Hydronic heat pump loops

A hydronic heat pump loop connects many water-to-air or water-to-water heat pumps to a common water circuit. Each heat pump draws heat from the loop when its zone needs heating and rejects heat to the loop when it needs cooling. When some zones heat while others cool, energy moves between them through the loop, and the central plant only needs to make up the difference.

This arrangement, often called a water loop heat pump or WLHP system, has been common in Canadian offices, schools, hotels and multi-unit residential buildings for decades. With the push toward electrification, it has gained new attention as a flexible way to combine heat pumps with geothermal fields, heat recovery chillers and waste heat sources.

How the loop works

The loop is a closed hydronic circuit maintained within a moderate temperature band. Typical components are:

• Heat pumps at each zone, usually water-to-air units in ceilings or closets, or water-to-water units serving radiant or fan coil systems.
• Loop circulating pumps, usually with variable speed drives.
• A heat rejection device that removes heat when the loop gets too warm: a closed-circuit fluid cooler, a cooling tower with a plate heat exchanger, or a geothermal field.
• A heat addition device that adds heat when the loop gets too cold: a boiler, an electric boiler, an air-to-water heat pump, a geothermal field or a waste heat source.
• Expansion, air separation and water treatment equipment like any closed loop.

Typical temperature band

Conventional boiler and tower loops often operate roughly between 15 C and 32 C (60 F and 90 F). The plant adds heat when the loop falls toward the lower limit and rejects heat when it rises toward the upper limit. Between those points, the loop floats and no central energy is needed.

Geothermal loops operate over a wider and lower band, often near or below 0 C (32 F) entering water in heating mode in colder Canadian climates, which requires antifreeze. See geothermal heat exchange for commercial buildings.

The heat pump's performance depends on loop temperature. A warmer loop improves heating efficiency, a cooler loop improves cooling efficiency. The band is a compromise.

Where the system works well

In a Canadian office with a large interior core, winter often brings significant simultaneous cooling of core zones and heating of perimeter zones, which is exactly the condition where the loop pays off.

Design considerations

Loop flow and pumping

Heat pumps are typically selected for a loop flow in the range of about 0.036 to 0.054 L/s per kW (2 to 3 gpm per ton). Older systems ran constant flow through every unit, whether its compressor was running or not. Modern practice is to install a two-way isolation valve at each heat pump that closes when the compressor is off, combined with variable speed loop pumps. This can cut pumping energy substantially, since many units are idle at any given time. See variable speed pumping and differential pressure setpoints.

Heat rejection

Closed-circuit fluid coolers keep loop water out of contact with air, which protects the small heat exchangers in heat pumps from fouling. An open cooling tower with an intermediate plate heat exchanger achieves the same separation at the cost of an approach temperature. Directly connecting an open tower to the heat pump loop is generally avoided.

In winter, fluid coolers outdoors need freeze protection: glycol in the loop, a glycol-filled fluid cooler isolated by a heat exchanger, or draining and dampering the unit.

Heat addition

Traditional systems used a gas boiler. For electrification, options include electric boilers, air-to-water heat pumps, heat recovery chillers and geothermal fields. Each has implications for peak electrical demand, which is often the deciding factor in Canadian climates. Resistance electric heat is simple but costly to run; heat pump sources keep overall efficiency high. See electrification of commercial heating.

Thermal storage

Adding a buffer tank or increasing loop volume lets the loop absorb daytime heat and release it at night or during morning warm-up, reducing plant cycling.

Ventilation

Water loop heat pumps are poorly suited to handling large amounts of cold outdoor air directly. Most designs provide ventilation through a separate dedicated outdoor air system with energy recovery.

Controls

A good loop control sequence:

1. Monitors loop supply and return temperatures.
2. Stages heat rejection (fluid cooler spray pump, fans) as loop temperature rises above the upper setpoint.
3. Stages heat addition as loop temperature falls below the lower setpoint.
4. Prevents heat addition and rejection from running at the same time.
5. Resets loop pump speed based on differential pressure or valve demand.
6. Alarms on loop temperatures outside safe limits for the heat pumps.
7. Interlocks each heat pump's isolation valve with its compressor.

Widening the floating band, within the limits of heat pump capacity and comfort, reduces plant energy.

Maintenance issues

Heat pump systems put many compressors and small coaxial heat exchangers throughout a building. Common issues include:

• Clogged unit strainers and fouled coaxial heat exchangers from poor loop water quality.
• Condensate pan and drain problems in ceiling-mounted units.
• Isolation valves failing open or closed.
• Noise from units located above occupied spaces.
• Filter changes at many locations.

Water treatment and filtration on the loop protect dozens or hundreds of small heat exchangers; see water source heat pump systems for unit-level considerations.

Frequently asked questions

Is a water loop heat pump system the same as geothermal?

Not necessarily. A geothermal system uses a ground heat exchanger as the heat source and sink. A conventional water loop uses a boiler and fluid cooler. Many systems combine both.

Can existing water loop heat pump buildings be electrified?

Often yes. Replacing the boiler with an electric or heat pump source is relatively straightforward because the zone heat pumps already run on electricity.

Key takeaways

• A shared water loop lets heat pumps move heat between zones, so the plant only handles the net load.
• Loops typically float between about 15 C and 32 C (60 F and 90 F), with geothermal loops running colder.
• Two-way isolation valves at each heat pump and variable speed pumping save significant pump energy.
• Heat addition choices drive carbon emissions and peak electrical demand.
• Loop water quality and unit maintenance determine long-term performance; geothermal systems can extend the loop's low-carbon potential.