Heating a small greenhouse

I can't find the post now, but I want to thank whoever it was who came up with the idea of a one burner stove top and a pot of water. I had racked my brains trying to think of a way to heat my greenhouse in a way that wouldn't burn my house down and run my electric bill through the roof. The greenhouse is a lean-to attached to the house. Space heater out of the question, not very efficient, and I have a wood chip floor, fire possibility, highly likely. I do have a crapload of 2 liter bottles painted black, donated by soda drinkers in my neighborhood, for passive solar heating.

We stacked a bunch of cinderblocks in the middle of the walkway, and put a single element burner on top, and put a huge stockpot (by huge I mean I scald turkeys in it) half full of water on it, and put the heat on medium. The temps this week dropped into the teens at night, and even though I have mostly cold tolerant stuff in there, I was worried about temps in the teens killing everything, including all the strawberries I had just planted in temporary pots. I unplug the burner during the day, as we don't really need it.

The pot keeps everything at about 40 degrees at night, which seems to be perfect. The heat spreads out evenly, and I'm beside myself that things are doing so great. All my cold weather stuff literally exploded into health, I don't know if it was the humidity, or what (high desert, really dry here). I have 2 comfrey plants in there that I've been barely able to keep alive for two years, and they've lost their freaking minds. I have never seen them so HUGE (although I'm sure the diluted urine and epsom salts didn't hurt). The purple bok choy has gone absolutely crazy. Arugula, radishes, spinach, everything doing great. Next winter is going to be fabulous. Finally I can accomplish my dream of being able to plant in the dead of winter.

So thank you, poster whose name I don't know and couldn't find, for your ever so helpful post, for which I will be eternally grateful.

This system sound great!
So, no condensation issues on the inside of the glazing?

The reason it works so good is because the heat of transition of water to vapor and water to ice is extremely high.
Farmers have been misting crops for decade on cold nights to protect them from frost.
What your doing is using heated water vapor which  more than doubles the protection of using water alone.

Great Idea!

William Bronson wrote: This system sound great!
So, no condensation issues on the inside of the glazing?

It's so dry up here that condensation isn't much of an issue. Everything dries up as soon as the sun comes out.

A compost-heated greenhouse might also work.  

Check out  Earthdwellers Homestead and how he has put an external compost box with a coil of water that heats up.  He uses a solar fan, but since it's working so well for him, at night the heat must rise out of there on its own.

There are variations on the hot water tube.  Hot air tubes can also be used.  YouTube has a lot of examples.

An internal compost bin would bring gnats in, which can drive you crazy.

Meant to say Check out Earthdwellers Homestead on YouTube...

:-)