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Thread: Large woodstove or furnace info

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    Default Large woodstove or furnace info

    I am wanting to heat a large greenhouse. I have been leaning towards the wood furnace at Lowes because I can run a vent down the GH to the other end which I think will spread the heat better than just fans. In the future I think I may use a boiler, but that a few bucks down the road.
    Any suggestions would be great. It is a 30x72, so I am thinking I need a fairly big one. I have gas backup, but I hate the thought of that thing kicking on.
    Thanks ahead Mark

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    Supporting Member Amigo Will's Avatar
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    I hate the thought of heating anything that big.How many cords a winter would that eat. For a house we always figured a cord a window.
    Now left only to be a turd in the forrest and the circle will be complete.Use me as I have used you

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    Member AKDoug's Avatar
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    In the spring or all winter? All winter for that greenhouse is most likely going to be 30 cords of wood minimum, maybe more depending how tight it is. Grow lights and hydroponics in an insulated space would work better.

    In the future we will be heating a greenhouse starting in March with a wood boiler and heat tubes flowing through the dirt in raised beds. That's our dream, we'll see.
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    Mark- You will find that no matter which model heaters you do the math based on, natural gas will be your cheapest btu, BY FAR!!! Go with a gas furnace and use wood as supplemental. I know several folks with commercial greenhouses, most are on the natural gas grid, and all that are use NG. Some supplement with a wood stove, you can wrap soft copper round a barrel stove and make a pretty good hydronic system to heat your soil, which is truly where you want the heat. Unless you are the homestead type where you don't earn a weekly paycheck, wood makes no sense as your primary heat source.

    Why pay more per btu for wood and then have to store it, haul it every day, etc. when your cheapest btu pipes itself right in?

    I do not like hydroponics because the potential for massive contamination is huge, but a well insulated (ideally underground) building with no windows and lights (i would grow in soil) would be an order of magnitude cheaper than a greenhouse if you are really wanting to grow from november - march

    p.s. there are much better quality furnaces for your money out there than can be gotten at a box store.


    Amigo- you just made me count, and my house has 17 windows, most are 3'x4' with a few bigger and a few smaller, so it serves as a good average size.The most wood I have ever burned in a winter is a touch over 3 cords (well-seasoned birch).

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    Supporting Member Amigo Will's Avatar
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    You have a much tighter house for sure,probably that new fangeled insulation also.
    Now left only to be a turd in the forrest and the circle will be complete.Use me as I have used you

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    Default wood

    Anrweav, Wood does make sense to me because I own hundreds of cords, so I think using it is commonsense. I am on NG and it will be my primary source, but... I'M a penny pincher, thanks to our economic climate. I settled on a wood furnace, that I can run duct with, we will see how my venting spreads heat. My lid is double pole and the walls corregated which will be doubled on the inside with plastic until May.

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    Default Large woodstove or furnace info

    I was going to add that caveat about wood! I am in the same boat (though lacking in access to NG).

    A boiler will work far better for transporting the heat, though I understand the cost to capitalize on that is much more. Can you rig up a water jacket to your furnace? A circ pump and copper loop are pretty simple to set up, and hot (warm) water running through your soil will yield HUGE benefits, and the furnace won't warm the beds very well unless you use elevated benches with ductwork below, which is a lot of work. Which model furnace did you end up with?

    A wood boiler / solar thermal combo is my goal for a future construction project that will involve cabins and hopefully several large greenhouses.

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    Default Woodstove

    As I stated in my original post; I plan on using a boiler in the future, but the one that was suggested is about 10,000.00 buckos, so I will wait until my project matures a bit. As far as the furnace; I will vent the heat along the bottom under a central table(heat rises) reducing the vent in size as I go and will also inject vents along the way. This should help spread the air more evenly along the GH then the fans will circulate it from there.
    I like your idea of using soft copper to spread heat in beds, however I purchased two 265 gallon water tanks and will stack them on one another, close to the heat source. The top will be feed well water, and will drip into the bottom as water is removed from it, via a pressure tank, and this should/will help alot. I am very interested in your copper idea to run pipes into or around the tank to supplement the residual heat from stove.
    I have searched and searched for woodstoves, etc for awhile now, and the amount of info is daunting to sort through. Andweav shoot me an email, lets not clog these guys forum with gardening info, I want to chat a bit, besides you boat too, don't ya.

