Is a hot water reset used to control the hot water supply temperature?

Published: August 24, 2017

By: CUNY BPL Staff and Interns


Determine whether a hot water temperature reset schedule based on outdoor air temperature is in use and operating properly. This requires monitoring the hot water supply temperature and the outside air temperature.

It may be the case that a hot water reset is being used; however, this measure will help to verify its performance.

Kit Contents (Equipment and Software)

For Hot Water Temperature:

  1. 4-Channel Thermocouple Logger: UX120-014M, one per boiler
  2. Surface Temp Self-Adhesive Thermocouple: two per analog logger
  3. Pipe insulation
  4. Pipe insulation tape

For Outside Air Temperature:

  1. Temperature/Relative Humidity Weatherproof Logger: MX2301, one per building


  1. PC-based device with USB port (e.g. laptop or Surface Pro)
  2. Mobile device with Bluetooth capabilities
  3. Hoboware® software installed on a laptop or Surface Pro
  4. HoboMobileTM app installed on a tablet or mobile device
  5. Microsoft Excel
  6. CUNY BPL-provided macro-enabled Excel file
  7. (Hot Water Reset Visualization.xlsm)

Initialization Procedures

Installation Procedure

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

Associated Kits