raychem Chemelex, heat trace cable, it is self temperature regulating along its length, if you tie it up in a knot, the centre remains at the rated temperature.
  4. #12


    Nov 30, 2010
    The usual beginner mistake is trying to micromanage every little thing while missing the big picture.
    Trying to make a resistance wire heating element is a lot more difficult than necessary. That is what Max is pointing at.
    If your seeds can survive light, light bulbs make excellent, cheap, heating elements.
    The size and insulation quality of every part of the outermost container are needed to apply math to this.
    The weight of the contents (and power applied) determine the rate of change of the temperature.
    You can add a fuse easily enough if the controller is not equipped with appropriate short circuit protection.
    Your temperature range is within the ability of common residential heating and air conditioning thermostats.

    I am not just making random comments. I am asking for information.
  5. wayneh


    Sep 9, 2010
    There are a lot of DIY incubators out there that use simple lightbulbs as heaters. You can see them working and you have a fairly accurate idea of how much power they are putting into the chamber. That's what I'd use - a lightbulb. You probably need the light anyway.

    Depending on the configuration, you may also want to add a couple computer "case" fans to quietly move the air around.
    #12 likes this.
  6. SeanO

    Thread Starter New Member

    Jan 14, 2015
    Thanks for the replies, everyone.
    The heat mat (underneath applied) is the standard approach, but I'll look into the lightbulb idea.
    I agree with #12, and I'm probably over thinking and overdoing this.
    I've decided to dial it back for this upcoming season, thanks to all your questions which made me look at the project from several different angles.
    Thanks again!