How deep does non-permafrost typically go ?

[theoldwizard1]

Areas like the U.P. of MI, northern MN, ME, ND and most of Canada get deep frost, at last 48". These areas obviously warm up enough in summer so that the ground (eventually) does melt.

Are deep vertical wells a reasonable source of "heat" for geothermal heat pumps ?

Most vertical GSHP don't see "stabilized" temperatures until at least 20'. For these colder climates, I suspect that is more like 40-50'. If the pipes are well insulated from that depth all the way to the heat pump, would that be adequate ?

[AC_Hacker]

Originally Posted by theoldwizard1

Areas like the U.P. of MI, northern MN, ME, ND and most of Canada get deep frost, at last 48". These areas obviously warm up enough in summer so that the ground (eventually) does melt.

Are deep vertical wells a reasonable source of "heat" for geothermal heat pumps ?

Most vertical GSHP don't see "stabilized" temperatures until at least 20'. For these colder climates, I suspect that is more like 40-50'. If the pipes are well insulated from that depth all the way to the heat pump, would that be adequate ?

These are very good questions.

You should consult some local GSHP installers, they would have exactly the information you are looking for.

I doubt that anyone on this forum actually has this information.

I hope that when you find out, you'll let us know.

Best,

-AC_Hacker

[theoldwizard1]

Originally Posted by AC_Hacker

These are very good questions.

Sigh. My life's story !

It usually takes a lot of digging on my part !!

[AC_Hacker]

Originally Posted by theoldwizard1

Sigh. My life's story ! It usually takes a lot of digging on my part !!

Yeah, but permafrost does not apply to that kind of digging.

-AC

[Mikesolar]

I remember one particularly bad year back in 93 (I think) when the weather was cold enough that the water lines 7ft down were freezing. That is rare but I don't remember ever hearing of building footings having an issue at 4ft. I would guess that you would have stable temps from 10' down or at least no possibility of freezing. Thing is, you have 500ft of tubing in one hole so the 10ft or so at the top won't affect the output much.

That number will be different in the far north tho. The number may be 20ft.

[theoldwizard1]

Originally Posted by Mikesolar

I would guess that you would have stable temps from 10' down or at least no possibility of freezing.
.
.
.
That number will be different in the far north tho. The number may be 20ft.

I was reading on another forum about folks who had installed horizontal loops at 12-15' deep, actually getting inlet water from their loop well below freezing ! Sure, it has been cold in the Midwest USA, with temps below 0F, but if your inlet water temp is below freezing, IMHO, you have a SERIOUS design/installation flaw !

[theoldwizard1]

Originally Posted by AC_Hacker

Yeah, but permafrost does not apply to that kind of digging.

-AC

Well, I did learn that permafrost is REALLY PERMANENT ! It can actually go 100s of feet deep !

[theoldwizard1]

Well, I sort of, kind of got the information I was looking for (perhaps I did not phrase the question well to begin with).

First, if you Google around you can find charts similar to this one from www.texas-geology.com/



The important data here is that soil temperature stabilizes at about 30'. I have not found a chart that shows deeper reading, but I am fairly certain the do not shift much (although I would still like to see readings down to say 200' if anyone has access to them !)

This chart is from TX, so the center point is pretty high (70F !).

This map from www.hotspotenergy.com shows the average ground water temperature in different parts of the US. I a pretty certain that these numbers line up with the center of the above graph.





So my conclusion is that GSHP are not as big of a "win" for heating and cooling in far northern and southern areas. This surprised me ! I thought below some reasonable depth (20-30') that all ground water (at least water not heated by some geological "feature") was about 50-60F REGARDLESS of what latitude !

Anecdotal evidence is welcome !

[where2]

Originally Posted by theoldwizard1

This surprised me ! I thought below some reasonable depth (20-30') that all ground water (at least water not heated by some geological "feature") was about 50-60F REGARDLESS of what latitude !

North Florida spring water is typically 72°F when it comes out of the ground. If I could pull 72°F water out of the ground in Maine with a simple pump, I'd be a millionaire.

Nice map, btw. I needed that info.

[ChrisJ]

Most GSHP's have performance charts which rate them at x BTU's at -1*C (30*F) for heating and 32*C (90*F) cooling.

A well designed loop (performance and cost) will get down to -1*C by the end of the heating season. To gain 1* or 2* can cost 75% to 100% more in upfront costs, not to mention the additional pumping costs for the life of the loop.

-10*C is not typical, most are only freeze protected to -9*C.

CJ