What a water heat pump does and why it works differently

A water heat pump pulls heat from a water source—a pond, lake, well, or ground loop buried in your yard—and moves that heat into your home. Unlike air heat pumps, which pull warmth from outdoor air, water heat pumps draw from a medium that stays more stable year-round. Water below ground stays around 50°F even in winter, so the pump doesn't have to work as hard to extract usable heat.

The core job is the same as any heat pump: it doesn't create heat, it relocates it. A refrigerant fluid circulates through a closed loop, absorbing heat from the water source on one end and releasing it into your home on the other. Because water holds heat better than air and doesn't get as cold, the pump runs more efficiently than an air-source model, especially in climates with harsh winters.

Key Takeaways

  • A water heat pump moves heat from a stable water source into your home using a refrigerant that absorbs and releases heat as it circulates.
  • The evaporator coil absorbs heat from the water source, the compressor pressurizes the refrigerant to raise its temperature, and the condenser coil releases that heat into your home.
  • Water stays warmer than winter air, so the pump works more efficiently in cold climates than an air-source heat pump would.
  • The water loop can be open (drawing from and returning to a well or pond) or closed (a buried loop that recirculates the same water).
  • Installation requires either access to a stable water source or space to bury a ground loop, which affects cost and feasibility.

The refrigerant cycle: how heat moves through the system

The refrigerant is the working fluid that does the actual heat transport. It enters the evaporator coil as a cold, low-pressure liquid. Water from your source flows past this coil, and the refrigerant absorbs that heat, boiling into a gas. The warmer the water source, the easier this happens—which is why water heat pumps are more efficient than air models.

The gaseous refrigerant then flows to the compressor, an electric pump that squeezes the gas under high pressure. Compression raises the refrigerant's temperature further—this is where the system adds energy. The hot, pressurized gas then enters the condenser coil, where it releases its heat into your home's heating system (usually water that circulates through radiators or radiant floor pipes). As the refrigerant cools, it condenses back into a liquid.

The liquid refrigerant then passes through an expansion valve, which lowers its pressure and temperature, and the cycle begins again. This loop runs continuously as long as your thermostat calls for heat. The compressor is the only moving part that uses significant electricity; the rest of the system relies on the pressure and temperature changes of the refrigerant.

Open-loop versus closed-loop water sources

An open-loop system draws water directly from a well, pond, lake, or stream, passes it through the evaporator coil, and returns it to the same source. This works well if you have reliable access to clean water and the source is large enough that removing and returning water doesn't harm it. Open-loop systems are simpler to install and often more efficient because the water temperature is stable. The downside is that water quality matters—sediment or minerals can clog the coil, and some municipalities restrict this approach.

A closed-loop system uses a buried loop of plastic pipe filled with water or a water-antifreeze mix. The loop is typically installed 4 to 6 feet underground (where temperature stays constant year-round) or coiled in a pond if you have one. The same fluid circulates endlessly through the loop and the heat pump. Closed-loop systems work anywhere and don't depend on local water sources, but they cost more to install because burying the loop requires excavation. A horizontal loop takes up more yard space; a vertical loop takes less space but costs more to drill.

Why water sources stay warmer than winter air

Ground temperature below the frost line (typically 4 to 6 feet down) remains nearly constant year-round, usually between 45°F and 55°F depending on your region. This is far warmer than outdoor air in winter, which might drop to 0°F or below. Because the evaporator coil always has a warm source to draw from, the refrigerant boils more easily and the compressor doesn't have to work as hard.

An air-source heat pump in the same climate would be pulling heat from 0°F air, which means the refrigerant barely boils and the compressor must run longer and harder to extract the same amount of heat. Over a heating season, this difference adds up to significantly lower electricity use for a water heat pump. In milder climates, the advantage shrinks, but in regions with long, cold winters, water heat pumps often use 30 to 50 percent less electricity than air-source models.

How the system handles heating and cooling

Most water heat pumps can reverse their cycle to provide cooling in summer. A reversing valve changes the direction of refrigerant flow, so the condenser coil (which released heat into your home in winter) now absorbs heat from your home and releases it into the water source. The evaporator coil becomes the heat absorber instead. This is why the system is called a heat pump—it moves heat in both directions depending on what you need.

In cooling mode, the water source acts as a heat sink, absorbing warmth from your home and carrying it away. Because ground or pond water is cool even in summer, the system cools efficiently without the high outdoor air temperatures that make air conditioners work harder. Some systems can also provide free cooling by bypassing the compressor entirely and letting cool water from the source circulate directly through your home's heating pipes—a feature called passive cooling that uses almost no electricity on mild days.

Installation requirements and site considerations

Before a water heat pump can be installed, you need either access to a reliable water source or space to bury a ground loop. If you have a well, pond, or stream on your property and local regulations allow open-loop systems, installation is faster and often cheaper. The contractor will test water quality and flow rate to confirm the source is suitable.

If you don't have a water source, a closed-loop system requires excavation. A horizontal loop needs roughly 1 acre of land per 12,000 BTU of heating capacity (a typical home might need 3 to 5 acres). A vertical loop uses much less space—often just a small area near the house—but requires drilling 150 to 300 feet deep, which is expensive. Some installers can place loops in a pond if you have one, which saves yard space. Before installation, confirm that your property's soil type, water table depth, and local regulations support the system you're considering.

Maintenance and common issues

Water heat pumps require less maintenance than air-source models because the water source doesn't accumulate dust or ice. However, the water loop itself needs monitoring. In an open-loop system, sediment or mineral buildup can reduce flow or clog the evaporator coil over time. Annual flushing or filter changes may be needed depending on water quality. In a closed-loop system, the fluid should be checked periodically for leaks, and the antifreeze concentration should be verified if you live in a cold climate.

The compressor and refrigerant circuit are sealed and rarely need service if the system is installed correctly. However, if refrigerant leaks, the system loses efficiency and must be recharged by a technician. The most common issue is a frozen or blocked evaporator coil in open-loop systems, which happens if water flow drops or water quality degrades. Regular inspection of water flow and coil condition can catch problems early. Most systems last 20 to 25 years with proper maintenance.

Frequently Asked Questions

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

Yes. A closed-loop system buried in your yard works without any existing water source. Horizontal loops need more land space, while vertical loops require drilling but use less yard area. Both are common in areas without ponds or wells, though vertical drilling adds to the installation cost.

How much electricity does a water heat pump use compared to an air-source model?

Water heat pumps typically use 30 to 50 percent less electricity in cold climates because the water source stays warmer than winter air. In mild climates, the difference is smaller. The exact savings depend on your local winter temperatures, the water source temperature, and how well the system is sized and installed.

What happens to the water after it passes through the heat pump in an open-loop system?

The water is returned to the same source it came from—the well, pond, or stream. The heat pump removes some warmth, so the returned water is slightly cooler, but the source is large enough that this doesn't cause problems. Some systems return water to a different location in the same source to avoid recirculating already-cooled water.

Can a water heat pump work in freezing temperatures?

Yes. The water source (especially underground) stays above freezing even when outdoor air is well below zero. The refrigerant inside the pump can be much colder than the water source, so the system continues to extract heat. In an open-loop system, the water itself won't freeze because it's moving and comes from below the frost line.

How long does it take to install a water heat pump?

Open-loop systems can be installed in a few days if the water source is accessible and permits are in place. Closed-loop systems take longer—horizontal loops may take a week or more depending on yard size, and vertical loops require drilling, which can add several days. Permitting and site preparation often take longer than the physical installation.