Cole have you ever tried using a drop in bucket heater in your greenhouse? Thought you may have some around and might be worth a try. In theory it makes sense to use heated water in the GH but I never tried it and most people I know use electric heaters.

Probably for me it would make the most sense to wire up a house water heater and connect it to my 220 gallon trough. Ideally, I would have run pex tubing through the gravel floor to pump the hot water through. These are ideas I had originally, but lacked the budget.

Data Collection
 

How did you collect your data? I assume you did not stay up all night. I am interested in an automated method of data collection.

I'm just using Cornell's data. Two years ago, I lost most of my plants on a night where the low was about 30 and a heater malfunctioned. It was a clear night, and I think the inside temp dipped well down into the 20's.

Clear nights are the ones that concern me the most. Tonight's forecast low is 36, but the sky will be clear. I'm still going to cover up my plants and run the heat.

Yeah the clear still nights are bone chilling here, too.
Mass makes a huge difference to the temperature in my greenhouse. I have lots of mass in the floor but that gives heat in the fall and winter, by spring it's well, stone cold. The real difference in spring and on a day-night cycle is above ground mass. I use buckets and other water containers, but what really makes a big difference is the soil in the containers. I can really notice the difference this year because there's none of that mass left, everything's been emptied out. It was noticeable in the fall as soon as I cycled the soil outside.

One thing about mass, the size of the item dictates how long is the cycle to acquire and then release heat. Smaller masses are great for the short cycle, larger masses for longer term cycles.

Here is one for you that you never see.
I well known double pane window seller had some information about the effectiveness of them.
What they said was people in the north got a lot more out of them in the winter than the people in the hot southwest.
It wasn't Anderson I cant remember who it was.
Basically the savings in energy cost for the hot south west wasn't worth the cost of the expensive windows.
Then there is the fact they are sealed and gas filled.
If this leaks out and you get air inside they fog up.

So here is my solution and common sense when you want it warm inside draw back the curtains.
When you want it to stay cool close the curtains.
The other one is called window awnings.
They have been around for years but no one uses them any more.
Now here is a wild idea of mine.
If you can get hold of a water cooled engine driven generator not only could you use it to power heaters but you could circulate the engine cooling water through the soil to warm it up.
You could also capture the heat coming from the exhaust to run it into the structure.
This is how the Volkswagen beetle warmed the inside of the car.
It was air cooled and they had an engine shroud and the cooling fan running air across the engine and into the cab to heat it.
The windshield wipers ran off the pressure from the spare tire.
It functioned just like a steam engine.

A big Stirling engine would be even better.
Back in the 70's when the oil crises was going on ford motor company was designing one for cars.
Then the cheap Arab oil started flowing in at low prices and killed the project.
[url]https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwi_7PS46tzLAhUISiYKHXIEBOUQFggkMAE&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FStirling_engine&usg=AFQjCNE99e6DNsN2T6ZwkM38_KG5TD-bzw[/url]

As for any of the engines the best thing to do is to run off diesel or natural gas.
The problem with the diesel is it isn't going to get as hot and they are expensive.
The natural gas engine will last almost forever.
Put your thinking caps on and do some research.
The big boys have been sticking it to us for years to keep the oil and power companies rich.
Why would they spend money and do research to cut their own throats.

Worth

I've lived in old houses with awnings. On a hot summer day, if it rains, the awnings are the only way to be able to keep the windows open, and with no air conditioning, that was important.

I had an unheated 20x30 ft high tunnel years ago and lost a whole bunch of 8' high tomatoes in Oct. when the outside temp got down to 30. Plants inside froze solid. Very discouraging to say the least.

I haven't tried to extend seasons much yet but always wondered if my old Christmas lights might be useful after all. They do warm, they're for outdoor weather so a little humidity shouldn't be an issue, and if strung along mid-height between plants might be just enough.....I'm not sure how big a plant that would work for but my main goal is to at least get tomatoes going early next year and try to have Chinese cabbage all year. Or at least fall through spring!

Even a little heat is worthwhile if you contain it under a cover with the plants.

My forecast for tonight is clear, low of 37, and patchy frost. I left a door open in the greenhouse. Tonight's conditions are perfect for the magic temp drop effect. I'm hoping if I leave the door open, it will be harder for the temp to get much lower than the outside.

Talk about counter-intuitive - it's going to be cold tonight, so I had better leave the door [I]open[/I] so it stays [I]warmer[/I] inside. sheesh.

This morning I noticed the temp. in the GH was 40. The outside temp was 45. Something too it for sure. Tonight, another clear cloud free night, is predicted to dip to as low as 36. Not a chance in the world ill risk it. Way too much time invested in the vegetable plants and flowers.

Needless to say I went to TSC to refill my propane, and the heaters are cranking now. Currently its 70 in the GH. I really need to get a thermostatically controlled heater. But now is NOT the time to be stingy with the propane.

I've got two small electric heaters going out there. Yesterday, we had a surprise snowstorm. The weather forecasters said that we would get 1-2" of snow overnight and we woke up to 12"!!! Last night it was 24° outside and 37° inside the gh. Everything is covered with polyester "garden quilts" and the 14 tomato plants I have going are inside Walls O'Water so there's yet another layer of protection. It's working! I've got tomatoes forming already! Crazy early for me and I'm so excited!

Wow. Nevada weather is even worse than Illinois. Good for you for making it through that.

Bobinb, I think you're low fabric covers are the key.
When I build another high tunnel I will definitely design so that I can use them. My climate is quite similar to yours and with bottom (in the ground) heat I think I could extend our wimpy grow season by 4 months.
a little snow last night and 23F for a low.
bob

Heavy equipment... that reminded me of something. Pit greenhouses:
[URL]http://www.inspirationgreen.com/pit-greenhouses.html[/URL]

Dave