Overview of Building Management Systems and its Role in Boiler Operations
Building Management Systems control the various heating / cooling zones by well tested and established methods, such as Outside Air Reset, controlling 3 port mixing valves on variable temperature circuits and cycling heating system equipment ON/OFF based on loop temperature set points. The BMS can also take into account varying space set points by throttling hot water supply to baseboard zones, VAV’s and AHU hot water heating valves. Although this is highly efficient, these settings only control the supply of heat to the occupied spaces. Regardless of the actual heating load within the building, the boiler continues to cycle ON/OFF. This is inherently inefficient for the following reasons:
1) A boiler system with low heating load will end up short cycling due to the low heating load. A boiler is most efficient when it is in steady state operation; when it cycles ON, it should stay ON and when it is OFF, the goal should be to keep it OFF. Short cycling of the boiler increases mechanical wear on motors and valves. The short cycling also introduces excessive purge cycles that ultimately cool the boiler, wasting energy.
2) With fixed cycling operational temperature bands (ON at 120F, OFF at 140F as an example), the boiler is programmed to deliver sufficient heat to the building during design day heating loads. This means that 99% of the time, the boilers are oversized; causing the system to be oversized. This will again cause short cycling and wasted energy.
How the Fireye NXM2G integrates with BMS
The Fireye NXM2G is fully compatible and complementary to all types of BMS/BEMS systems. The M2G is interlocked by fail-safe relays that are connected in series to the burner/boiler control or stat circuit; the BMS is also connected into this circuit via volt free contacts.
The Building Management System will give a “call for heat” or enable signal, the M2G will in turn check for boiler demand and the boiler memorized supply and return profile. The M2G’s adaptive algorithms will ensure that the best firing stage is established for the current load conditions and invoke a “save mode”. The Fireye NXM2G has been designed specifically to overcome and avoid unnecessary “standby cycling” of the boiler operation under low load conditions, such as “standing losses”, etc.
To further complement the operation of BMS with Fireye NXM2G, it is possible to interface the inputs/outputs of the unit, to standby contact relays and in this way it would be possible for the BMS system to directly monitor the Fireye NXM2G operations if required.
For systems with more than 1 boiler, the Building Management System typically controls the boilers’ operation from a blended system temperature from a common header. During these cases, the Fireye NXM2G will monitor and control each boiler’s operation on an independent basis.
Documented Savings when the Fireye NXM2G is installed on systems with BMS equipped with Outside Air Reset Schedules
Hydronic Heating Systems operated by a BMS systems that employ outside air reset schedules operate on a floating cycling band (140F ON – 160F OFF schedule can be reset to be 120F ON – 140F OFF). This allows the boilers to operate more efficiently as the heating loop drops in temperature, the boiler achieves more efficient heat transfer, which reduces boiler stack losses. Employing this technique also decreases the heating value within each gallon of water delivered to the occupied space, which allows the boilers to Stay ON longer. Although, the reset schedule delivers gains in operational efficiency, additional efficiency improvements can be achieved through the use of the Fireye NXM2G. As the boiler(s)’ cycling band is reset, it is still cycling ON/OFF based on preset conditions that still over delivers heat with no feedback on the actual demand for heat. The control logic within the Fireye NXM2G measures actual demand for heat by measuring the rate of temperature change within the heating loop.