What Is a Water-Source Heat Pump? How It Works, Benefits, and Costs

A water-source heat pump uses water as the heat source and heat sink to move heat efficiently for space heating and cooling. This guide explains what a water source heat pump is, how it works, typical performance, costs, and when it makes sense. It highlights key design choices, maintenance needs, and U.S. codes and incentives so buyers and builders can make informed decisions.

What Is A Water-Source Heat Pump (WSHP)?

A water-source heat pump is a reversible HVAC unit that transfers heat between indoor air and a circulated water loop. In cooling mode, it rejects heat to the water; in heating mode, it extracts heat from the water to warm the space.

The core technology is the same vapor-compression cycle found in refrigerators and air conditioners. What changes is the medium: instead of exchanging heat directly with outdoor air, a WSHP uses water at controlled temperatures.

WSHPs are common in commercial and multifamily buildings because they combine individual zone control with the efficiency of a central hydronic loop. They can also serve single-family homes that have access to wells or a suitable body of water.

Key Terminology

• Water-to-Air WSHP: Heats or cools indoor air with a fan coil inside the unit.
• Water-to-Water WSHP: Produces hot or chilled water for hydronic distribution or domestic hot water.
• Boiler–Tower Loop: A closed water loop maintained by a small boiler and a cooling tower.
• Open-Loop System: Uses well water or a surface water source directly, with proper filtration and discharge.

How A Water-Source Heat Pump System Works

Boiler–Tower (Closed-Loop) Systems

In many U.S. buildings, WSHPs connect to a closed water loop that circulates through all units. The loop runs within a typical range of about 60–95°F to keep all heat pumps in their efficient window.

During summer, a cooling tower rejects excess heat from the loop to the outdoors. During winter, a small boiler adds heat to keep loop temperatures above the minimum setpoint. The WSHP in each room or zone reverses to meet local demand.

This configuration enables heat recovery: heat rejected by units in cooling mode can help units needing heat elsewhere in the building, reducing boiler and tower energy.

Open-Loop And Surface Water Systems

Some WSHPs use groundwater or surface water directly as the heat source/sink. With proper permits and water treatment, open-loop systems can achieve high efficiency because water temperatures are stable compared to outdoor air.

Open-loop designs must address water quality, fouling, and discharge requirements. In the U.S., wells and reinjection may fall under the EPA’s Underground Injection Control rules for Class V wells, and local water rights regulations.

Water-To-Air Versus Water-To-Water

Water-to-air units resemble small furnaces or fan-coils with compressors inside, providing supply air via short duct runs. They are popular for zoned comfort in offices, hotels, and apartments.

Water-to-water units produce hot and/or chilled water for radiant floors, fan coils, or domestic hot water preheating. They work well with dedicated outdoor air systems and radiant designs focused on comfort and energy efficiency.

Efficiency, Ratings, And Performance

Water-source heat pumps are rated primarily by EER (cooling efficiency) and COP (heating efficiency). In the U.S., WSHPs are commonly tested to AHRI/ISO 13256-1 rating conditions.

• Cooling rating condition: Entering water 86°F; entering air 80°F dry-bulb/67°F wet-bulb.
• Heating rating condition: Entering water 68°F; entering air 70°F dry-bulb.

At these conditions, quality WSHPs often deliver EER in the mid-teens to 20+ and COP around 3–5, depending on compressor type, unit size, and controls. Variable-speed compressors and variable water flow improve part-load performance.

Typical Temperature Windows

• Loop temperature: ~60–95°F (some designs target 55–100°F).
• Delta-T across unit (water side): ~8–12°F typical.
• Air off-coil temperatures: Similar to air-source systems, sized by load and airflow.

Representative Performance Range

Important: Efficiency depends on loop design. Poor water treatment, excessive pumping energy, or unstable loop temperatures can erode savings. Good controls and commissioning are essential.

