July 8, 2020
By: Matthew Crump, PE
When designing a multi-tenant residential or hospitality hi-rise building there are several different choices for the air conditioning system. The system selection must consider how the decision affects all the interested parties over the life of the building, including the developer, contractors, building management, owner, and/or dwelling owner in the case of a condominium. Ultimately, all these groups have common goals, but how they prioritize these goals may differ. The developer is concerned with return on investment, the contractor is concerned with ease of installation and reduced call backs, the building management wants to reduce maintenance, and the owner wants a reliable and comfortable space.
An easy choice to meet all these needs is a Water Source Heat Pump (WSHP) system. This system provides a refrigerant based heating/cooling system in each unit that is served by a building level water loop. Fundamentally the dwelling unit-based heat pump rejects/draws heat from a building water loop that has also has a heat addition or rejection system. The building system includes a cooling tower (heat rejection), boiler (heat addition), and piping to transport the water. The unit system includes a compressor, reversing valve, mixing valve, coil, heat exchanger, and fan.
WSHP Systems benefit all interested parties
From a developer standpoint the WSHP system provides a lower up-front cost than the traditional chilled water system. A chilled water system would require a chiller, (possible cooling tower), boiler, 4-pipe insulated system, and fan coil systems. The WSHP system eliminates one set of pipes and doesn’t require insulation. Also, the cooling tower is less expensive, less complex, and easier to maintain than an air-cooled chiller system. A WSHP eliminates the need for dwelling unit based external equipment as you would need from an air-cooled heat pump system, decreasing sound pollution and increasing the aesthetics of the building exterior. These benefits increase the return on investment of the building.
For the contractor a WSHP system is less complex and easier to install. Without the need for insulated pipes there is no worries of condensation causing an issue and a call back. In the case of the Nailor Serenity Vertical Stacked heat pumps, the assembly is modular. This allows the cabinets to be delivered and installed separately from the cooling cool and compressor chassis. This reduces risk of damage while on the job site.
A less complex system also benefits the building management through reduced maintenance costs and worry. The modular design on the Serenity Vertical Stacked WSHP makes it easy to service, the building maintenance can easily replace the chassis while maintenance is performed. This reduces downtime of the dwelling air conditioning. Concentrating most of the cooling work, and electricity used, in the unit allows the conditioning costs to be individually monitored. For a condominium development, it makes it easier for each unit to be responsible for these costs, which is not easy for a centralized chilled water system.
Finally, for the occupant or dwelling unit owner the highly efficient, reliable, and quiet system reduces their costs while providing a comfortable space. A study published in 2011 found that a WSHP system reduced overall building energy use by 8% when compared to an air-cooled chiller system and 20% compared to a water-cooled chiller system. Individual owners who understand the implications of this will be attracted to a building that makes a low impact on the environment.
Alternative System Comparison
When selecting the overall system design for a high-rise residential development there are fundamentally two alternatives to the WSHP system: chilled water fan coil units and air-cooled heat pumps. Each of these designs have benefits and challenges when compared to the WSHP system.
The most “traditional” HVAC system selected for large buildings is a chilled water system. With this design a chiller cools water that is delivered to the individual building units to cool the air in the space. This requires pumps, insulated supply piping, and insulated return piping to connect the chiller to the individual units. Each unit will have a fan coil that will transfer the heat from the space to the water from the chiller. There will also be boiler system that delivers heated water to the individual units for transferring heat to the air in each space. This will require an additional set of pumps and insulated piping.
Within the individual dwelling there will be a single or multiple fan coil units. The chilled and hot water will be distributed to these fan coils for conditioning the room. The fan coils consist of heating/cooling coils with a valve package, fan, and controls. When compared to the water source heat pump system, fan coils will operate quieter because there isn’t a compressor generating sound.
The challenges with the chilled water system mostly come from the complexity of the system. With multiple pipe systems, pumps, and the chiller it is a system that will require a more skilled maintenance program. The chilled water pipes can pose a condensation issue if they are not properly insulated throughout the building. With a centralized heating/cooling plant the maintenance can affect the entire building and could require some redundancy built into the system, further increasing cost and complexity.
During cooling chiller does provide some efficiency gains over the water source heat pump system when operating at full load, this however is rarely the case. For the heating a boiler system is only 80-90% efficient, much less than the 500% efficiency of the heat pump systems. In many detailed energy studies, the WSHP system has proven to provide better energy efficiency than the traditional chilled water system.
From a building management standpoint, having a centralized system requires the measurement and bill back of the heating/cooling used at the unit level. This can be a challenge because unit owners aren’t used to getting billed for this expense.
Air Cooled Heat Pumps
Another option is to put air-cooled heat pump systems at each dwelling. The biggest benefit of this selection is that all cooling/heating charges can be monitored for each unit and all building level systems would be eliminated. Because the conditioning is distributed throughout the units, the air-cooled design.
The challenge with this selection is mostly aesthetic. Each unit will require a condensing unit on the exterior. For low rise multifamily these can be located on the roof for less interruption to the external look of the building. This will centralize all the noise generated by these condensers and could be loud for the residents on the top floor. If the condensing unit is located on a balcony of the individual dwelling it could affect the look of the building and could cause noise interruption to the dwelling. This would be much louder than the compressor associated with the WSHP system.
The selection of the mechanical system for a high-rise residential building is a major investment in the future operations of the building. Understanding how this decision can affect those that touch the building throughout its lifecycle, will help to make the best choice. This is a first in a series of articles about Water Source Heat Pump systems, applications, considerations, and equipment. If you’d like to know more or have any questions please reach out to Matthew Crump, PE; Product Marketing Manager at firstname.lastname@example.org.