Most residential heat pumps use a primary refrigerant alone, but some designs require a secondary loop

A secondary refrigerant is a heat-transfer fluid that circulates through your home's ducts or pipes instead of the primary refrigerant itself. Most air-source heat pumps—the kind mounted on your roof or beside your house—move the primary refrigerant directly through indoor coils and don't need a secondary loop at all. But ground-source (geothermal) heat pumps and some water-source systems do use a secondary refrigerant because the primary refrigerant cannot safely or efficiently travel through underground pipes or water loops.

The reason comes down to pressure and distance. Primary refrigerants like R-410A operate at high pressures and require sealed, specialized copper tubing. Running that tubing hundreds of feet underground or through a water loop would be expensive, difficult to maintain, and risky if a leak occurred in an inaccessible location. A secondary refrigerant—usually a water and glycol mixture—can travel through standard plastic or metal piping, making it practical for longer distances and outdoor loops.

Key Takeaways

  • Ground-source heat pumps use a secondary refrigerant loop buried underground because primary refrigerant cannot safely travel through long buried pipes.
  • Water-source heat pumps that pull heat from a pond, lake, or well also use a secondary loop to isolate the primary refrigerant from the water source.
  • Air-source heat pumps (the most common residential type) do not use a secondary refrigerant; the primary refrigerant circulates directly through indoor coils.
  • Secondary refrigerants are typically water mixed with propylene glycol or ethylene glycol, which can flow through standard plastic tubing without the high-pressure containment primary refrigerants require.

Ground-source heat pumps and their underground loops

A ground-source heat pump (also called a geothermal heat pump) exchanges heat with the earth by circulating a secondary refrigerant through pipes buried 4 to 6 feet deep or deeper. The secondary fluid absorbs heat from the ground in winter and releases heat into the ground in summer. This loop can extend hundreds of feet across your property, and that distance is why a secondary refrigerant is necessary—the primary refrigerant would require expensive, high-pressure copper tubing and would be nearly impossible to service if it leaked underground.

The secondary loop is sealed and pressurized, but at much lower pressures than a primary refrigerant system. Inside your home, the secondary fluid passes through a heat exchanger where it transfers its heat (or cold) to the primary refrigerant, which then circulates through your indoor coils and ducts as usual. This separation means a leak in the buried loop does not expose your home's primary refrigerant to the soil, and repairs can be made without draining the entire system.

Water-source heat pumps and open-loop systems

A water-source heat pump pulls heat from a well, pond, lake, or recirculating water tank. Some of these systems use an open loop, where water from the source flows directly through the heat pump's coils and then returns to the source or drains away. Others use a closed secondary loop filled with water and glycol that circulates between the heat pump and the water source.

When a secondary loop is used, it protects the primary refrigerant from contamination and corrosion caused by minerals, sediment, or biological growth in natural water sources. The secondary fluid is easier to treat chemically and easier to replace if it becomes fouled. If the system uses an open loop instead, the primary refrigerant never contacts the source water—the water itself acts as the heat-transfer medium, and no secondary refrigerant is needed.

Why air-source heat pumps do not need a secondary refrigerant

An air-source heat pump moves the primary refrigerant directly through an outdoor coil (the condenser in cooling mode, the evaporator in heating mode) and an indoor coil. The distance is short—usually just the length of refrigerant lines running through your wall—and the outdoor coil is accessible for maintenance. There is no need for a secondary loop because air is free to circulate around the coil without containment, and any leak can be found and repaired without excavation or water system disruption.

The primary refrigerant in an air-source system is sealed in copper tubing and operates at high pressure, but that design works well for the short distances and outdoor exposure involved. The system is simpler, less expensive to install, and easier to service than a ground-source or water-source system with a secondary loop.

What secondary refrigerants are made of

Secondary refrigerants are typically a mixture of water and an antifreeze agent. The most common choices are propylene glycol (food-grade, safer if ingested) and ethylene glycol (more efficient but toxic). The ratio of water to glycol depends on your climate—colder regions use higher glycol concentrations to prevent freezing, while milder climates may use mostly water.

