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Heating a 40 x 25 greenhouse (double poly)

Posted by Slimy_Okra 2b (My Page) on
Fri, Jan 17, 14 at 8:11

I start all my tender plants (tomatoes, peppers, etc) on April 1st. Our mean lows are 30*F in April, but it can get as low as 0*F on the coldest nights. I would need to maintain an interior temp. of 50*F.
The only heating system I have is electric. I have a 100 amp panel with 15 amp circuits.
I looked at the heating requirements for a greenhouse of this size and it appears that six 1500w space heaters are insufficient to maintain 50 F on cold nights.

I am going to use heating pads for the seedling trays both during and after germination. Besides this, are there any ways in which I can keep the plants warm after they have germinated? Would convection-type heaters placed below the benches help?


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RE: Heating a 40 x 25 greenhouse (double poly)

  • Posted by fruitnut z7b-8a,4500ft SW TX (My Page) on
    Fri, Jan 17, 14 at 10:19

I'd be thinking about ways to cover the plants on cold nights. Could you cover them with heavy duty frost blanket? If that's not enough then find something with even more insulation.

If you concentrate your heat under the frost blanket seems to me you could save a lot on heating costs. Heat the plants rather than the whole greenhouse. Often soil temperature is more important than air especially for damping off and such.

I can get melons up in 4-5 days outside in March with lows in 30s by covering with heavy insulation at night. This maintains a soil temperature in the 70s.


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RE: Heating a 40 x 25 greenhouse (double poly)

I would look into getting one of the heaters that screws into a tank of propane.


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RE: Heating a 40 x 25 greenhouse (double poly)

propane catalytic heater as described might be OK for marginal nights but for a 25x40' structure and 0*F nights you had better invest in a unit capable of 80K BTU or better just to be safe.

Since yoiu don't count on more than 3-4 nights like that you might want to have a propane turkey fryer base handy or even a gas grill that you could set in just for those nights. I've never had any night below 20*F to contend with but I have scrambled everything available on some nights.

Also get a couple of cheap box fans, or better versions if you can, so that you can move the air to avoid cold spots within the greenhouse. When I experimented with Fruitnuts suggestion years ago (plants under row cover under a tunnel within the tunnel) I lost about 10% of my tomato plants because I was monitoring temperatures at on spot and other spots just got too cold.

Even the best of setups fail if the temperature drops below capabilities of your system. A Minn. researcher reported on a project using solar panels to heat water pumped under greenhouse soil during a very cold winter period. Although the circulated water reached very high temperatures during many days the extreme nights were just too cold for the setup to succeed.

A few things you might consider are to develop a backup plan which may include moving your plants into your basement, etc. Also consider starting them in your house anyhow since 50*F is much too cold to germinate seedlings. From a comfortable vantage point you can look at weather forcasts and determine if conditions are favorable to move plants into your greenhouse. I have 3 greenhouses yet my house setup of 28 shoplights is maxed-out even as late as April 1st. when my greenhouses are either planted full or reaching capacity.


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