Advanced Boiler Control
By David A. George
In a few short years, boiler control technology has made dramatic advancements. Boiler controls have become smarter, leaping out of the dark ages and into an era of complex boiler operation. In fact, smart boiler controls have advanced beyond boiler operation and into related operations, such as the management of multiple boilers, boiler pump and system pump control, domestic hot water production and more. System control functions once only available via a remote control or Building Automation System (BAS) are now “on board” the boiler.
First and foremost, advances in boiler control were made due to advances in combustion systems. One such combustion system, Negative/Regulation (Neg/Reg), operates using blower speed and matching gas flow to modulate the main burner across a broad input range. Neg/Reg is unique, because changing negative air pressure informs the gas valve on the amount of gas to supply. In other words, the blower and the gas valves are smart now too.
Soon, PID logic was employed to control the modulation of new combustion systems. PID combines proportional, integral and derivative logic into a set of directions for the boiler to operate. Proportional is the difference between the actual water temperature and the target temperature or setpoint. Integral is the difference between the actual water temperature and the setpoint over time. Derivative is the rate at which change is occurring in the difference between the actual water temperature and the setpoint. These three ways of calculating the boiler’s modulation rate work together to achieve control that is more accurate than any single calculation.
Add to this the idea of outdoor reset, which changes the setpoint temperature based on outside air temperature. As the outside temperature drops, a reset curve in the control logic will increase the setpoint temperature. This puts higher water temperature in the heating system to offset the greater heat loss created by the lower outside air temperature.
Multiple boiler operation was the next logical step in onboard boiler control. In most commercial applications, multiple boilers or banks of boilers are installed. This provides boiler redundancy to deliver a level of safety against total system shutdown if only one large boiler were installed. Previously, multiple boiler control was provided by a separate modulating or stage controller. By building this into the boiler’s operating control, an important function is offered at no extra cost. Plus, on-board multiple boiler control is specially configured to match the particular design and operating characteristics of the boilers.
Often referred to as “cascade,” the boilers will be wired together or “daisy chained.” One boiler will be assigned the task of group leader. If there is a building automation system, it will communicate with the lead boiler. Appropriate sensors required for boiler operation or group operation will be connected to the leader. The actions of the group will be decided by the leader.
The follower boilers will fire upon a call for heat directly from the leader. Depending on the programming, the follower boiler will fire at a given input rate chosen by the leader or by its own onboard logic. Typically, the leader and the follower boilers will “take turns” being the first one to come on each day. This operation gives equal cycle time to all the boilers over a long period of time.
Another new feature is domestic hot water prioritization (DHW). For DHW, a boiler would be piped to an indirect water heater as well as to the space heating system. An indirect water heater is a storage tank for potable water. Inside the tank is a coil of copper or stainless steel tube. Heated water from the space heating system passes through the coil, transferring its heat into the potable water. With this design, a low cost indirect fired tank is connected to an existing space heating system to provide domestic water. This reduces system cost by eliminating a separate domestic water heater and storage tank.
The control logic will monitor the space heating system and the DHW system. If the DHW system needs heat, the control will shut off flow to the space heating system, direct flow over to the indirect tank and fire the boiler for appropriate heating. Typically, the boiler will fire at a high input rate with a high setpoint temperature. The idea is to quickly satisfy the domestic water needs and get back to the job of space heating.
Smart controls can now manage more operations beyond the boiler’s internal operations. Night setback is a popular cost saving function built into many boiler controls. As the name implies, this allows the control to alter the setpoint during hours and days when the building is unoccupied. The boiler will fire at a lesser rate or shut off entirely, which saves fuel costs.
In addition to operating the boiler, a smart control program can reach out beyond the boiler to control the operation of other related devices in the system. The most obvious is pump control, such as pumps that flow water through the boiler and pumps that flow water throughout the space heating and DHW systems. The smart control can coordinate the firing of the boiler and the flowing of water with the pump, turning the pump on or off in time with the call for heat. More advanced controls can modulate variable speed pumps in synchronization with the modulation rate of the boiler.
In addition to improved boiler control, these new smarter control programs can collect and transmit a large amount of operational information. Status of the boiler is primary. Is there a call for heat? Is the main burner on to answer the call for heat? Is the boiler shut down on a fault? What is the fault?
Control programming can collect other operational data, such as Hours of Main Burner operation. Hours of Main Burner operation can be captured at low or high input rates, such as less than 25%, 25% to 50%, 50% to 75% and 75% to 100%. The controls can record the number of attempted ignitions and the number of successful ignitions. The control can count the firing time, the firing cycles and inform the customer that the boiler is due for service.
Smart boiler control is a natural progression, and it’s not over yet. There is a future out there for even smarter boiler control — wireless communications, Internet access, email reports of faults. Super smart boiler control is a lot closer than you think.
David A. George is a product manager for Lochinvar Corp.
