Water source heat pump systems

A water source heat pump (WSHP) system uses many small heat pumps distributed throughout a building, all connected to a common water loop. Each heat pump either extracts heat from the loop to warm its zone or rejects heat into the loop to cool its zone. A central boiler or other heat source adds heat when the loop gets too cold, and a cooling tower, fluid cooler or ground loop removes heat when it gets too warm.

WSHP systems have been used in Canadian offices, schools, hotels and multi-unit residential buildings for decades. They provide individual zone control and recover heat naturally between zones. When paired with a geothermal loop or another low-carbon heat source, they are also a practical route to electrified heating.

How the system works

1. Each zone has its own heat pump, typically a horizontal unit above the ceiling, a vertical unit in a closet, or a console.
2. Each heat pump has a compressor, a refrigerant-to-water heat exchanger, a refrigerant-to-air coil, a reversing valve and a fan.
3. A pumped water loop runs through the building, connecting every heat pump.
4. In cooling mode, a heat pump rejects heat into the loop. In heating mode, it absorbs heat from the loop.
5. When interior zones are cooling and perimeter zones are heating, heat moves through the loop from one to the other, reducing the need for central heating or cooling.

Loop temperature control

The loop is kept within a temperature band suitable for the heat pumps. For a conventional boiler and tower loop, a typical range is about 15 to 32°C (60 to 90°F). Ground-coupled loops operate over a wider range, often from near freezing (or below, with antifreeze) to about 35°C (95°F), because the ground acts as both heat source and sink.

Rating standards for water-to-air heat pumps, such as ISO 13256-1, report performance at standard entering water temperatures for boiler and tower and for ground loop applications, which helps compare products for each type of loop.

Advantages

• Individual zone control with each heat pump responding to its own thermostat
• Built-in heat recovery between zones with different loads
• Small distribution with water pipes rather than large ducts
• Failures are local: one heat pump failure affects only its zone
• Flexible electrification: the loop can be connected to ground loops, heat recovery or air-to-water heat pumps instead of a boiler
• Simple tenant metering in some multi-tenant buildings, since each heat pump has its own electrical supply

Limitations

• Many compressors in occupied areas create noise and maintenance points throughout the building.
• Ventilation is separate: WSHP units recirculate air, so a dedicated outdoor air system or other ventilation is required.
• Maintenance is distributed: filters, condensate drains and coils are spread across ceilings and closets.
• Loop temperature tradeoffs: a cooler loop helps cooling efficiency but hurts heating, and vice versa.
• Boiler and tower energy: in conventional loops, the central plant still uses fossil fuel and water unless replaced with low-carbon alternatives.

Design considerations

Pumping and flow

Older WSHP systems often use constant-speed pumps circulating full flow 24 hours a day. That wastes a lot of energy. Modern practice uses two-way motorized valves at each heat pump, which open only when the compressor runs, and variable-speed loop pumps controlled from differential pressure. Our guide to variable speed pumping explains how to set the pressure setpoint.

Design flow is typically around 0.045 to 0.055 L/s per kW of heat pump capacity (about 2.5 to 3 US gpm per ton), but always confirm with the manufacturer's data.

Heat rejection

Closed-circuit fluid coolers are common because they keep the building loop clean. Open cooling towers with a plate heat exchanger are also used. In Canadian winters, heat rejection equipment needs freeze protection: glycol in the outdoor circuit, basin heaters, or isolation and draining. See cooling tower selection and winter operation.

Heat addition

Boilers can add heat directly or through a heat exchanger. For electrification, options include air-to-water heat pumps, heat recovery chillers, connections to district energy networks or ground loops.

Ventilation

Outdoor air is usually supplied by a dedicated outdoor air system with energy recovery. Supplying untempered outdoor air directly to heat pump inlets in winter can cause low entering air temperatures and freeze trips.

Water treatment

Each heat pump has a small water-to-refrigerant heat exchanger that can foul quickly if water quality is poor. A closed loop with good water treatment, filtration or strainers, and air and dirt separation is essential. See water treatment for closed and open loops.

Retrofit opportunities in existing WSHP buildings

Many older Canadian buildings have WSHP systems from the 1970s to the 1990s. Common upgrades include:

1. Replacing heat pumps with higher-efficiency, variable-speed models as they reach end of life
2. Adding two-way valves at each unit and variable-speed pumping
3. Replacing open towers with closed-circuit fluid coolers or adding plate heat exchangers
4. Adding or upgrading a dedicated outdoor air system with energy recovery
5. Replacing boilers with heat pumps or connecting to a ground loop for electrification
6. Rebalancing the loop and verifying flow at each unit

Maintenance

Key maintenance tasks include:

• Filter changes at every heat pump, usually quarterly or more frequently
• Condensate pan and drain cleaning, with safety switches checked
• Water-side heat exchanger performance checks, using entering and leaving water temperatures
• Loop water treatment testing and strainer cleaning
• Fluid cooler or tower maintenance and freeze protection checks before winter

Good access is critical. Heat pumps installed above hard ceilings without access panels are often neglected.

Frequently asked questions

Is a WSHP system the same as geothermal?

Not necessarily. A WSHP system can use a boiler and tower loop with no ground connection. When the loop is connected to a ground heat exchanger, it becomes a ground source or geothermal system.

How efficient are WSHP systems?

Efficiency depends on loop temperatures, heat pump efficiency, pumping energy and how much heat moves between zones. Systems with variable pumping and modern heat pumps can be quite efficient; older constant-flow systems often waste significant pump energy.

Do WSHP systems need balancing?

Yes. Each heat pump needs its design water flow, and the loop needs correct differential pressure. Water balancing verifies both.

Key takeaways

• WSHP systems connect many zone heat pumps to a common loop, moving heat between zones and to or from a central plant.
• A boiler and tower loop typically runs about 15 to 32°C (60 to 90°F); ground loops operate over a wider range.
• Variable-speed pumping with two-way valves at each unit saves significant energy over constant flow.
• Ventilation must be provided separately, often with a dedicated outdoor air system.
• Water treatment, filter changes and accessible units are essential to long-term performance.