Benefits And Use Cases

WSHPs appeal to building owners and designers seeking flexible zoning and strong efficiency across a wide range of outdoor conditions. They are especially competitive in multi-zone buildings with simultaneous heating and cooling.

• Zoned Comfort: Each unit serves a zone, improving comfort and reducing overconditioning.
• Heat Recovery: Waste heat from cooling zones can serve heating zones, cutting boiler run time.
• Stable Efficiency: Water temperatures fluctuate less than outdoor air, boosting performance in extreme climates.
• Modular Design: Units are standardized and scalable, simplifying phasing and tenant fit-outs.
• Serviceability: Most maintenance occurs at the unit or in mechanical rooms, minimizing refrigerant piping runs.

Common applications include offices, schools, hotels, multifamily, senior living, and retail. In residential settings, a WSHP can be attractive where a reliable water source exists or where a building provides a shared loop.

Design Considerations For U.S. Buildings

Sizing And Hydronic Flow

Correct sizing ensures efficiency and comfort. Designers typically select WSHP capacity using detailed load calculations and specify water flows around 2.0–3.0 gpm per ton for constant flow systems.

With modern variable flow and two-way valves, design flow rates as low as ~1.2–1.8 gpm per ton are common, reducing pump energy. Maintain loop delta-T in the 8–12°F range and verify adequate condenser water pressure at part load.

Ventilation And Humidity Control

Because water-to-air WSHPs mainly recirculate zone air, most buildings pair them with a Dedicated Outdoor Air System (DOAS) to meet ventilation codes and manage latent loads.

In humid climates, consider DOAS with energy recovery and enhanced dehumidification features. Some WSHPs offer hot-gas reheat or reheat coils to improve humidity control and comfort.

Controls, Setpoints, And Commissioning

Effective controls increase savings. Key strategies include loop temperature reset based on building load, variable-speed pumping with differential pressure control, and occupancy-based scheduling.

Commissioning should verify unit airflow, water flows, valve operation, and sensor calibration. Continuous monitoring via a BAS can alert staff to fouling, leaks, or inefficient sequences.

Water Treatment And Materials

Closed loops need corrosion inhibitors and periodic testing; towers require biocide and scaling control plus blowdown management. Open-loop systems must filter particulates and manage scaling potential.

Material selection matters. Cupronickel heat exchangers resist fouling better for open-loop use. For closed loops, copper and standard brazed-plate exchangers are common, but chemistry must be maintained.

Acoustics, Vibration, And Architecture

Packaged WSHPs contain compressors, so designers should specify vibration isolation, acoustic lining, and thoughtful placement away from sensitive spaces.

Short duct runs reduce fan power and noise. Mechanical closets often house units in apartments or hotel rooms, improving service access while protecting occupants from sound.

Installation, Costs, And Payback

Total cost depends on building type, number of zones, loop infrastructure, and whether a cooling tower and boiler are added or reused. Cost ranges vary widely by region and project complexity.

• Equipment: Small water-to-air WSHPs (2–5 tons) often run about $3,000–$8,000 per unit, depending on features and brand.
• Installed Cost (per unit): Frequently $7,000–$15,000 or more when including controls, piping, and electrical.
• Central Plant: Cooling tower, boiler, pumps, and controls can add significant cost but serve many zones.

Compared to variable refrigerant flow (VRF) or distributed air-source systems, WSHPs can be cost-competitive in mid- and high-rise buildings where vertical distribution with water risers is efficient and refrigerant routing is complex.

Payback depends on energy prices, operating hours, and heat recovery. Buildings with simultaneous heating and cooling often see shorter paybacks due to loop heat sharing. Utility incentives can further improve economics.

Maintenance And Reliability

WSHPs are reliable when maintained properly. Routine tasks include air filter changes, coil cleaning, and condensate management. Central systems need tower and boiler maintenance year-round.

