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I installed a new Honeywell TH8321WF1001 thermostat to replace a Carrier programmable thermostat. This is a dual fuel Carrier system (heat pump with gas backup). The new HW stat is working fine except that the outdoor temperature sensor is reading about 15-20F low when compared to the temperature reading on the adjacent heat pump. The temperature indication was accurate on the old stat. No changes were made other than to move the sensor wire to the new stat; 2-wires are used for the thermocouple to the stat. The soft settings are configured correctly to specify the outdoor sensor is wired. Interestingly, the new stat has terminal designations S1 and S1, whereas the old stat had designations S1 and S2. But I understand this does not matter nor does it matter which wire from the outdoor sensor is installed to which S terminal (I have not switched the wires to see what may happen). I prefer to use the local reading instead of the internet reading option available in the new stat. Any ideas what to check to resolve the sensor inaccurate reading? Thank you. Patrick

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After inspection of the thermocouple located just below and outside the heat pump I discovered the tape used to hold the sensor to the heat pump was caked with light dirt and mud. I removed the crusted coating from the sensor and the temperature appears to be matching up nicely when compared to my IR handheld and a weather T/C placed adjacent. This accurate reading is confirmed on the new stat display.

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    no need to apologize a self-answer is fine in this context. After the timeout expires you can accept this answer (by clicking the checkmark) to mark the solution. Commented Nov 23, 2023 at 14:14

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