Heating a 40 x 25 greenhouse (double poly)

10 years ago

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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Comments (3)

fruitnut Z7 4500ft SW TX
10 years ago
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.
cole_robbie
10 years ago
I would look into getting one of the heaters that screws into a tank of propane.
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I have a central boiler with a maximum 500,000 BTUs. The firebox is 5'x4'x3' and the 400 gallons of water surround the firebox. I am only heating the ghs with this stove. I am in zone 3 and the coldest morning of the winter is often near -30 and only rarely -40, but I am quite sheltered from the wind and we usually have enough snow for some insulation around the bottom. Yes, I have poly on both the inside and outside of 4" walls. A 30'x30' pyramid is 900 square feet and the cubic feet is found by multiplying 1/3 of the 20 ft. height. So the volumn of air is much less than a building with vertical sides. The 51,000 BTU figure is something I can work with. I'll see what the 8' finned tubes are rated at or what a plentum in a furnace blower is. A was going to try the first one with manual ball valves with a bypass and the second one I was going to try to make it thermostatic and automaticly controlled with a pump or zone valve.
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Refreshg, you brought this thread back to life and there are many aspects to consider: film type, film installation, inflation setup, etc. You will likely find many recommendations on each subject. I'll only give you my choices and you can get other feedback from others. I went with 2 different layers, the inner layer an anti-drip, light dispersing film. It doesn't cost much more than the regular outer layer. When installing I find that you are better to not stretch the film as tightly to allow for inflation. Inflation will "Balloon" the outer layer and snow slideoff is better than with an unflated structure- thus you want to run inflation fan year round. To install the inflation fan I only use 1' square plywood sheets (4" hole cut in center) and carriage bolts. After installing the inner film layer you just sandwich it between plywood sheets and tighten nuts (facing inward) on bolts before installing outer layer. Any small stove blower fan will work and the filling is best if not tight- you don't want to over-inflate. Some growers have "jumper" ducts to also inflate rollup sides. I don't go that far and I'm testing wether to even have 2 film layers on the rollup sides. You can always either cut off the second layer or install a replacement side curtain only with a second wiggle wire overtop of the first. I'm sure you wanted detail and I didn't provide much but you don't need to be exact in any of this. Just when you think you are an expert someone will come up with a better way or more advanced product line.
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barrie2m_(6a, central PA)
10 years ago
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.

Heating a 40 x 25 greenhouse (double poly)

Comments (3)

fruitnut Z7 4500ft SW TX

cole_robbie

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barrie2m_(6a, central PA)

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