The struggle to obtain maximum temperature comfort for building tenants at an affordable cost has long been a challenge in the commercial real estate market. With new commercial HVAC technology coming into the marketplace every day, the innovative performance of two-stage water source heat pumps are starting to become more widely accepted for achieving maximum comfort at minimum cost.
While the two-stage heat pump has been on the market for eight years, demand is now surging. This is due in part to a better understanding of how a two-stage outperforms a single stage heat pump. Having been installed in the market place, building owners and managers are noting the lower operating costs, as well as the better comfort conditions, giving them an advantage in the highly competitive commercial real estate market where owners need a differentiator from other buildings. Manufacturers of two-stage components have also made it more applicable to buildings by including commercial and residential voltages, as well as making it available for new construction and expansions or replacements. The two-stage heat pumps are perfect for office buildings and schools or in places where occupant comfort is a priority.
It is important to note that these heat pumps are often geothermal systems. They work by conditioning the air with water, but are capable of making both cold and warm air. This capability is possible because the system not only draws heat from the earth for warmth but also rejects the heat to the earth for cooling through polyethylene plastic pipe.
Two-Stage versus Single Stage Heat Pumps
The two-stage versus single stage heat pump has caused some confusion as to their differences. The primary difference between the two is the type of compressor they use.
With a single stage, the compressor runs only at full load capacity, meaning once the temperature is satisfied, the compressor turns off, causing noticeable swings in room temperatures. On the other hand, the two-stage compressor can operate at part load or full load capacity. When cooling is needed, the two-stage heat pump compressor turns on, but when stage two of the thermostat is satisfied, the compressor continues to run at a lower level, stage one. This allows tenants to enjoy a more even room temperature.
The ability of the two-stage unit heat pump to run at part or full load capacity gives it significant operating cost benefits over traditional single stage pumps. Because the two-stage unit consumes less energy and does not have to work as hard to adjust to swings in room temperature, it results in a lower operating cost while keeping occupants comfortable.
When you consider that the part load output on a two-stage compressor operates at 67 percent, the result is a 22 percent decrease in cooling and heating capacity. At part load, the amount of kilowatts consumed is less with a two-stage unit. Because the unit can operate at either full or part load capacity, the kilowatts fall off faster than the capacity. The end result is a 40 percent increase in efficiency.
On average, the two-stage unit costs $250 more than a premium single stage unit, making it a very affordable unit for the payback received. When considering the benefits building owners and managers will receive in energy savings and in room temperature comfort, the additional fee becomes nominal, especially for Class A office space.
Factoring Cost Savings
To determine cost savings it is important to have an engineer or contractor look beyond the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) ratings. Instead, look at real life conditions based on building loads and design parameters. Summer design water temperatures for ground loops are around 90 degrees. AHRI uses 77 degrees as its rating point, but that is not a real design temperature. In the winter, AHRI rates at 32 degrees, while the real life design point is more commonly 45 degrees. All specifiers have access to software where they can enter the loads into a computer program and simulate the geothermal system. Once these are gathered and put in the software, you will have a more accurate sense of the operation cost comparison and compare that to the unit cost. This comparison will be able to tell you the amount of dollars saved per year and thus the payback period. Most often a building owner will receive $600 in annual energy savings, a seven year payback and a $4,000 cost add for two-stage units.
To evaluate the potential energy savings, Daikin McQuay used eQuest™ to determine the operating cost for a small, single level, 57 ton cooling load, 35,000 square foot elementary school in St. Louis. The building was divided into 16 zones, each having a 4-ton heat pump. With the 16 single stage units, the school consumed 166,552 kWh annually, while the same school consumed 160,847 kWh annually with the 16 two-stage units. At a 10 cent per kWh electrical rate, the annual operating cost savings for the two-stage units is $570.50. The cost premium for two-stage units is $4,000 based on $250 premium per unit. Simple payback calculation yields a seven year payback, which is acceptable in the school market. The graphs below show the monthly energy consumption for each system.
Single Stage Units Two-Stage Units
Life cycle cost (LCC) is another way to analyze the cost savings. Using a 20 year project cycle, eight percent discount rate and two percent energy escalation cost, the LLC for the single stage unit is $122,183, compared to $114, 625 for the two-stage unit. The $4,000 investment yields a $7,558 savings after 20 years.
Actual Operating Conditions
Another important consideration is that operating conditions for two-stage and single stage heat pumps vary throughout the year. In a building, depending upon the outside weather, air conditioning could be needed in the month of March or heating could be needed in the month of October. In March, because the water temperature in the ground loop may be cold, capacity of the unit increases. In October, because the water temperature in the ground loop is warmer, the capacity also increases. The run time for a single stage unit is shorter, leaving building occupants with poor air comfort. A two-stage unit reduces capacity so the run time is longer, leaving the building with an end result of better comfort during those transitional weather seasons. Because the unit can operate at a full or part load capacity, it makes it perfect for those times when ground water and air temperature don’t match.
Two-stage heat pumps also have the ability to vary airflow. The electronically commutated fan motor (ECM), can be programmed at a lower airflow volume, cubic feet per minute (CFM). At lower airflow, the two-stage unit will produce more latent cooling, which results in lower humidity in the space. The compressor runs longer, which allows for lower temperature swings and improved control of humidity. This will then push the comfort level higher and the energy savings will also subsequently increase.
With the advances in today’s HVAC technology, building owners and managers should be less tolerant of high electric bills and noticeable swings in room temperature. With almost eight years’ worth of successful applications of two-stage heat pumps being applied to commercial properties, it is time to begin investigating into what a more technologically advanced heat pump can mean for your building. When choosing heat pumps for your next project, the two-stage heat pump unit is the clear choice for all around comfort and energy savings.
Craig Fischbach has over 30 years of experience in the HVAC business. He is a product sales manager for Daikin McQuay