• At Each Unit: Replace filters regularly (1–3 months), clean coils annually, check condensate drains, verify refrigerant charge, and test safeties.
• Pumps And Valves: Inspect seals, actuators, and strainers; verify differential pressure setpoints.
• Cooling Tower: Maintain water treatment, conduct inspections, and implement a Legionella water management plan per ASHRAE 188.
• Boiler: Verify combustion (if gas), inspect controls, and maintain water chemistry.

Typical service life is 15–20 years for WSHP units. Towers, boilers, and hydronic piping often last longer with good water treatment and periodic refurbishment.

Comparing WSHP To Air-Source And Geothermal Heat Pumps

WSHPs sit between air-source and ground-source systems. They leverage the moderating effect of water while avoiding the extensive drilling required for deep geothermal borefields.

In mixed-use or tall buildings, WSHPs can be easier to distribute than long refrigerant runs and can outperform air-source during weather extremes. Where land and budget permit, ground-source typically delivers the lowest energy use but at higher upfront cost.

Codes, Standards, And Incentives In The U.S.

Designers should confirm compliance with current energy codes and standards. WSHPs are addressed in equipment efficiency tables and hydronic system requirements.

• Efficiency Ratings: AHRI/ISO 13256-1 provides rating procedures and conditions for water-to-air WSHPs.
• Energy Codes: ASHRAE 90.1 and the IECC set minimum EER and COP values by capacity. Project specifications should reference the applicable edition.
• Open-Loop Permitting: EPA Underground Injection Control (UIC) regulations and local water authorities regulate wells and discharge.

Incentives vary by state and utility. Many programs offer prescriptive or custom rebates for water-source heat pumps, cooling towers with VFDs, and advanced controls.

• Commercial 179D Deduction: Federal tax deduction for energy-efficient commercial buildings achieving modeled energy savings, including efficient HVAC.
• State/Utility Rebates: Programs may reward high-efficiency WSHPs, variable-speed pumps, and heat recovery.
• Residential Credits: Some residential WSHPs may qualify for incentives if they meet program-defined tiers; eligibility depends on program rules and listings.

Always confirm eligibility with the utility or program administrator and gather AHRI certificates, submittals, and commissioning documentation to support applications.

Selecting And Specifying A WSHP

A clear specification helps ensure performance, longevity, and ease of maintenance. Include rating standards, minimum efficiency, water quality requirements, and controls sequences.

• Performance: AHRI/ISO 13256-1 ratings; specify minimum EER/COP at standard and application conditions.
• Compressor: Prefer variable-speed or staged compressors for part-load efficiency and comfort.
• Fan And Filtration: ECM fans; specify MERV filtration to meet code and IAQ goals.
• Water Side: Two-way valves; variable flow; strainers; pressure/temperature taps; isolation valves.
• Heat Exchanger Materials: Cupronickel for open-loop; copper/brazed plate for closed-loop with proper chemistry.
• Controls: BAS integration via BACnet; loop temperature reset; occupancy schedules; fault detection.
• Acoustics: Vibration isolation, sound-absorbing liners, and noise criteria by space type.
• Service: Access panels, condensate overflow protection, and clearances for coil cleaning.

Request factory performance data at expected entering water temperatures and flows, not just at standard rating points. Verify compatibility with DOAS, reheat strategies, and dehumidification needs.

Trends And Future Outlook

Electrification and decarbonization are reshaping WSHP designs. Many teams are replacing gas boilers on loops with heat pump boilers or heat-recovery chillers to maintain loop temperatures using electricity.

Manufacturers are transitioning from R‑410A to lower-GWP refrigerants such as R‑454B or R‑32 in line with the U.S. AIM Act HFC phasedown. Controls now feature advanced analytics and automated fault detection to sustain savings.

Hybrid systems combining WSHPs with thermal storage, solar PV, and high-efficiency DOAS are emerging. Expect more water-to-water units providing domestic hot water and hydronic heating as building codes push for higher efficiency and lower emissions.

