The pool is not warm enough.

To heat the pool to 86F from the latest pool temperature reading (85F), the heater would need to run for 0.6 hours, costing $2.69 (USD) in natural gas and generating 0.013 metric tonnes of CO2e (which is approximate to 32 miles driven by the average passenger vehicle). Source
[Or, approximate to 64 miles driven by a Tesla in California, Gavin.]

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* Pool and Spa temperatures are only logged by the pool controller when the pumps are running in those modes. When the pool and spa pumps are off, the water temperatures are logged by Ambient Weather TX-3107 floating pool thermometers. Also, the air temperature sensor is currently poorly place. As a result, the air temperature sensor is affected by the exhaust from the heater (which will be remedied).

[2021-02-01 Update] The air temperature sensor has been relocated. It remains to be seen if it will still be subject to the pool heater exhaust.

[2021-02-08 Update] The air temperature sensor is too sensitive to the sunlight. I've started using air temperature data from an Ambient Weather sensor and Open Weather Maps current weather data.