Smart temps makes the control-panel number an estimate of the water temperature in the tub. There is no separate sensor in the tub. The controller uses the HydroMate probe, the selected temperature, whether heat is requested, and how long the pump has been running or stopped.
The displayed estimate does not normally decide when heating starts or stops. Those decisions use the HydroMate probe. With the default Fahrenheit settings, heat is requested when the probe reads 3°F below the set temperature and is released when it reads 2°F above it. The probe is in the pump unit: its temperature can change faster than the large volume of water in the tub. After a long pump run, Smart temps subtracts 2°F from the probe reading to estimate tub temperature. That 2°F difference is the rule the controller uses, not a direct measurement of the water in the tub.
A 100°F example
For this example, the set temperature is 100°F, Smart temps is on, the pump has been off long enough for the display hold to end, and the display starts at 100°F. Only the ordinary heat call runs the pump, and the probe reaches 102°F between two and 15 minutes after the pump starts.
| What is happening | HydroMate probe | Heating decision | Smart temps display |
|---|---|---|---|
| Resting at the selected temperature | 100°F | No heat requested | 100°F |
| The probe cools to 97°F | 97°F | Request heat and start the pump | 99°F while heat is requested |
| First two minutes of circulation | Changing | Keep heating | Hold at 99°F |
| The probe reaches 102°F | 102°F | Release heat | 100°F: 102°F at the probe minus 2°F |
| The pump stops | 102°F at the stopping point | Wait for the next heat request | Hold 100°F, subject to the early-release rule below |
The 97°F on / 102°F off readings are a 5°F span at the probe. In a stable tub, the actual water may vary by less than 1°F over that cycle. The table’s display values are estimates, not measurements of that water-temperature change.
What the estimate actually does
These rules describe the default Fahrenheit behavior. The controller also supports Celsius in half-degree units.
- For the first two minutes after the pump starts, it holds the previous estimate. Newly circulating water can move the probe reading sharply. If heat is requested and the held value is at or above the set temperature, the display changes to one degree below the set temperature. The controller also records a starting estimate for the rest of the run.
- After those two minutes, a heat request holds the display one degree below the set temperature while the probe is in the near-set-temperature range. With a 100°F setting and the default thresholds, that range is 97–101°F at the probe. If the pump starts with substantially colder water, or the probe moves outside that range, the display follows the probe-based estimate instead of holding at 99°F.
- As the pump keeps running, it relies more on the probe. For the first 15 minutes, the rules above determine the display. From 15 to 75 minutes, the display blends toward
probe − 2°F. At 75 minutes, it usesprobe − 2°Fdirectly. This also applies to pump runs for filtering or jets. - When the pump stops, it holds the last estimate for up to 30 minutes. After two minutes, the hold ends early if
probe − 2°Ffalls below the held display. Once the hold ends, an idle pump displays the set temperature (one degree below it if heat is requested) while the probe remains in the expected range. Outside that range, the estimate follows the probe.
The Fahrenheit display uses whole degrees, so smaller changes can be hidden by rounding.
Optional early heat request
The “Improved” setting also enables proactive heat. During a pump run, the controller remembers the highest qualifying probe reading. If the reading then falls by more than 1°F, probe − 2°F is below the set temperature, and heat is not already requested, it requests heat before the usual 3°F-below threshold. It lowers the displayed estimate by the detected drop. This is an additional heating behavior, separate from the ordinary heat thresholds. An external Sonoff heat controller can own the heat request instead.
Thus 97°F is the normal heat-on threshold in the example; proactive heat can call earlier. A set-temperature change, external heat command, or another operating mode can also change when the pump runs. Filtering and manual jets can run the pump without a heat call.
Choosing what the panel shows
The stock-like display can show P while the pump is off or just starting; otherwise it shows the HydroMate probe temperature. Hold Up and Down together for at least 4 seconds to enter “Improved” mode, which hides P, enables Smart temps, and enables proactive heat. Repeat the shortcut to return to the stock-like settings.
After a configuration reset, Smart temps and proactive heat are disabled by default; saved settings can differ. The shortcut also changes other behavior, including the allowed set-temperature range.
Service-menu adjustments
To change the display options separately, P16 controls P (1 hides it), P21 controls Smart temps (0 enables it), and P28 controls proactive heat (0 enables it). Those are independent of each other so you can hide P but still disable the Smart temps so the tub would just show the sensor temp..
The example uses the default Fahrenheit heat thresholds. PP4 changes the normal heat-on threshold (default 3°F below the set temperature). PP5 changes the heat-off threshold and the running-probe offset (default 2°F). These settings change heating behavior as well as the estimate.
What the display can and cannot tell you
A steady Smart temps reading is expected near the set temperature, but it does not prove that the water is exactly at that number. If the tub feels too warm or too cool, check it with a reliable thermometer in the water.