Pool Heater Temperature Controller

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This project is extensively detailed on the Parnell Engineering website here. The high-level details of the project are mirrored on this Wiki page.


The pool heater temperature controller was created to enhance control and monitoring over the unit used prior to 2020 (a Johnson Controls A419).

It provides thermostatic (on/off) control of the pool temperature via the boiler.

The key features are as follows:

  • Clear indication of the sensed (feedback/input) temperature and setpoint (process) temperature on LED displays.
  • Monitoring of the boiler operation (response to a 'call for heat').
  • Provides network services via WiFi:
    • A mobile-friendly website indicating operating status and temperatures.
    • A monitoring-friendly service (i.e. JSON via HTTP) for monitoring trends, etc. via external software tools.
    • On-call connection to an SMTP server to provide notification messages.
    • The ability to post the present temperature to a website and/or digital signage (optional).

Furthermore, the following principles are used in its design:

  • The core function (temperature control) shall be performed by a proven, durable controller. In this case, a Red Lion T48 was chosen.
  • The auxiliary function (monitoring) shall occur in a separate controller/core (NodeMCU/ESP8266) and shall NOT affect the core function in the event of failure or malfunction.
  • While the device is connected to a restricted network (access limited via strict firewall rules), the monitoring controller should NOT be able to modify parameters on the temperature controller. The reasons for this are two-fold:
    • This adds additional security risk, as a security exploit could allow the boiler could be controlled remotely.
    • There is no apparent need; changes to the setpoint are rare. Changes can be done on-site via the front panel.


A drawing has been created per the present as-built configuration.

Additional changes are proposed (shown as "optional") in the drawings and may be implemented in a future build. Specific connections to the boiler (wire colors) are pending an as-built update.

FFSC-002 Temperature Controller.png


The software is stored on GitHub, in a private repository located at https://github.com/fairwayfarms/PoolHeaterControl.

As the source contains private information for WiFi and SMTP, it is not shared. For FFSC members or staff: contact B&G for access to the repository. In the future, a sanitized version may be made public.