A heating, ventilation and air-conditioning (HVAC) piping system is designed to convey a heat transfer fluid to multiple air handlers, providing cooling (chilled water) or heating (hydronic heating) loads in a building. A typical HVAC system consists of a primary loop and a secondary loop. The primary loop contains the chiller or boiler along with primary circulating pump(s). The secondary loop draws fluid from the primary loop and consists of a secondary pump along with the supply header piping, various air handlers and return-header piping. The primary and secondary loops are connected, so fluid mixes between the loops.
The key to designing an HVAC circulating water system is to control the air handling flow rates to maintain the temperature in the conditioned spaces. There are two methods to achieve this goal; one is a constant volume system, the other a variable demand system. Below is a case study of how a project in the Pacific Northwest achieved cost savings by using a variable demand system versus a constant volume system.
Modeling the System
Before you design a new system, or make major modifications to an existing piping system, it is a good idea to create a computerized hydraulic model of the HVAC piping system. Piping software can provide the information needed to properly select the pumps and control valves. The model also provides a good understanding of the interaction of pumps, pipelines and control valves.
The information from the computerized model can also be used to perform an economic analysis of pumping costs for both types of control systems. Image 1 is a rendering of a modeled HVAC system.
To perform this analysis, look at the differences between constant volume control and variable demand control, and perform a pumping cost analysis for each.
Constant Volume Control vs. Variable Demand Control
Under constant volume control for the piping layout in Image 1, both secondary pumps are operating just over 660 gallons per minute (gpm). Since this is a constant volume system, both pumps will run constantly at this flow rate year-round. To keep the volume constant, some flow will go through the air handlers, and the remainder will go through a bypass line.
In a variable demand system, the flow rates to the loads vary based on the required heating or cooling needs. The first requirement is to estimate the required flow rates. They are based on the heating or air conditioning loads and may vary by the time of year and time of day.
In this analysis, it has been assumed that the average flow requirements in the winter months will be 30 percent of the maximum design flow, spring and fall is 45 percent of the maximum, and in the summer the design flow is 85 percent of the maximum. Note that the maximum design load is based on the needs for the hottest or coldest day of the year.
Constant Volume Pumping Costs
For this project, the yearly pumping costs for the system were an energy rate of 8 cents per kilowatt hour (kWh). This should provide a conservative estimate of costs elsewhere as the national average is close to 13 cents per kWh. The operating costs for pumps SP1 and SP2 are $10,012 and $10,062. Together, the operating cost for the secondary pumps at a constant volume and fixed speed is $20,074.
Variable Demand Pumping Costs
The variable demand costs are slightly more complicated as the loads vary more throughout the year. Here, we are using the flow rates mentioned above and considering summer, winter, spring and fall to each account for 25 percent of the year. The spring and fall demands are the same so they have been combined for a total of 50 percent of the year.
The system is capable of meeting the demands year-round using only one secondary pump. The operating costs for this secondary pump are $3,080 in the summer, $140 in the winter and $560 in the combined spring and fall seasons for a total of $3,780 for the year.
The annual operating cost for the secondary pumps is less than the cost associated with the constant demand system. The reduction in cost can be brought about by installing a variable frequency drive (VFD). It is important to factor in the cost of the VFD in your life cycle pumping cost calculations.
Constant Volume System
The constant volume system is such that a constant volume of fluid is flowing to each air handler regardless of the load. The piping to the air handler consists of a three-way valve that directs flow to the air handler or to a bypass line (Image 4).