These fluids are chosen because they flow easily through standard plastic (PEX, PVC) or metal (copper, steel) tubing without the high pressure and specialized sealing that primary refrigerants demand. They also have good heat-transfer properties and can be treated with inhibitors to prevent corrosion and biological growth. Unlike primary refrigerants, secondary fluids do not require EPA certification or specialized handling during installation, though they should still be disposed of properly at the end of the system's life.

How the secondary loop connects to your home's heating and cooling

Inside your home, the secondary loop passes through a heat exchanger (sometimes called an intermediate heat exchanger). On one side of this device, the secondary fluid from the ground or water source flows in; on the other side, the primary refrigerant circulates. Heat transfers between the two fluids through a metal wall without them mixing. The primary refrigerant then travels to your indoor air handler or fan coil, where it heats or cools the air that flows through your ducts.

This two-stage design adds a small amount of complexity and cost compared to a direct air-source system, but it allows the heat pump to work efficiently over long distances and in environments where the primary refrigerant would be unsafe or impractical. The secondary loop is also easier to balance and adjust—technicians can add or remove secondary fluid to fine-tune the system's performance without touching the sealed primary refrigerant circuit.

Maintenance differences for systems with secondary loops

A system with a secondary loop requires periodic checks of the secondary fluid's concentration, pH, and condition. Over time, water in the mixture can evaporate or the glycol can degrade, especially in ground-source systems that cycle frequently. A technician can test the fluid with a refractometer or pH meter and top it up or replace it if needed—a task that is simpler and less expensive than servicing the primary refrigerant side.

The buried or submerged piping in a secondary loop should be inspected for leaks during routine maintenance. Small leaks in the secondary loop are less critical than primary refrigerant leaks (because the secondary fluid is not as tightly regulated), but they still reduce efficiency and should be repaired. If your ground-source or water-source system is losing secondary fluid regularly, a technician should locate and seal the leak before it allows air into the loop, which can cause corrosion and reduce heat transfer.

Frequently Asked Questions

Can I convert an air-source heat pump to use a secondary refrigerant?

No. A secondary loop is built into the system's design from the start and requires a different heat exchanger, piping layout, and control strategy. Retrofitting an air-source system would mean replacing the entire unit. If you are considering a ground-source or water-source system, you would install a new heat pump designed for that process.

What happens if the secondary loop develops a leak?

A slow leak in the secondary loop will reduce the amount of fluid circulating, which lowers the system's heating or cooling output. You may notice your home takes longer to reach the set temperature. A technician can locate the leak using dye or by inspecting accessible piping, and repair it by patching or replacing the damaged section. The secondary fluid can then be refilled and the system recharged with air removed.

Is the secondary refrigerant toxic?

Propylene glycol is food-grade and low-toxicity; ethylene glycol is toxic if ingested. Most residential systems use propylene glycol for safety. Either way, the secondary fluid is sealed in pipes and not meant to contact skin or be inhaled. If you suspect a leak or spill, ventilate the area and contact your technician. Do not drain secondary fluid into storm drains or soil.

Do I need to replace the secondary refrigerant regularly?

Not on a fixed schedule. A sealed secondary loop should hold its fluid for many years. However, if testing shows the fluid's concentration or pH has drifted, or if the system has had a leak and been repaired, a technician may recommend a partial or full fluid change. This is much less frequent than primary refrigerant service and is usually part of a routine maintenance visit.

Why is a secondary loop more expensive than an air-source system?

Ground-source and water-source systems require excavation or well drilling, longer piping runs, a heat exchanger, and more complex controls. The secondary loop itself is not the main cost driver—it is the infrastructure needed to access the ground or water source. However, these systems often have lower operating costs over their lifetime because ground and water temperatures are more stable than air temperature, making the heat pump work more efficiently.