Using a multimeter set to Ohms (Ω), disconnect the thermostat and measure resistance between R and W (heat call). A short (0 ohms) or very low resistance (under 10 ohms) indicates a stuck contactor or zone valve solenoid. This forces high current through the TT91’s internal switch, generating extreme heat.

To allow the device to calculate optimal control parameters automatically, access the "Auto Tune" setting (often labeled AT ).

The PV (Process Value) shows current temperature, while the SV (Set Value) shows your target.

Despite its age, the TT91 is beloved for its brutalist simplicity: a thermocouple input, a relay output, and a PID algorithm that just works—until it doesn't.

Essay: The Role of Thermotech TT91 Systems in Radiological Safety

For complete technical documentation, including specific parameter settings and underfloor heating applications, you can visit the Thermotech Documentation Center pid temperature controller - INSTRUCTION SHEET