How is the boiler cycling, and what is the stack temperature?

Published: August 24, 2017

By: CUNY BPL Staff and Interns


Look at both burner motor cycling and stack temperature to obtain a deeper understanding of the heating plant’s efficiency.

The burner motor(s)’ on/off status is logged to determine boiler staging performance. It is also used to determine whether short cycling is occurring, when used in combination with stack temperature. Short cycling not only indicates inefficient operation, but also that the motor longevity may be compromised. Stack temperature(s) also helps to deter-
mine whether the burner(s) is modulating.

Note: If you have a variable frequency drive on the burner motor, the kit described in this module is not capable of accurately assessing on/off status. In such cases, rely solely on stack temperature to help detect and diagnose performance issues.

Kit Contents (Equipment and Software)

For Boiler Cycling:

  1. HOBO® Motor on/off data logger: UX90-004 or UX90-004M, one per boiler

For Stack Temperature:

  1. Data logger: UX120-014M, up to one per boiler
  2. Thermocouple probe, TCP6-K12, one per boiler


  1. PC-based device with USB port (e.g. laptop or Surface Pro 3)
  2. HOBOware® software
  3. Microsoft Excel
  4. CUNY BPL-provided macro-enabled Excel file (Boiler Cycling Visualization.xlsm)

Initialization Procedures

Data Acquisition Procedures

Data Visualization Procedure

Suggested Actions

Burn time is determined by looking at stack temperature cycles on the trend chart. The duration commences when stack temperature begins to rise, and it ends when stack temperature drops precipitously.
Look at stack temperature for the individual boilers. If a boiler’s stack temperature is relatively constant during firing, no modulation is occurring. If the burners are capable of modulation, they should be serviced to enable modulation.

If the burn time is very short without a significant variation in stack temperature, consider the following options. For a central plant with only one boiler, check pressuretrol (aquastat) settings for steam (hot water) boilers. Settings should account for a broad enough operating differential to satisfy the two following criteria: first and foremost, such settings creating the variation in pressure (hot water temperature) should ensure the boiler’s ability to provide steam (hot water) appropriately and effectively to the distribution system. Second, the operating differential should be sufficiently large to minimize boiler cycling, but not so large as to cause too broad a range of steam pressure (hot water temperature).

For boilers used in lead-lag mode, ensure that the pressuretrol (aquastat) settings are properly set on the respective boilers to prevent short cycling. Short cycling can occur in either boiler, but is more common for the boiler in lag mode. It is common to rotate the role of a boiler (whether in lead, lag or standby position), and the changes must be made to pressuretrol (aquastat) settings when the boiler roles are changed. Alternatively, installing a programmable lead-lag sequencing controller capable of changing pressuretrol (aquastat) settings will automatically apply the appropriate
settings when the boiler roles are changed.

For central plant configurations with burners capable of modulation and multiple boilers set in lead-lag mode, short cycling of the lag boiler may persist. To avoid short cycling, consider using an agastat to delay the call for the lag boiler to fire.

Due to pre-purge and post-purge, motors are on longer than burners. The trend chart may indicate excessive motor cycling based on the rate at which the motor changes from “off” to “on” status and/or by noticing very short off cycles.

Roughly speaking, a good stack temperature at high fire is under 400°F for a gas-fired boiler and under 500°F for an oil-fired boiler.

A high stack temperature may indicate that the tubes have soot or scale buildup, inhibiting heat transfer and therefore reducing efficiency. Clean and tune boilers/burners, including a check on the burner modulation and the air fuel ratio.

Associated Equipment