When A Water-Source Heat Pump Makes Sense

WSHPs shine where buildings have many zones, experience simultaneous heating and cooling, and can support a hydronic loop. They often offer a strong balance of first cost, efficiency, and serviceability for mid- and high-rise projects.

In climates with extreme temperatures, WSHPs maintain efficiency because water temperatures are easier to manage than outdoor air. Where drilling is impractical, WSHPs can capture many benefits of geothermal performance without a borefield.

Practical Tips For Owners And Designers

• Right-Size Units: Oversizing hurts efficiency and humidity control; use load calculations and diversity analysis.
• Minimize Pumping Energy: Specify variable-speed pumps, two-way valves, and thoughtful differential pressure control.
• Design For Service: Provide access for filter changes and coil cleaning; include isolation and balancing valves.
• Mind Water Quality: Implement a water treatment program from day one; monitor chemistry and biological activity.
• Plan For DOAS: Meet ventilation codes and dehumidification; consider energy recovery ventilators.
• Leverage Heat Recovery: Sequence controls to reuse internal heat before firing boilers or running towers hard.

Frequently Asked Questions

What Is A Water Source Heat Pump In Simple Terms?

It is a heat pump that moves heat to or from a water loop instead of directly to outdoor air. The water loop temperature is controlled so each zone unit can heat or cool efficiently.

Is A Water-Source Heat Pump The Same As Geothermal?

Not exactly. A boiler–tower WSHP uses a building water loop with a cooling tower and boiler. Geothermal uses ground loops to exchange heat with the earth. Both use water, but their heat sources are different.

Can A WSHP Make Domestic Hot Water?

Yes. A water-to-water WSHP can produce hot water for domestic use or hydronics. Pairing with storage tanks and heat recovery can raise efficiency and provide stable hot water supply.

Will It Work In Very Cold Climates?

Yes. WSHPs perform well in cold climates because loop temperatures are maintained. The boiler or heat pump boiler keeps the loop within range so the zone units run efficiently.

How Noisy Are WSHP Units?

They contain compressors, so acoustics matter. With vibration isolation, lined cabinets, and careful placement, WSHPs can run quietly in offices, hotels, and apartments.

What Maintenance Is Required?

Change filters, clean coils, and test safeties at the unit. Maintain tower water treatment and inspect pumps and valves. A BAS with alarms helps catch issues early.

What Are Typical Efficiency Numbers?

At standard rating conditions, EER in the mid-teens to 20+ and COP of 3–5 are common. Actual efficiency depends on loop temperatures, water flow, and controls.

Are There Rebates Or Tax Credits?

Many utilities offer WSHP incentives for commercial projects. Federal 179D can apply to efficient building retrofits. Residential eligibility varies by program; confirm current rules and required certifications.

What About Environmental Impact?

WSHPs enable electrification and heat recovery, cutting emissions, especially on cleaner grids. Newer models use lower-GWP refrigerants to reduce climate impact further.

Quick Checklist For Evaluating A WSHP Project

• Loads: Do zones experience simultaneous heating and cooling?
• Water Source: Closed-loop with tower/boiler or viable open-loop/pond?
• Distribution: Space for water risers, unit closets, and DOAS ducting?
• Controls: BAS integration, loop temp reset, and variable flow specified?
• Water Treatment: Plan and budget for ongoing chemistry management.
• Acoustics: Isolation and lining appropriate for space types?
• Incentives: Confirm utility rebates and documentation needs early.
• Compliance: Reference AHRI/ISO 13256-1 and applicable ASHRAE/IECC editions.

The Bottom Line

A water-source heat pump is a flexible, efficient HVAC approach that suits many U.S. buildings. By using a managed water loop, it delivers zoned comfort, heat recovery, and solid performance in hot or cold weather. With careful design, commissioning, and maintenance, WSHP systems can lower energy costs and support decarbonization goals without sacrificing comfort.