How deep does non-permafrost typically go ?
 

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

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

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.

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

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 !

You can still get reasonable performance from a HP when the installation temps are below freezing. Just don't let the installers skimp on tubing. Remember that and ASHP starts losing efficiency well below that.

If I ever had a GSHP system installed, there would be a "performance" clause in the contract. If my inlet water temp EVER fell below 40F, the installer would have to repair/replace/agument the ground loop. I would try to hold 50% of his installation fee in escrow for at least 1 year.

If you are going to pay the premium of having a GSHP, you want one that is designed and installed for optimal performance !

You wouldn't have to go that far. Lots of systems have input water temps that low and still perform well. It just has to be designed for it. No problems.

Don't forget, the system has glycol in it so it won't have a freeze problem and it won't hurt the equipment.

Do you think there would ever be a contractor willing to provide such a performance clause? ..or allow themselves not to be paid in full for that long? Most contractors can't afford to provide such a guarantee unless their business was more of a sure thing or health related like radon mitigation.

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/

http://www.texas-geology.com/AC_%20E...vs%20depth.jpg

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.

http://www.hotspotenergy.com/ground-...rature-map.jpg



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 !

From my research, in general, most heat pumps start to loose efficiency around 35-45F. Why not pay a few extra $$$ to have a system that will stay ABOVE that threshold. ?

I understand a extra well hole could be thousands of dollars, but it could mean the difference between sitting comfortably or having to install some kind of secondary heat source !

It is all about trade offs. If that secondary heating system only costs a few hundred to install and $100-$200/month to operate 3-4 months a year. maybe it IS a a more cost effective solution ... short term ;) !

I'm pretty sure that you are kidding here... but if you found someone that would agree to that, you would know that you had a fool for a contractor.

"Optimal" by what measure? Yes, it is possible to build a loop field that will do what you are asking for, but it will be expensive... maybe more expensive than you are willing to pay.

Rather than wearing out your keyboard by asking these serious questions of people who have never actually designed a loop field (but are wearing out their keyboards answering you), why don't you read the design and installation manuals that are written by people who actually do design and install loop fields. These manuals are a compilation of theoretical and practical knowledge gathered over half a century, from thousands of jobs, from all over the world, for all manner of applications. This information has been distilled, so as to be organized and accessible to someone like you who wants a working system.

The information is all there for you to design and build a loopfield that will function correctly at the lowest price point, if that is what you want, or you can design and install a loop field that will meet a criteria such as you have set... or anyplace in between.


If you bought every book they have, it would cost you $692, if you joined IGSHPA, it would be cheaper. Then you would have at your disposal more knowledge than anyone you could ever hire.

Why waste your time with fantasies of escrow, when you can take full responsibility, and become the very best designer?

Best,

-AC

If the specs on my GSHP showed CoP falling off below 4C, I would surely want a field that could produce above 4C !

Thank you !

Although I would likely never install a loop field, I would rather PAY for ACCURATE knowledge then what people with a keyboard (and too much time on their hands) espouse as "truth".



This (and my other recent thread) all started because a person on another forum was complaining about poor heating performance of his GSHP. He finally admitted that is inlet water was -10C and he felt that was "typical" in winter in the midwest US.

COP with More Good Picture
 

Dear Wiz,

If someone gave you the impression that COP falls off (like a buffalo falling off a cliff) at some particular temperature, they have seriously misled you.

The truth is that in a heat pump situation, COP depends on the temperature of the SOURCE, no matter if it is air, or water, or concrete.

When the temperature of the source declines to any extent, so does the COP.

EDIT: I went to the Carnot Efficiency page of Wikipedia, and generated a graph that will best indicate what I am trying to say... the legends are bogus, but the shape is right.


When your source temperature goes down at all, so does your COP, and it will continue to do so at an increasing rate.

The actual problem is really an economic issue, because there will be a point when the electricity to run the unit, is worth more in watts of heat than your heat pump can deliver.

Now, it is possible that you could build a loop field so vast, that you would no longer be able to notice any temperature drop (and COP decrease)... and that is because you are old and your eyes are not those of an Olympian god so you would not be able to see it.

-AC

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

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.