Literature on Geothermal

Check with IGSHPA…I took some classes with them in the fall of 2024

https://igshpa.org/

If you are looking for information on cost savings of a geothermal vs a standard forced air system, let me tell you about 2 neighbors of mine. They both replaced the geothermal systems that were 20 years old with a Lennox high efficiency gas furnace and a high efficiency A/C system. They are both very pleased with the results. They claim to be saving money on the cost of operation, both summer and winter, and have less service costs. Their geothermal systems consisted of coils of black plastic pipe buried 6' underground. I have no specifications on the geothermal systems installation. They are located in south western Pa.

Fracking really killed the cost benefit of ground source heating. I think a vertical well with closed loop would be much better than coils at 6' but still I couldn't show an operating cost difference between mine and natural gas forced air.

Of course I got very efficient summer cooling also for no additional up-front cost, and I loved how nice the heat pump was.

What type of soil are the pipe runs buried in?

Do the 7 loops of pipe consist of individual 500 foot long rolls/reels of High Density Polyethylene pipe with no butt fused pipe connections except at the header pipes?

What is the depth that the tubing loops are buried in?

Were the loops buried below the frost line if that applies here?

What is the separation distance between in feet between each pipe run?

What machinery is/was used to excavate the trenches?

Was the pipe pushed to one side edge of the trench and dirt piled on it every 10 or 20 feet to hold the HDPE in place prior to thier burial?

Was the rock in the trench dirt separated from the soil before the burial of the pipe?

I wish those were deeper

We are good here Paul. It is a world of difference compared to NJ or even east of the Cascades in Washington. It gets wicked cold there.

Take a look at the design standards and Manual J information. It is just mild. I have never observed over 100 or below 23 myself.

We could get by with good shades and fans and no AC but we are wimps.

I did my own Manual J with Lennox Design pros Cool Calc.

I came up with 25018 BTUH heating and 21002 cooling, and a little less than 1000 CFM

Then I paid an engineer to go through it with me, do his own Manual J and design a system for me.

He came up with 31091 heating and 27443 cooling and ~1100 CFM, a little more conservative.

I had already set my sights on a Bosch 3 ton unit with an air handler for cooling.

I went with a 4 ton inverter that should be loafing, and gave it almost 6 ton equivalent of loops.

That said, I am going to call them and opt for a 6 ton AHU that includes 2x5KW strip heaters and about double the coil capacity. I think it will be a better match for my planned 105 SWT on design heat day and I can kick in the strips to take the edge off quickly if ever needed.

The 3 ton Bosch became unavailable, unfortunately. It was not a DC inverter unit like the 4 ton, and therefore was not a good match with my11.4Kw inverter. It had big Locked Rotor Amps (LRA), where this 4 ton is quite gentle on the uptake.

I had not planned on having the heat pump on the critical loads panel but now I can. The heat will work, the lights, and the fridge during a power outage. That covers all the main bases.

It is all down 6' +. I moved 300 tons of clay several times back and forth in different motions, the goal being to not disturb all the grass up there.

Here you can see the loops as they arrive at the house, 6' + down. If they are located down the hill, they are dug down an appropriate amount. It is just hard to visualize it so I marked the corners of the loops with 8' boards. That big ridge on the right is just stored there to backfill the house when I can. The flattish surface is the first time I could visualize grade.

I need to add some retaining walls and more dirt on the loop to the right. That was the hardest digging on the site there.

The back yard will never be flat. It will get stepped retaining walls to make a flat-ish spot with what I have.

It's come a long way since the rough Ex in September.

looking to stay 40+ deep into the season by alternating loops.

Is cost the reason you did not use gravel bank concrete sand for the entire excavation instead of the local clay?

is that 1" ?

Curious how big the pump will be to keep the laminar flow

The PPI website has a useful calculator for plastic piping.

https://plasticpipecalculator.com/PressureDropHeadLoss.aspx

This shows a 500' loop of 3/4" pex. 2.25 gpm of 30% glycol gets you over 2 fps velocity.

It will take a circulator sized for that 24' head, at the gpm total of all the loops.

If all the tube is protected from freezing plain water really helps decrease the circ requirement, 9.8' with water

I think that’s what he meant

OK, let's spitball some numbers. The manual J above has a heating load of 31K BTU/hr at 49F temperature difference, that's 632 BTU per degree hour or 15K BTU per degree day. I don't know where you are but you say it's pretty mild, so let's say 4,000 degree days per year, or 60 million BTU per season. That would be about 435 gallons of heating oil per year.

You said that you moved 300 tons of dirt and that the trench is 6' deep. So that's 600,000 pounds of dirt; one sixth of it will be within a foot of the tubing so 100,000 pounds of dirt within a foot of the tubing. Your mean annual temperature is 49.8F, soil at depth will be at that temperature, let's call it 50F to keep the math easy. You've stated you don't want to let the soil temperature drop below 40F.

Let's say the soil has a really good heat capacity, say 0.5 BTU/pound/F. So that 100,000 pounds of dirt requires 50,000 BTU to get it to change temperature by one degree. Cooling all of it from 50F to 40F will require extracting 500,000 BTU. At design load that's about 16 hours of operation. Another way of looking at it is that in order to store a year's worth of heat in that layer you'd have to store 600 BTU per pound, which would require a temperature change of 1200F!

Clearly, you need to be pulling heat from a much, much greater area than just the trench. The storage capacity of the dirt in the trench is minimal compared to your heating needs. The most important attribute of the soil in contact with the tubing is that is be able to conduct heat from the surrounding area.

"I doubt you will be at design load 31K? for more than a week or two per year." Design temperature should be 1-percentile, so one percent of the hours in a year are at or below that temperature. So 88 hours per year.

"Really a GEO field is basically a giant solar collector/ storage. Suns energy will drive into the ground to recover the field also."

In most places at about ten feet the soil stays at the year-round average temperature all the time. Shallower than that it follows the seasons, warming in summer and cooling in winter. Deeper than that it gets warmer. The earth's core is at about 11,000 F but it's 4,000 miles away, so it's a pretty gradual increase.

For earth source you want your coils in that soil that stays at constant temperature, you don't want them cold in winter and hot in summer. True geothermal goes much deeper and actually uses the heat of the earth.

Deeper than that it gets warmer. The earth's core is at about 11,000 F but it's 4,000 miles away, so it's a pretty gradual increase.

This is why "geothermal" is the wrong term. The heat this close to the surface does not come from the core, it comes from the sun.

And the trench, whatever size it is, does not define the heat he will have at his disposal all winter. Heat will warm the "trench zone" from surrounding soil (below and from the sides) all winter.

Was simple shallow trenching for a single continuous run of pipe considered for thIs at all?

My neighbors geothermal system was run in a single shallow trench loop with black polyethylene pipe in the grass airport runway, they heat and cool a huge home with forced air heat exchanger using glycol.

Any colloidal clay which is what you have is a net insulator when it comes to geothermal heating and cooling.

"This is important, because our observations with my brother's field, a big 30x60 with 1800 feet of tubing in the 1800SF would cold soak pretty badly deep into the season. His soils were drier and less dense and not good conductors as @DCContrarian was talking about. We did not get much edge effect and that large contiguous field got below freezing by the end of the season."

Frozen soil is a much better insulator than unfrozen soil. If the soil freezes then the capacity of the field drops precipitously and it's pretty much game over.

Knowing the load, using a GSHP design software, all the data is built in. Once you add location it knows typical weather data and ground temperature at various levels. So adding the soil type the program can size the loop tube and field size.

NORA has soil temperature by depth data for the US.

Those are the three leading theories. Nobody really knows.