# At what depth does the underground begin to warm up?

Spring water comes out colder from being underground. But if you get deeper the temperature goes up. At what depth does the underground stop getting colder and begins getting warmer?

Is there a map for that point like a geothermal table map? I ask this because I want to build a underground water cooling air conditioner in the same manner that pipes are heated on roof tops.

• It depends on where you are, in many places the answer is zero.
– John
Jun 11, 2018 at 5:58
• Are you asking how far the influence of the atmosphere penetrates into the ground? Jun 11, 2018 at 11:35

Your question has an incorrect assumption built in. Near surface ground water temperatures are not generally colder, but rather reflect the average annual temperature. This will be colder than surface temps in summer, but warmer in winter. There is an an additional effect if your rainfall isn't evenly distributed over the course of the year, as percolating rainfall will be initially warmed by warm soil temps in summer. As an example, in central Alberta shallow well water temps run about 10 C, while our average temp for the year is around 4-5 C

A graph on that page shows how soil temps vary with depth depending on seasonal variation, and moisture levels. Ball park: 30 feet gets out out of the seasonal variation, but 12 feet eliminates about 2/3 of the annual variation.

If your climate is cold enough in winter, it may be worth your while to store coolth in the form of brine. In effect build a well insulated building with insulated foundation with what amounts to a swimming pool in it filled with either salt water brine, or calcium chloride brine. Have a few thousand feet of 1/2" plastic pipe (drip irrigation pipe is cheap) both outside and in the pool, and circulate antifreeze through the pipe when winter temps are below the brine temp. In the summer, a secondary loop circulates between the pool and the heat exchangers for the house.

If you are in a desert climate, you may be able to do something similar on a smaller scale. Under a calm clear sky, you can often get frost on the side of an insulator facing the dark sky at night. Modifying this idea, create a large bottom insulated surface coated with a good IR emitter (Titanium dioxide white is one such.) , and run shallow water over it. If you can routinely get 30 C colder than day time air temps, then you only have to store a few days worth of coolth instead of a years worth.

You can make the system above colder by pre-chilling the water with an evaporative cooler

Edit: One source claims 1 degree F per 30 feet starting at around 100 feet. Temperatures above this however are highly variable from water movement.

• Even though the answer is correct regarding to the OP project, it does not give an answer to the depth where temperature starts to rise (several hundred meters I suppose). Jun 12, 2018 at 12:59

The temperature would stabilize to a steady temperature when you go down 15 ft or so. The point where it gets significantly warmer is probably much deeper than you want to go. A better system would probably be to bury two separate loops that are 6 or 8 ft deep. One is the heat loop on one side of the house and the second could be the cool loop on the other side of the house. In the winter, cool the ground down with outside air. this should freeze the ground in the cold loop. In the summer, use the cool frozen loop to connect to your house instead of the outside air. The opposite is done with the heat loop on the other side of the house. Heat the ground all summer with outside air. You could even have a solar powered fan blow hot air through a solar collector then underground. In the winter, close the outside connection and hook it up to blow through a closed loop that draws the stored heat from the ground to your basement all winter. I don't really have any university studies to back up what I am saying. It just makes common sense.