    I'm all ears on any of this stuff thanks, and( Amigo Will) Yes I am wondering how much wood I will consume starting March 1st

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    Member AKDoug's Avatar
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    While it doesn't transfer heat as well, heat pex plastic is FAR cheaper than copper. Buried in a bed of dirt it will do a fine job transferring heat to the bed. For a coil system for heating water in a barrel, the copper is the way to go.
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    Supporting Member bullbuster's Avatar
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    Quote Originally Posted by AKDoug View Post
    heating a greenhouse starting in March with a wood boiler and heat tubes flowing through the dirt in raised beds.
    I built mine with Pex in the poured slab floor. LOVE IT. My buddy in Kenai wanted a similar, but cheaper way to go. He excavated the dirt down 3 feet. He lined the hole with 2" blue board. Then he backfilled with compacted D-1 and layed in Pex tubing. He heats his water with a wood/oil heater. Maybe not as efficient as concrete, but he is still growing this late in the year.

    Here's mine hard at work.
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    Default Curiosity

    Quote Originally Posted by mark oathout View Post
    I am wanting to heat a large greenhouse. I have been leaning towards the wood furnace at Lowes because I can run a vent down the GH to the other end which I think will spread the heat better than just fans. In the future I think I may use a boiler, but that a few bucks down the road.
    Any suggestions would be great. It is a 30x72, so I am thinking I need a fairly big one. I have gas backup, but I hate the thought of that thing kicking on.
    Thanks ahead Mark

    Just out of curiosity, I am familiar with anyone who heats their greenhouse to this extent. Are you heating it extend the growing season on vegetables or for flower? This thread has really got me interested. I use a greenhouse for cucumbers and have wondered what the options are for as getting as many out of the plant as possible before the cold weather takes over. And if it's worth the effort for the crop to use heat to start in March and also extend past fall.

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    We used ours to get an early start on both vegetables and flowers. Plants need heat and light. Our winters are dark and artifical lighting is expensive (they catch pot growers by the size of their electricity bills). By March we have plenty of light but not so much heat. We started a bunch of flats, dahla tubers, and other stuff in the green house in March. Things moved outside as appropriate throughout the spring and by early summer just the heat loving vegetables were still in the greenhouse, where they stayed. I usually quit trying to grow stuff in the greenhouse around mid October.

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    Supporting Member bullbuster's Avatar
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    I am still picking cherry toms and they are SWEET! Also have limes and lemons coming.

    I am about to shut it down as work has gotten to be too much.
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    What is your source of heat for the greenhouse in those early months such as March?

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    Quote Originally Posted by bullbuster View Post
    I am still picking cherry toms and they are SWEET! Also have limes and lemons coming.

    I am about to shut it down as work has gotten to be too much.
    That's awesome! And I can understand how much work it must becoming. What is the size of your greenhouse and heat source if you don't mind me asking?

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    Default Heat source

    There are several advantages. First, we can extend our growing season, second we can grow plants that will not grow outside in AK. We can also change zones by throwing alittle heat out in the greenhouse during those aweful cold nights, and therefore can experiment with perenials like blackberries, etc. that cannot survive a winter exposed in AK.
    For myself; the wood furnace provides extra cost cutting means because I own lots of wooded land. Without having everything sealed tight, I can strip about 20-30 degrees out of the equation, which accounts for an immense cost savings towards my main heat source, which is natural gas.

    My interest is in southern tomatoe varieties that taste like a tomato should, but require warm soil to perform, and with my system, I can put the heat right where it needs to be with water coils or ductwork. While we can warm the air, Alaska's soil is cold by normal standards, so we go through great lengths to curtail the problem.

    I also branched off the venting and ran a 4" up and under a 250 gallon water tank which will be blueboarded to trap the 120 degree heat. If one BTU will raise a pound of water one degree, then it will take 81,000 BTU's to raise the water 20 degrees which puts it at about 60 degrees, and that is perfect for plants. I should be able to do this over night because only a branch of the system is for this, and I'm not sure how many BTU'S that branch emits, I just divided it by four, based on the entire system, for an estimate. If I am close to correct four hours should be close.

    This is only logical if you own your own wood because natural gas and wood are very comparable in cost per BTU

    Anyone can come over and check this out, and... I'm always open to advice. I'm close to Wasilla

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    Thanks for the information Mark. That would be interesting to see how your greenhouse is set up, as I am building a new one this spring along with some new beds. Having an almost unlimited wood supply would be a huge bonus is heating such a structure.

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    Member pacific23's Avatar
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    Could a person not raise some rabbits in the a green house to help heat?

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    They could but the rabbits would eat everything.
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    Supporting Member bullbuster's Avatar
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    Then I would eat the rabbits.
    Might be the perfect solution. I am ready to shut it down for the winter and I am hungry.



    My GH is heated with these Pex tubes that I ran to my gas furnace. It is 8x22'. 60 degrees as I type. One thing I have is a remote read thermometer. Great for monitoring highs and lows.
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