Same length, very different shape
The defaults ask for 1,000 feet of exchanger. As boreholes at 200 feet that is five holes, laid out three by two at 20 foot centres, occupying a rectangle 60 by 40 feet once a 10 foot margin is allowed — 2,400 square feet. As trenches at 100 feet each it is ten trenches at 12 foot centres, a rectangle 120 by 128 feet, which is 15,360 square feet. Six and a half times the ground for the same thousand feet.
That ratio is the whole reason vertical loops exist. It is also why the horizontal option keeps coming back on rural sites, because 15,360 square feet is nothing on five acres and a mini excavator on hire for two days is a fraction of what a drilling rig costs to bring in and set up.
The pipe goes the other way. Five bores at 200 feet with a single U is 2,000 feet of pipe in the ground, plus interconnect and header. Ten trenches at 100 feet with two passes is 2,000 feet as well — identical, because both are two passes per foot of exchanger. Switch the trench to a six-pass slinky and the trench option buys 6,000 feet of pipe instead, three times as much, in exchange for shorter trenches for the same heat transfer. Pipe is cheap and trenching is not, which is why slinkies exist at all.
Rounding up is not free
Bores come whole. Ask for 1,000 feet at 200 foot depth and you get exactly five. Ask for 1,010 and you get six, which is 1,200 feet drilled — 190 feet more than the design wanted and, at drilling rates, a meaningful number. The page prints the drilled length beside the required length for exactly this reason, because the step is invisible until you look at it.
The same step exists on the trench side but costs less to be wrong about, since a trench can be dug to whatever length the ground allows. In practice the last trench is short rather than full, which is why the field shape here should be read as a count of runs rather than as a plan.
Spacing is a design number, not a geometry one
The spacings on this form look like layout decisions and they are not. Two bores 10 feet apart pull from overlapping volumes of ground, so the pair performs worse than the sum of two isolated bores, and the shortfall grows every year the loop runs. Working out how much closer they can sit before the interference matters is part of the design, done against the ground data and the annual load balance, and it produces a number this page then uses. Putting a spacing in the box does not make it a good one.
The same applies to bore depth, which reads like a free choice and is not. It is constrained by what the rig can reach in the formation under the site, by what the groundwater authority permits, and by what the drilling contractor will quote. Halving the depth doubles the hole count and roughly doubles the land, and that is often the actual constraint rather than anything about heat.
What the area figure leaves out
It is the rectangle the loop itself sits in, plus whatever margin you typed. It does not include the room a drilling rig needs to set up and move between holes, the space spoil takes while a trench is open, the setbacks the permit requires from boundaries, wells, septic fields and the building, or the fact that nothing can be planted with a deep root over the loop afterwards. On a tight urban plot the access to the back garden is usually the constraint that decides the whole question, well before the arithmetic on this page gets a look in.
Questions people ask
How much land does a horizontal ground loop need?
At the defaults here — 1,000 feet of exchanger in 100 foot trenches at 12 foot centres with a 10 foot margin — it is 15,360 square feet, about a third of an acre. The three inputs that move it are the trench length the site allows, the spacing the designer sets, and the exchanger length itself. Longer trenches cut the count and the width; wider spacing costs width directly. None of the three is a free choice, which is why they are all fields rather than assumptions.
How many boreholes will I need?
The exchanger length divided by the bore depth, rounded up to a whole hole. A thousand feet at 200 foot bores is five; at 150 foot bores it is seven. The rounding is not free, because a bore is drilled whole: 1,010 feet at 200 foot depth is six holes and 1,200 feet drilled. The depth itself is set by what the rig can reach in the formation under the site and by what the authority governing the groundwater permits, not by what makes the arithmetic tidy.
Is a vertical loop or a horizontal loop better?
They are not ranked against each other in general and this page does not try. Vertical takes roughly a sixth of the land on these defaults and needs a drilling rig on site with the access that implies, and it is permitted drilling work near groundwater. Horizontal takes a lot of ground, digs shallow where the seasonal temperature swing still reaches, and can be done with excavation plant. Which one is available is usually decided by the site rather than by preference — access, land, geology, and the permit between them normally leave one option standing.
Why does a slinky trench use so much more pipe?
Because it puts several passes of pipe in each foot of trench instead of two. Six passes per trench foot is three times the pipe of a straight two-pipe trench for the same trench length. The trade is deliberate: pipe is cheap by the foot and trenching is not, so overlapping the pipe gets more heat out of less digging. It does not get proportionally more, because the passes sit close together and compete for the same ground, which is why the heat transfer per trench foot is a design output rather than something you can scale by the pass count.
Can I put the loop under the driveway or the garden?
That is a permit question and a services question, not a calculation. Setbacks from boundaries, wells, septic fields and the building are set by the authority with jurisdiction; existing water, gas, electricity and drainage runs have to be located before anything is dug; and once the loop is in, deep-rooted planting and any future excavation over it become a problem for whoever owns the house next. The area figure on this page is the rectangle the pipe occupies and nothing else.