Green House

So, in simple terms, you have a 12 square foot (bottom) radiant surface at 50° trying to heat a 3,200 cubic foot area at 30°.

Throw in the sides of the hole and you've got 48 square feet of surface.

Not much help.

You could always roast a pig. That hole's about the right size.

Rather than digging a hole, you may want to consider using black 6mill, both on the floor, and any other siding areas that won't block the sunlight to the plants. The black will warm it up considerably during the day. I don't know how cold at night though. You may want to consider a small space heater.

I don't know where you are, but you might be able to take advantage of the solar light and use thermal massing to retain and radiate the heat.

Essentially, I would insulate the floor and use either rocks or concrete as a heat sink for the sun inside the green house.

Maybe the walls could be stone with an airspace and then clear glass or clear plastic as the outside wall. The airspace between would act as rising warm air during the day and the wall would radiate heat at night.

SolarEgal, I didn't think of having it open on the top,Your right. as for everyone else, yes i understand about the thermo Mass, like cement floor,I figure the Brown dirt floor would do the same. ok how about this. I know this gets hot from living on a farm when i was a kid. In that hole put in chopped up corn stalks,leaves, kind of like a compost pit.

John

No, with a dirt or concrete floor the mass is in direct contact with the cold ground and therefore sinks away the heat.

You could use compost as a heat source, but you would still need a heat exchanger....Because the compost heap would have to be outside the enclosure, composting vegetation gives off methane....You could build an aerobic composter that fed the airspace inside the enclosure directly, this would supply heat and CO²(boost growth) but this would take careful planning and a complicated control system, as well as nasty odors and energy usage...Perhaps a large compost pile on the northern exposure would work....with a clay brick wall to act as a heat sink? Dug down in the ground a couple feet? might work...

I was wondering why you were concerned whether 30°C would heat your greenhouse...then you mention 50° ground, so I guess you're talking °F. Please remember that this is an international board, and that ISO units are Centigrade...so always state your units (darned good practice whatever!). _{Or are you building on a slope?}

Something I have seen done is to make the walls of the greenhouse from empty clear plastic drinks bottles. You may find the extra insulation helpful whilst the light and radiant heat loads inside are not diminished.

You haven't said to what temperature you want to raise the greenhouse air. Using bottlewalls and black ground sheet should keep you frost free, assuming you're in a sunny position (I mean the greenhouse is not shaded during winter days).

For more information, google Radio 4 Gardeners' Question Time and Bob Flowerdew (yes, he's real and yes, he's a well-known UK gardener - organic and eco-friendly).

You might avoid a lot of cost and effort by simply installing some ground heating cables designed for greenhouses. Here's a link to a website that offers various solutions, but the ground heating cables might be your simplest solution. One 200-foot long cable, costing less that US $220 will supply 1000 watts. If one is not enough, it would be easy to add more cables.

http://www.littlegreenhouse.com/accessory/heaters3.shtml

You could insulate your hole and use an old heating oil storage tank or some other tank. Heat the water or oil stored in it with a solar panel. Use this fluid at night to warm the interior of the green house.

I highly recommend that you visit Mother Earth News online regarding how to build a DIY Greenhouse. There's lots of invaluable information and tips available!

http://www.motherearthnews.com/search.aspx?search=greenhouses

Have a safe sunny day!

I was just trying to see about how many degrees would be made from the earth , like i said dig a hole in the floor about 4 foot deep and use the earths heat at that depth.But one person did say that I would have to cover the hole or it would BECOME part of the floor with no dirt on it.He was right. so I would have to lay a coil of pipe in the hole or something and then Cover it with dirt.So now say if I were to put a 50 gallon barrel in the ground with a pipe going in one end and the other end coming back up to the top of the floor ,Would i get any usable heat from this.

Yes, when the air temp dropped below ground temp....

In order to take advantage of the heat of the earth you would need to insulate the outside of the foundation walls with a minimum of 2 inches of closed cell rigid insulation. The depth of insulation would be determined by your location, as in Texas the 50°F temperature gradient would occur much closer to the surface than in Minnesota (in Minnesota it is below 8 feet). Also you would want to insulate the north wall - presuming you're in the northern hemisphere. Make the north wall of stone, brick, or cement block and paint it black to absorb the sun's heat. You could paint the outside of the masonry wall with insulating paint, which would make it superinsulated so the masonry would be a large thermal mass, helping minimize temperature swings during the night. Cover the greenhouse with at least two layers of poly with a small air space between (less than 0.75 inches to prevent conduction and convective currents from transferring the heat to the outdoors). A single layer of poly has a very low R value and it would be difficult to maintain adequate heat within the greenhouse.

Or......you could use "cool tubes" - a network of tubes buried at the 50°F depth - and circulate the warmer air via a fan. With "cool tubes" you would need to slope the tubes downward away from the greenhouse and provide a drain a the low point to prevent condensation from building up in the tubes and promoting mold growth. Cool tubes also act as air conditioning in the summer by removing moisture as well as supplying much lower temperatures.

Hi bud.

I don't think you would get a lot of heat out of it. I never saw a mechanic dive into a pit to get warm, but I think it would be just a tad warmer.

Apart from the good suggestions already given, I did see, (as in TV) a system a guy made where he filled a pit with glass fragments, and gently blew warm air from a black painted wall through it during the day, and of course at night, the warmth came out, either blown or not.

It's a system i operate with the cellar door. It's always one temp down there.

Refinements might be insulate the pit first, put a wall of glass in front of the black wall,

If you crack inter-seasonal storage, let us know.

CNC jim

I have been looking at green houses for a couple years now. The way that I have seen them commonly heated is through a black open top water tank exposed to sunlight. They are inexpensive and easy to make. It also helps regulate the moisture in the green house.

http://forums2.gardenweb.com/forums/load/strucs/msg011445279873.html is a nice discussion of thermal mass. One comment even explains how to calculate how much thermal mass is needed.

Another option is to dig an old style well lined with brick or rock, divide the well in two, and blow air into the well. Not sure how deep it needs to be. I saw a house design use this to stabilize a room in a house.