A few feet under your lawn, the ground in Northern Virginia and Montgomery County sits near 57 degrees in January and near 57 degrees in July. Every geothermal system is built on that one fact.
This is the explanation I give at kitchen tables, written down. No sales pitch, and nothing in it that you would have to take on faith. If you come out the other end deciding geothermal is wrong for your house, that is a good outcome, and it is cheaper to reach now than after a drilling rig is booked.
The whole idea, in one sentence
A furnace makes heat. A geothermal system moves heat that already exists. That is the entire difference, and almost everything else about these systems follows from it.
Burning something is a one-way trade. You buy a fuel, you set fire to it, and you get out somewhat less heat than the fuel contained, because some of it goes up the flue. You can get closer to the theoretical limit with a better furnace, but you cannot get past it. Moving heat is a different kind of trade. You spend a small amount of electricity running a compressor and a couple of pumps, and in exchange you relocate a much larger amount of heat from one place to another. The heat was already there. You paid for the moving, not the making.
A refrigerator does this in your kitchen. It does not make your food cold, it moves heat out of the box and dumps it into the room through the coil at the back. A geothermal system is the same machine, scaled up, with the ground as the place it takes heat from in winter and the place it puts heat into in summer.
Why the ground, and not the air
An air-source heat pump does the same trick, and it is a good machine. Its problem is the thing it is working against. On the coldest morning of the year, when your house wants the most heat, the outdoor air has the least of it to give. The machine has to work hardest exactly when conditions are worst, and that is why a cold snap is when people notice their heat pump struggling and the backup electric strips coming on.
The ground does not have that problem, because it does not care what the weather is doing. Soil is slow. Below about six feet it stops following the daily swing entirely, and below about twenty feet it stops following the seasonal swing too, settling at roughly the average air temperature for the year. In this part of Virginia and Maryland that average lands in the mid to high fifties, which is where the 57 comes from.
Read that chart as a design problem and the conclusion falls out. In January the air is about twenty degrees colder than the ground, and in July it is about twenty degrees warmer. A machine working against the red line is fighting a moving target twice a year. A machine working against the blue line is doing the same job in the same conditions every day it runs.
The three parts, and what each one actually does
The loop
Buried pipe, filled with water and an antifreeze, running in a closed circle between your house and the ground. Nothing is pumped out of the earth and nothing is pumped into it. The fluid goes down at one temperature, comes back at another, and the difference is the heat that changed hands.
The loop is the part that gets sized, and it is where most of the engineering lives. How much pipe you need depends on how much heat the house needs, what the soil and rock under the lot conduct like, and how much space there is to put it. This is the number that is expensive to get wrong, because it is underground and under your lawn by the time anybody finds out.
The heat pump
A cabinet, usually in the basement or a mechanical room, about the size of a large furnace. It contains a compressor, two heat exchangers and a fan. It is the part that does the actual moving, and it is the last thing we choose, not the first.
There is no outdoor unit. Nothing sits beside the patio, nothing runs under a bedroom window, and there is nothing outside to rust through a winter or get hit by a mower. That is a real quality-of-life difference and it is the thing homeowners mention back to us most often.
The distribution
Ducts, in most houses, or radiant floors and a hot water coil in some. This is the half of the system that is already in your house, and whether it is any good decides a surprising amount. A perfectly designed loop feeding an undersized duct system gives you an expensive machine and rooms that are still uneven.
What it is not
Four things get attached to the word geothermal that do not belong to it, and clearing them up saves everyone time.
- It is not free energy. The system runs on electricity. What changes is how much work you get for each unit of it, because you are paying to move heat rather than to create it. Your electric bill goes up and your fuel bill goes away.
- It is not volcanic. Power stations in Iceland tap deep heat from the earth's interior. A house system does nothing of the kind. It uses the shallow ground as a place to park heat, at a temperature that comes from last year's weather rather than from the earth's core.
- It is not a water well. A closed loop is sealed. The fluid in it is the same fluid for the life of the system, and it never touches groundwater.
- It is not an off-the-shelf swap. This is the one that costs people money. A geothermal project needs a load calculation, a loop design and a drilling date, and none of those fit inside the forty-eight hours you have when the heat has already gone out in January.
What your lot has to have
Less than most people assume, and the honest answer for any particular property takes a site visit. Broadly, there are three questions.
- Can a rig get in? A drilling rig needs a route to the drilling position. Gates, slopes, mature trees and retaining walls are what rule a lot out, far more often than the size of the lot does.
- Is there room for the boreholes? Vertical boreholes take a fraction of the surface a horizontal field would. Townhouse lots are tight and some of them do not work. Plenty of suburban lots that look too small are fine.
- What is under it? Soil and rock conduct heat at different rates, which changes how much pipe the design calls for. This part varies across Northern Virginia and across Montgomery County, and it is why a loop design is a calculation rather than a rule of thumb.
If a lot cannot host a loop field, you should hear that on the first visit and not after a deposit. It is a day-one answer and we treat it as one.
What changes inside, and what does not
On a replacement, the indoor work is about the same order of magnitude as changing out a furnace and an air conditioner. The difference is what the new equipment connects to: a buried loop instead of a gas line and an outdoor condenser. In most houses the existing ductwork stays.
What leaves is the combustion side of the house. No flame, no flue, no fuel deliveries, no annual burner inspection, and nothing outside. If you are on oil or propane, that is also the end of having a tank and of watching a price you do not control. There is more detail on what a changeover involves on the GeoSwap page.
What it costs to run
Running cost is the question everyone actually means when they ask what it costs, and it is answerable from your own bills rather than from a brochure. What you are comparing is the price of the fuel you burn now against the price of the electricity the heat pump will use to move the same amount of heat.
Two things make that comparison move in your favour here. The first is that the machine delivers several units of heat for each unit of electricity it draws, because it is moving rather than making. The second is that your heating cost stops tracking the price of oil, propane or gas. It does not become zero and it is not immune to electricity prices, but it stops being exposed to the fuel market that has caused most of the nasty surprises on winter bills.
Anyone who gives you a payback figure before looking at your fuel bills and your house is guessing. We would rather do the arithmetic on your actual usage and tell you if it does not work.
Where the energy credits come in
There is one more piece that is specific to this region and surprises people. A geothermal system in Virginia or Maryland earns renewable energy credits for the clean heat it produces, and those credits have a cash value. They are not a rebate and not a discount on the installation. They are earned every year the system runs, and they are paid to whoever owns the system.
That last clause matters more than it looks. If you buy the system, the credits are yours. If you put it in through a financing partner who owns it during the term, they keep the credits. Neither choice is wrong and the right one depends on whether you would rather keep cash in your pocket now or collect the credits later. The G-REC page explains how the credits are scored and paid, and the financing page covers the other route.
Credit prices and program terms change, and nothing on this site quotes you a rate. Any figure in a proposal from us is an estimate tied to the day it was written, not an offer.
How to tell if it is worth a conversation
The clearest signal is timing. If your current system is near the end of its life but still running, that is the moment, because you have the weeks a designed project needs. The worst moment is the week the furnace dies, when the only thing available is whatever can be fitted on Friday.
The second signal is your fuel. The arithmetic is at its most convincing for houses on oil or propane, good for houses on electric resistance heat, and more of a real comparison for houses on natural gas, where it depends on your usage and your rates.
The third is whether you are building. On a new house the ground is already open, the access is already there, and the mechanical room can be drawn around the equipment instead of the other way round. Every decision costs less the earlier it is made, which is covered on the new construction page.
If none of that describes your house, geothermal is probably not the next thing you should spend money on, and anybody who tells you otherwise without seeing your bills is selling rather than designing.