What a water source heat pump does and why it works

A water source heat pump moves heat from a body of water — a pond, lake, well, or ground loop — into your home instead of pulling it from outdoor air. Because water stays warmer than air in winter and cooler than air in summer, the pump needs less energy to extract usable heat or reject unwanted heat. The result is higher efficiency than an air source heat pump, especially in cold climates where air temperatures drop below freezing for months.

The core principle is the same as any heat pump: a refrigerant circulates through a closed loop, changing state between liquid and gas to move heat from one place to another. The difference is where the heat comes from. Instead of an outdoor coil exposed to winter air, a water source pump has a coil submerged in or connected to water, which acts as a more stable heat source.

Key Takeaways

  • Water source heat pumps extract heat from a pond, lake, well, or buried ground loop instead of outdoor air, making them more efficient in cold climates.
  • The system requires either an open loop (drawing water directly from a well or lake) or a closed loop (circulating fluid through buried pipes), each with different installation costs and maintenance needs.
  • Water stays at a steadier temperature year-round than air, so the pump works harder less often and uses less electricity to heat or cool your home.
  • Installation is more expensive upfront than an air source heat pump because it requires drilling, pond work, or extensive ground excavation, but lower operating costs can offset this over time.

Open loop systems: drawing water directly from the ground

An open loop system pumps water directly from a well, pond, or lake, runs it through the heat pump's coil to extract or reject heat, and returns the water to its source. This approach is the simplest to install if you already have a reliable water source on your property — no digging trenches or drilling multiple boreholes.

The catch is that the water must be clean enough not to clog or corrode the coil. Well water with high iron content, sediment, or mineral deposits can damage the system or require frequent filter changes. Many municipalities also restrict open loop systems because returning warmed or cooled water to a lake or stream can affect the ecosystem. Before choosing this route, check with your local water authority and have your water tested.

Open loop systems are cheaper to install than closed loops if you have a suitable water source, but they carry ongoing maintenance costs and the risk of water quality problems. They also use more water than a closed loop, which matters in areas where water is scarce or metered.

Closed loop systems: circulating fluid through buried pipes

A closed loop system buries plastic pipes in the ground or submerges them in a pond, then circulates a heat-transfer fluid (usually water mixed with antifreeze) through those pipes and back to the heat pump. The ground or water absorbs or releases heat to the fluid, which carries it to the pump. Because the loop is sealed, no water is drawn from or returned to any source.

Closed loops come in two main configurations. A vertical loop involves drilling boreholes 100 to 400 feet deep and inserting U-shaped pipes that circulate fluid down and back up. This takes up minimal land but requires specialized drilling equipment and costs more per foot of pipe. A horizontal loop buries pipes in trenches 4 to 6 feet deep across a larger area of your property — cheaper per foot but demanding more land, typically 1 to 2 acres for a residential system.

Closed loops are more expensive to install than open loops because of the excavation or drilling work, but they require almost no maintenance, work in any climate, and avoid water quality or environmental concerns. Once buried, the pipes last 25 to 50 years with no moving parts in the ground itself.

How the refrigerant cycle extracts heat from water

Inside the heat pump unit, a refrigerant circulates through four main components: an evaporator coil (where heat is absorbed), a compressor (which pressurizes the refrigerant), a condenser coil (where heat is released), and an expansion valve (which reduces pressure). In heating mode, the water-side coil acts as the evaporator.

Cold, low-pressure refrigerant enters the water-side coil and absorbs heat from the water circulating through it. Even water at 40°F contains heat energy that the refrigerant can extract. As the refrigerant absorbs heat, it evaporates into a gas. The compressor then pressurizes this gas, raising its temperature further. The hot, high-pressure gas flows to the condenser coil inside your home, where it releases heat to your indoor air and condenses back into liquid. The liquid returns to the expansion valve, pressure drops, and the cycle repeats.

In cooling mode, the cycle reverses: indoor air passes through the condenser coil (now acting as an evaporator), and heat is rejected to the water loop instead. Because water temperature is more stable than outdoor air temperature, the refrigerant never has to work as hard to complete the cycle, which is why water source systems use less electricity.

