Modeling Hydronic Efficiency

By Peter Biondo

System efficiency in a hot water heating system or chilled water system is best carried out through the balanced distribution of energy. Water, in this case, is the carrier of energy. Therefore, to balance the energy within a building is to control the flow rate of water through the production side, starting at the power plant, and to manage the proportional flows through the distribution of sub mains, branches, and ultimately through the terminal units. The benefits of calibrating flow are comfortable and even temperatures throughout the building, plus energy efficiency at the production plant and throughout all parts of the system is maintained. Distributing energy within a hot or chilled water system is often referred to as “hydronic balancing.”

Without hydronic balance, underflow or overflow within piping distribution can undermine the performance and efficiency of a system by up to 40%. Without balanced flow, temperatures are difficult to control. Boilers and chillers may short-cycle or run more often. As a result, some rooms may have considerable temperature swings, while other rooms farthest from the boiler or chiller may not get the energy required to meet thermostat demand. Balancing can correct the problem that some troubleshooters might regard as an issue of boiler sizing.

Uneven temperatures are compounded, especially during low load conditions in spring or fall when controls work to strike a balance between heating and cooling. Even the slightest swings of temperature can lower operating efficiencies significantly. If the average temperature in a building exceeds the nominal value by 2°F, energy consumption is increased by 6 to10%. In cooling systems, temperatures that are 2°F too low will result in an increase in energy consumption of about 15 %.

Hydronic balancing is an energy-saving feature carried out through the use of balancing valves. ASHRAE recommends placement of a balancing device at the following locations: pump discharge, chillers, cooling towers and boilers, risers and branch circuits, and the emitters or fan coils. Balanced distribution was once painstakingly engineered and constructed by tapering down pipe diameters and also through reverse return piping. The designer still has to work with correct pipe sizing, but with the use of balancing valves, presetting and measuring make it possible to insure energy balance throughout the entire system.

Balancing flow begins at the power plant. The boiler or chiller will not operate at its rated efficiency and output if the flow rate is not calibrated to design. Balancing at the boiler or chiller eliminates the problem of overflow, which can cause short cycling and loss of performance. Regulating flow is particularly important for multiple chillers or boilers controlled for staged operation. Balancing valves would correct the problem of one unit robbing flow from another during a combined operation. Without balancing valves on each unit, overflow through one appliance may cause underflow through another. While overflow causes short cycling and lowers the unit’s rated efficiency, it also has a tendency to reduce the expected service life of the equipment. Underflow, on the other hand, can damage a boiler or chiller. A boiler’s heat exchanger may crack due to temperature stress. Underflow for a chiller increases the chances of tubes freezing and causing damage to the unit. Balancing at the power plant eliminates flow issues and insures delivery of the rated output into the water from each unit.

From the pump discharge, getting the right balance and flow to all the branches of piping and into each terminal unit is a matter of pushing the flow out to the farthest reaches of the building with balancing devices. The process of hydronic balancing is conducted in an organized procedure done by an experienced commissioning agent. Whether automatic valves, handwheel calibrated balancing valves, or circuit setters are installed, the agent’s responsibility is to insure that the hydronic system is operating at design specifications. Understanding how a system is brought into balance is best explained through the work of a commissioning agent.

Pushing the flow out to the farthest branches and terminal units is a matter of creating resistance in the sub mains, branches, and terminals that are closest to the boiler or chiller. The balancing valve, by means of a handwheel-turned dropped stem or an automatic venturi, adjusts the resistance to flow and enables the commissioner to have a reference place to measure the flow rate. The commissioning agent begins with the balancing valves at the power plant and then works with the balancing valves at the sub mains, starting from the closest valve from the pump and ending at the farthest valve. This process is carried out for balancing valves on the risers, the branches and finally at the terminal units (such as fan coils). The commissioner always begins at the closest balancing valve to the pump discharge and works his way out to the farthest balancing valve for each group and sub group.

Calibrated balancing valves (CBVs) have many practical features that are serviceable to the commissioning agent and the service technician. CBVs give the commissioning agent the ability to preset the valve for the desired flow rate, and then to adjust and measure at the CBV the precise flow rate within 3% measuring accuracy. The multi-turn handle allows for a wide range of flow adjustment. CBVs are designed to have low pressure drops across the valve (1psi or 2.3 ft of head minimum drop for test and balance) and offer high Cv values. This affords lower energy consumption at the pump over automatic balancing valves. A service feature unique to CBVs is the built-in memory position at the top calibrated setting, and the valve can be used as a positive shut-off valve. Also, adapters can be put in place of the measuring ports for draining and filling. These features allow for the CBVs to be utilized for servicing the hydronic system. With the memory stop in place, opening back the valve handwheel to memory resets the balancing valve as the commissioning agent left it. Handwheels can also be locked.

Balancing valves are particularly important at fan coils. Energy output at the fan coil is dependant on flow, water temperature, and air velocity. All three must be in balance for the coil to operate at its rated output. This is particularly important in a variable flow system, when a two-way proportional control valve is installed at a fan coil. The relationship between the control signal and the resulting thermal power from the coil, referred to as the circuit characteristic, determines the controllability of the system. In variable flow systems, differential pressure across the control valve must not vary too much in order to prevent the circuit characteristic from distorting. Without pressure stabilization valves, small changes in the control signal, even tiny alterations, may cause large swings in thermal output. On the other hand, any actions from the control signal could result in marginal changes of thermal output. Balancing valves stabilize differential pressures at the control valve and improve the circuit characteristic. The result is a better control relationship from changes in the proportional control valve to the thermal output at the coil. No matter how advanced the controls, an effective measure to improve the circuit characteristic across a fan coil includes a means to set the design flow rate for the fan coil, at the control valve fully open so the thermal output will respond more linearly to proportional flow.

Balancing valves for fan-coil units and surface heating systems are also manufactured in valve combinations with controls. These valves offer the benefit of a control valve with a linear flow characteristic, which is advantageous when using proportional actuators that have linear stroke behavior. These combination valves are space savers in air handler units and require less labor cost to install than building individual components together. The flow range is easily viewed and can be set with the help of a lockable handwheel. These valves can include test points for measuring the regulated flow.
Heating and air conditioning technology has come a long way. Energy efficient boilers and chillers, intelligent controllers and building management systems have replaced yesterday’s crude and often oversized systems. But one problem remains — complaints about comfort persist, while at the same time HVAC systems are using 40% more energy than intended. Even when the most recent technology has been implemented, property managers can’t determine why a building cannot achieve temperature balance. Hydronic balancing isn’t just a good idea, it is good practice. Balancing valves are an important component throughout the hydronic system to evenly distribute the energy for heating or cooling a building. Quite frequently, insufficient attention is paid to balancing a hydronic system—inexpensive valves are chosen, poor choices are made, and efficiencies that could be gained are lost. You wouldn’t drive circles around the block to get where you want to go—you want to find the most efficient route to get to where you want to be. Hydronic balancing delivers the energy throughout the piping network or roadways where it needs to go efficiently and right on time.

Peter Biondo is the Technical Sales Coordinator for Oventrop Corporation. He has been involved in solar hot water and hydronic heating for more than 25 years. His primary work is assisting mechanical engineers and contractors with hydronic heating systems, as well as solar domestic hot water and heating for residential and commercial applications. He is a workshop instructor for Solar Energy International, and a Webinar instructor for the Radiant Panel Association. His solar and hydronic workshops are featured at trade show conferences throughout the country.