HONEYWELL Tech Tip: T87K and T87N Field Failures

Honeywell has received some customer feedback that the T87N and T87K Round thermostats are not calling for heat. Honeywell expects this to be regionally isolated due to the types and age of the equipment that are more commonly found in that region. These no heat call issues are showing up when the thermostat is connected to dirty AC power, DC power, Carlin controls, or when there is an abused use of range stops. Below is a summary of the investigation results and how to address the situations. These issues to date have been isolated to the Northeast region of the U.S.
NO CALL FOR HEAT FINDINGS:
    DIRTY AC POWER, LARGE DC POWER, CARLIN CONTROLS
The Round technology requires that the AC power cross the zero point and then rise with part of the normal AC sign wave. The presence of (or lack of) this wave-shape tells the thermostat that it is installed and to 'wake up'. If the power has a large enough DC power signal or has a severely distorted AC signal, the thermostat may not 'wake up'. Possible causes of dirty power are primary controls (such as gas valves) beginning to fail or actual DC applications such as millivolt applications.
    FIELD ACTION:

The occurrence of extremely dirty or large DC power is rare so typically no field action is required. Applications with dirty AC power, if the source of the dirty power cannot be identified and corrected, the FocusPRO TH5110 should be used. The TH5110 is very universal and can be setup for heat only (including normally open, normally closed or power to open and power to close zone valves), cool only or heating and cooling applications. It will also operate 750 mV applications. When installing on Carlin controls if the result is no call for heat, switch the R and W wires at the thermostat.
    NO CALL FOR HEAT FINDINGS: RANGE STOPS
If range stops are installed and the thermostat setpoint is forced beyond the range stop limits, the setpoint will be pushed to and stuck beyond the limits. A large amount of torque is required to break past the range stop limits.
    FIELD ACTION:
When operating the thermostat with the optional range stop accessory installed, do not force the dial setpoint past the range stop limits.