Why water temperature matters more than air temperature

Ground temperature below the frost line (typically 6 to 8 feet down) stays between 45°F and 55°F year-round in most of North America, regardless of whether the air above is 0°F or 95°F. A pond or lake temperature fluctuates more than deep ground but still remains warmer than winter air and cooler than summer air. This stability means the heat pump operates in a narrower, more efficient range.

An air source heat pump must work much harder when outdoor air drops to 0°F because the temperature difference between the refrigerant and the air is larger. A water source pump extracting heat from 45°F water faces a smaller gap, so the compressor uses less energy to move the same amount of heat. Over a heating season, this difference compounds into significantly lower electricity bills, especially in climates where winter temperatures stay below freezing for weeks or months.

The efficiency advantage shrinks in mild climates where winter air temperatures rarely drop below 30°F. In those regions, the extra installation cost of a water source system may not pay back as quickly as it would in a colder area.

Installation requirements and what to expect

Before installation, a contractor must assess your property for water availability, soil composition, and space. For a closed vertical loop, they drill boreholes and insert pipes — a process that takes a few days and requires access for a drilling rig. For a horizontal loop, they excavate trenches, lay pipes, and backfill — typically a week or more depending on property size and soil conditions.

For an open loop, they locate and test the water source, install intake and return lines, and set up filtration if needed. All systems require the indoor heat pump unit itself, which is similar in size to a furnace and usually placed in a basement, utility room, or garage.

The total installation cost for a closed loop system typically ranges from $15,000 to $30,000 for a residential home, depending on loop type, property size, and local labor rates. Open loop systems cost less if a suitable water source exists but may require permits and water testing. Air source heat pumps cost $8,000 to $15,000 to install, so the water source premium is real — but lower operating costs can recover that difference over 10 to 15 years in a cold climate.

Maintenance and long-term durability

Closed loop systems require minimal maintenance because the loop is sealed and buried. You should have the indoor unit serviced annually — the same as any heat pump — to check refrigerant charge, clean coils, and inspect electrical connections. The buried pipes themselves need no attention and typically last 25 to 50 years.

Open loop systems demand more oversight. Water quality must be monitored, filters changed regularly, and the intake and return lines inspected for corrosion or sediment buildup. If your well or pond water is poor quality, maintenance costs can add up. Some open loop systems also require a permit renewal every few years, depending on your jurisdiction.

Both types benefit from a backup heating source — usually an electric resistance heater or a gas furnace — for extremely cold days when the heat pump alone cannot keep up. This backup rarely runs in mild winters but provides security in severe cold snaps.

Frequently Asked Questions

Can I install a water source heat pump if I don't have a pond or well?

Yes. A closed loop buried in the ground works on any property with enough space. Vertical loops need only a small area for boreholes, while horizontal loops require more land but no water source. If you have neither space nor water, an air source heat pump is your alternative.

What happens to my pond if I use it for a heat pump?

A closed loop submerged in a pond circulates sealed fluid, so the pond water itself is not drawn out or altered. An open loop that draws water from the pond and returns it warmed or cooled can affect fish and plants if the temperature change is large. Check local regulations before using a pond for an open loop.

How much electricity does a water source heat pump use compared to a furnace?

A water source heat pump typically uses 30 to 50 percent less electricity than electric resistance heating and costs less to operate than a gas furnace in many climates, though the comparison depends on local electricity and gas prices. The exact savings vary by system size, climate, and how efficiently your current heating works.

Do I need a permit to install a closed loop system?

Most areas require a permit for any ground excavation or drilling, and some require a licensed contractor to do the work. Check with your local building department and well authority before you start. Open loop systems often need additional permits because they involve groundwater.

What is the lifespan of a water source heat pump?

The indoor unit typically lasts 15 to 25 years, similar to an air source heat pump. Closed loops buried in the ground last 25 to 50 years with no maintenance. Open loop systems depend on water quality but can last 20 to 30 years if maintained properly.