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Average temperature of shallow ground water

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Geothermal hot spots map
Some areas are warmer underground than other areas due to deep underground volcanic activity

geothermal  heating forum
forum on fireplaces, stoves & chimneys

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Geothermal resource map

shallow ground water temperature map Shallow ground water temperature map

Four ways to look at underground temperature:
1) Underground water pipes are geothermally heated. Each house receives some measure of geothermally heated water when pipes are buried underground. Typical pipes are 8-20 inches deep, so map on left is not good predictor of cold water temperature entering water heater. Use thermometer.

2) Year-round, shallow ground water temps start at 8-12 feet deep, and vary by latitude as shown on map. Other variables can include elevation and nearby volcanic activity.

3) Deep underground temps, up to 5 miles in some diamond mines exceed 130+. read1 read2

4) Geothermal hot spots (map at top) from volcanic activity cause high temps near surface.
54 Approx annual average outdoor temperature for northern US states
65 Approx annual average outdoor temperature for southern US states
Water pipes entering home
"All water used in our homes comes from either ground water source or a surface water source. Ground water sources include a well; surface water sources include lakes, streams or reservoirs. According to U.S. Geological Survey in 1990, about 44% of domestic water comes from ground sources. The remaining 56% comes from a surface water source." Page 6 Rheem .pdf    USGS site
The 1990 percentages shown above for domestic water have changed over time as subsidence of local ground level in many regions and increased draw of saltwater into aquifers in coastal areas has caused some cities to switch from underground to surface source. Surface water would more closely match the average seasonal temperature of air.

Underground water pipes entering your home are geothermally heated.
Water line from city supply or water well is usually buried underground to prevent freezing. The depth varies by latitude and other factors such as elevation, with colder regions requiring deeper trenches to protect pipes.
The temperature of incoming cold water affects water heating, with colder water requiring more energy to bring up to thermostat set point.
What is the temperature of incoming cold water at water heater?
It depends on many factors such as season, latitude, elevation, source of water (water well pumps groundwater with temperatures shown on diagram above), depth of pipes, length of pipes, ambient temperature of room, type of piping material (galvanized pipe will bring colder water farther into home than pex or pvc), amount of insulation on pipes inside home etc.
Best answer is to use thermometer to measure  water temperature at nearby sink or water spigot at different times of year.
Family chart for setting electric water heater timer based on incoming water temperature
Tempering tank Tempering tank to preheat incoming cold water
Adding a tempering tank in naturally warm area will help preheat cold incoming water before it enters main water heater or tankless,. However, the preheating uses energy from the furnace unless tempering tank is set in naturally sunny area or crawlspace etc that is partially heated by another source of energy.

Tempering tank
9-ways to save

Coons Cave Bloomington Indiana
Coon's Cave Monroe County
Cave temperature = equals shallow ground water temperature
Casual proof: As former caver or spelunker, I observed year-round underground temps.
Once past the entrance, caves remain same temp year-round. Caves get temperature from average underground temperature in region. For example, limestone caves in Indiana are 54F year-round, summer or winter. Southern caves are warmer. The map shows same temps I saw inside caves.

For cave info: http://en.wikipedia.org/wiki/Caving see links at bottom

For data, search US Geological Survey:
waynes cave Wayne s Cave 1965 / Monroe county Indiana
We used carbide lamps, and coveralls to climb down into and crawl through caves in Monroe County in the late 60s..
Most area caves had been thoroughly explored in the 1950 s and early 1960 by National Geogical Survey teams.
They closed the caves in Monroe County around 1985 to avoid injury, death, and public expense, except for a few privately owned caves that are still open for tour.
Caving is dangerous. The chance of falling, flood, exhaustion, getting lost, exposure are alwys present.
Sitting on a cold rock in total darkness, waiting for uncertain rescue must be terrifying.
We never had problems... but we were lucky.
Caves of Indiana book at Amazon
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