Water Source Heat Pump Loops

Water source heat pumps are slightly less common than other HVAC systems; however, are among the most efficient. Water source simply means that the condenser rejects into a water loop, not directly to the outside air. Water source can sometimes imply geothermal (since geothermal also involves a water loop), but this article will consider those as separate systems.

A water source heat pump operates as a normal heat pump. There are the same four stages of a normal refrigeration cycle: compression, condensation, expansion, and evaporation. While a normal (air source) heat pump will use a fan to reject heat from the condenser (and thus reject heat from the building), a water source heat pump will reject heat from the condenser into a circulating water loop by means of a shell and tube heat exchanger. Heat pumps, by their very nature, can also provide heat. For this to happen, the refrigeration system need only operate in reverse. More information about refrigeration cycles and differences between air-cooled and water-cooled systems can be found in separate articles.

A building will typically have water source heat pumps located along the perimeter of the building, for example, in offices and meeting rooms. The interior of the building will typically be conditioned through air handlers, also responsible for delivering outside air. This is not always the case, as is anything in the HVAC field, but happens often enough to consider it the standard. For this setup, the heat pumps will be responsible for the majority of the heating/cooling load.

These heat pumps will be provided water via a circulating condenser water loop. Water is delivered in parallel, meaning that the water leaving the heat pump goes directly back to the source and not into another heat pump down the line. The source is a combination between a cooling tower and boiler.

In the winter, the heat pump system will operate along the following procedure:

1. Hot water leaves the boiler, typically between 60-80 °F

2. This water is then pumped throughout the building to every water source heat pump

3. The hot water acts as a heat source for the condenser, allowing the compressor to operate under ideal conditions and deliver heat into the space

4. This process removes heat from the water loop

5. The water from each heat pump is pumped back to the boiler

6. The boiler heats the water back up to the desired setpoint

In the summer, the heat pump system will operate along the following procedure:

1. Cold water leaves the cooling tower, typically between 75-85 °F

2. This water is then pumped throughout the building to every water source heat pump

3. The cold water acts as a heat sink for the condenser, allowing the compressor to operate under ideal conditions and remove heat from the space

4. This process add heat to the water loop

5. The water from each heat pump is pumped back to the cooling tower

6. The cooling tower rejects heats from the water in order to deliver the desired setpoint

In many situations, there will be a heat exchanger between the cooling tower and the boiler. If the building is calling for heating, the condenser water will flow directly to the boiler. If the building is calling for cooling, the heat exchanger loop (connecting the cooling tower and condenser loop) will operate. The water still passes through the boiler, but the boiler is off. The condenser loop rejected heat into the heat exchanger/cooling tower loop.

The main benefit of the water source heat pump system is what happens during mild weather conditions. In milder conditions, portions of the building may be calling for cooling while other portions are calling for heating. For example, an office building with a lot of windows. The south side will receive a lot of sun light and will heat up. Although it is decently cold outside, the south side will need to be cooled. However, the north side does not receive the same amount of direct sunlight. So, the north side will need to be heated. The heat pumps will then operate along the following procedure:

1. Water enters the heat pumps on the south side of the building

2. The heat pumps reject heat into the water loop (for example, + 2 °F)

3. Water simultaneously (to step 1) enters the heat pumps on the north side of the building

4. The heat pumps remove heat from the water loop (for example, - 2 °F)

5. The water from all heat pumps comes together to flow back to the source

6. The water loop temperature remains unchanged, meaning the boiler and cooling tower do not need to run

7. Water is then delivered back to the heat pumps

In a sense, the building was able to use the heat on the south side of the building to heat the north side of the building. The ability to move heat within the building increases the overall system efficiency.