August 1, 2010
By Dave Hudson And Dwayne Squires Victaulic
The symptoms of indoor climate problems within buildings usually surface as complaints from building tenants. The living or working spaces are too cold in winter, too hot in summer, or some combinatio...
The symptoms of indoor climate problems within buildings usually surface as complaints from building tenants. The living or working spaces are too cold in winter, too hot in summer, or some combination of both extremes year-round. In response to these temperature variations, building occupants often compensate by using space heaters, opening windows and adjusting thermostat settings.
There are adjustments that can be made to the HVAC system to correct these issues. These could include the installation of larger pumps, the resizing of components, the changing of night setback and morning start-up times, and flow adjustments in the system’s mains, branch lines, and circuits.
However, such “fixes” are costly and ineffective — and they ignore the root of the problem. Resetting a workplace HVAC-system start-up time to 5:30 a.m. from 7:30 a.m. means that a plant operates at capacity for two additional hours a day. This is a 25% increase in energy consumption, which negates the energy and cost savings that night setbacks were designed to achieve. Even minor changes — such as a one-degree change in the thermostat setting — can be very costly. Each degree Fahrenheit increase in a thermostat’s setting can add 6% to a building’s heating costs, while each degree Fahrenheit decrease can add 8% to a building’s cooling costs. Additional consequences of changing an operating system can include increased wear on pumps and HVAC components, and reduced controlvalve authority. Finally, the changes may cause problems with other tenants who begin to complain.
The root of most indoor temperature and climate problems can be traced to incorrect flow rates due to improper balancing in the terminal units. Therefore, the key to an HVAC system’s effectiveness and efficiency is to control and balance the flows to terminal units properly. Because consulting engineers typically design HVAC systems with excess capacity, the ability to provide necessary heating or cooling energy is present. The challenge is getting that energy to terminal units and air-handling units (AHUs) in the most efficient and effective manner possible.
Dealing with underflow or overflow conditions
The flows in an HVAC system change over a 24-hour period. Due to heat gain from the sun and changes in building occupancy, the demand for heating and cooling in a commercial structure varies not only throughout the day, but also by the position and segment within the building. An effective and efficient HVAC system must provide the correct energy output when and where required.
Hydronic balancing is essential in an HVAC circuit. When a system is unbalanced, sections of a building will have underflow or overflow conditions that impact controlvalve authority and, thus, indoor climate. For instance, the terminal units closest to chillers, boilers and pumps could receive excess flow, resulting in excessive heating or cooling. Terminal units farthest away from those units could receive insufficient flow, resulting in inadequate heating or cooling.
The building controls in a large commercial structure are the thermostats or sensors in each room or space that take in and read temperature information. These readings electronically trigger the mechanical systems to open/close or to modulate valves, which changes the flows in the cooling or heating circuit to maintain the temperature at the specified level.
A consulting engineer will typically specify both the building controls and the hydronic controls/system, and thus needs to make sure that the two systems are integrated and compatible to achieve the desired goal of climate control.
The challenge with integrating the hydronic systems with the building controls is that the building controls are dependent on the hydronic systems in order to function correctly. The control system can only be as good as the hydronic system. If the system is not properly designed, installed or balanced correctly, the system design flows will not be achieved. Thus, it is very important that the consulting engineer’s initial plans are followed in order to achieve the design flow.
Three principles for effective design
There are three important principles to address when discussing effective hydronic balancing.
First, the design flow must be available at all terminals. This need can be compromised for a number of reasons, but mainly because the balancing valves are not properly adjusted. If the balancing valves are not in the right places, or if there are not enough valves, then ultimately the system will not perform to its specified parameters. There will be a greater cost to the owner because of underflow or overflow conditions.
One can ensure the availability of the design flow by choosing and set ting the balancing valves according to the design flow specified rather than the pipe line size. Installing a balancing valve based upon line size and not flow rate may result in the inability to obtain the correct circuit flow rate. For example, the design line size to a specific coil may be ” and the coil has a required flow rate of 1 gpm. However, a flow rate of 1 gpm is in the recommended flow range for a ” valve. As a best practice, it is always a good idea to consult the valve manufacturer or its sales representative to ensure the valves are being used and installed correctly.
The second key is to make sure that the differential pressure across the control terminals doesn’t vary too much. The control valve authority should be 0.25 minimum, with 0.5 or greater being optimal.
This requirement means that within an HVAC circuit the pressure across the whole loop should be the same. This is also where building controls become an issue, because if that pressure is constant, then the controls will have an easier time doing their job of sensing and regulating the temperature. The best approach is to make sure that the differential pressure control valves and differential pressure sensors for variable speed pumps are in the right locations from the start, so that there are no spikes at the control valves which would turn them into an on/off mechanism, rather than a modulating system. Differential pressure control valves are located on the branch lines to specific system modules in the area where the isolation valves would be located. The differential pressure sensor is located on the mains near the index circuit of the system.
The final key is to make sure that the water flow rate and temperature from the production side is compatible with all the system interfaces. For example, in the summer you need to make sure that you have enough chillers and capacity to provide flow to cool the building. If you haven’t done the first two steps above, then this step is difficult, as they all tie together.
Far too many buildings are unnecessarily plagued by temperature variations that can lead to tenant complaints and high energy and operating expenses for owners. In most cases, these issues can be easily resolved by properly balancing the heating or cooling system to conform with the original design performance specifications from the consulting engineer.
In addition to observing the three keys to hydronic balancing above, a comprehensive hydronic balancing program should be integrated into commissioning to save time and energy and to improve the long-term value of a building.
Dave Hudson is a senior engineer with Victaulic in Euston, PA. Dwayne Squires is hydronic balancing manager for Victaulic in Canada, based in Burlington, Ont.