7) Geothermal- Ground or Air?


"No fossil fuels ever!" To paraphrase Joan Crawford. When it came to deciding on a heat source we concluded pretty quickly that a geothermal heat pump was the way to go. Unless you live in Iceland or Yellowstone on top of a geyser, there are few ways to avoid using oil, natural gas or propane to heat your home. OK, there's nuclear, but they just shut down Vermont's only nuclear power plant!

Anyhow, armed with our photovoltaic tracker, and thus free electricity from the sun, what we plan on doing is pumping the free heat that lives deep down in Mother Earth. 

It turns out that there are two types of heat pumps: air-source and ground-source.

The former draws the latent heat from the air, and brings it inside, exactly like an air-conditioner, except in reverse.


People cool their houses, or cars, or refrigerators, by pumping heat out of the place they want cold, and blowing it elsewhere. With heat pumps we're talking about the opposite: taking the heat in the air ( or ground) and moving it inside where it's needed. Air-sourced heat pumps look like this:

You have an external unit that extracts the heat in the air, and one or more units inside the house that blows it into the living space. Either hot air or through liquid, say using embedded radiant heat tubes or radiators. The added benefit of air-sourced heat pumps is that they can act as air-conditioners, pumping excess summer heat outside when it's sweltering inside.


With ground-based heat pumps we're talking geo-thermal, i.e. 'earth heat'. The pump takes warmth from the ground itself. At 12 feet under the surface, the temperature is practically constant all year 'round. It varies by latitude, obviously, the ground is warmer under the Sahara than it is at the same depth in the Antarctic. In Vermont that deep down constant temperature is about 48°. If you have a well, you can figure what the constant ground temperature is where you live. Just run the tap for awhile and measure how cold it is. Since wellwater comes from more than two fathoms down, it's constant. But it depends where you live. Wellwater is warmer in Tampa than it is in Anchorage!

The way I like to think about it is to speed up time. Imagine that a year was 7 days long. On Monday the sun is blazing and warming the ground surface to 80° throughout the day. By conduction the heat sinks lower and lower warming the earth. But long comes Thursday (winter) and it's freezing out, heat is leaking upwards, the ground cools. But a couple days later, it's a hot summer Monday again and the earth is warming up once more. It's pretty easy to see that somewhere down deep it averages out. Temperature changes don't come fast enough to heat or cool things down there. This graph sums it up very well, I think: 














You can see in the sine waves that the ground surface temp swings drastically from 82 ° in August, to 40 something in February, but the farther down you go, the less it varies. At 12 feet it barely changes. What's really interesting is the delay. At 12 feet down, the highest temperature is in November, it doesn't hit the low point until late-April. What's key here is that when we want to start sucking heat out of the ground, in November, there's plenty of heat down there. And by the time it gets chilly down below, in late March, we hardly need any of that free geothermal heat.

So based on such things, we decided to go for ground-based geothermal heat, though it had pros and cons. On the negative, it's more expensive, secondly you can't use it for cooling. (Which we already felt was unimportant at our locale.) One advantage is that there are no noisy, ugly fan boxes outside the house. It's not susceptible to damage by falling ice or freeze ups, or getting backed into by the snowblower. But by far the most important, it's significantly more efficient. 


But we're not done yet. It turns out there are two types of ground-based geothermal: coils and wells.


Coils involve several hundred feet of piping buried 7 feet down in a field. Liquid (usually glycol to prevent freeze-ups) is circulated through the coil field to absorb the heat. 


Something like this: 


With wells, you don't go horizontal, you go vertical. Wells are drilled 200 feet down and heat is absorbed from the bedrock and groundwater.

We decided to go with the coils, with the idea that we would bury them deeper than normal to take full advantage of the more abundant heat and the time delay. Amazingly we couldn't find a company in Vermont to handle our installation, we had to reach out to our neighbors in New Hampshire. We fixed on Ultra-Geothermal out of Concord, New Hampshire. Click their link for more info on their systems work.


Next week, I want to talk about a strange Frenchman and free wood chips and what that could mean to ensuring that the SunCottage is the most efficient geothermal home on the planet.



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Previous Posts:

36) Available for Rent!

                   February 13, 2020 

35) Furnishings & Triskelions

                   April 23, 2019 

34) Green Roof & Appliances

                   August 15th, 2018 

33) Between Snow & Summer

                   May 27th, 2018 

32) Sauna, Painting, Tile & Nest

                   April 1st, 2018 

31) Tesla, Geothermal ... Plasterers

                   February 17, 2018 

30) Solar Tracker is Online!

                   December 31, 2017 

29) Grid Connection & New Panels

                   December 31, 2017 

28) Interior Work

                   December 24, 2017 

27) Views of the Exterior

                   December 22, 2017 

26) Woodchips & a Bifacial Tracker

                   December 15, 2017 

25) Well Capping & A Minor Disaster

                   December 7, 2017 

24) Deep Wells, Green Roof & Chips

                   November 24, 2017 

23) Roofing, Windows, Bridge & Time

                   November 14, 2017 

22) The Crew & the SolaFlect Pad

                   October 31, 2017 

21) Walls & the Earth Tube

                    September 30, 2017 

20) Framing & Plan Change 

                    August 31, 2017

19) Foundation, ICFs & Radiant 

                    July 31, 2017

18) Demolition, Firemen & Footings 

                    June 15, 2017

17) Floor Plans   

                    June 2, 2017

16) The SunCar   

                    May 31, 2017

15) The Green Roof   

                     May 30, 2017

14) Energy Recovery- ERV or HRV?   

                     May 25, 2017

13) Triple Pane Windows, ICFs

                     May 24, 2017

12) Hot Tub-Small but Beautiful  

                     May 20, 2017

11) Solar Chimney 

                     May 18, 2017

10) The Tesla Battery- The Heart     

                     May 3, 2017

9) The Bottle Wall   

                     May 1, 2017

8) On Whey and Woodchips   

                     April 27, 2017

7) Geothermal  

                     April 13, 2017

6) Key Elements of the Building 

                     March 2, 2017

5) A True Passive House?                         

                     Jan. 26. 2017

4) The Systems: Solaflect Tracker   

                     Dec. 14, 2016

3) What to Build  

                     Nov. 23, 2016

2) The Story of the Old Cottage  

                     Oct. 22, 2016

1) SunCottage Genesis                         

                     August 29, 2016

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