Central Ducted Fujitsu Heat Pump: Use the Wall Thermostat Sensor instead of the Air Handler's Sensor
Sunday, January 12, 2025
If you have a central ducted Fujitsu Heat Pump with a wired remote control unit (wall thermostat), you may know that the central air handler’s temperature sensor is used by default instead of the wall thermostat’s temperature sensor. If your wall thermostat doesn’t have the thermometer icon at the display’s bottom center, then your Fujitsu is probably using the air handler for temperature. You may have noticed that the wall thermostat’s scheduled temperature almost never matches the home’s actual air temperature. In my case, temperature readings at the air handler were very unreliable at ~±5ºF. So if you want to use the wall thermostat for a (very likely) more representative sampling of your home’s air temperature to better control the heat pump, here are the instructions from a post on Reddit:
Here are a few other settings you should make sure you have set as well:
Use indoor controller to measure indoor temperature instead of air handler sensor:
Room temperature sensor switching (function 42) set to "01" (both).
Room temperature sensor switching (function 48) set to "01" (wired remote controller).
After setting 42 and 48. Turn off heat pump and switch the breaker off for air handler and heat pump for 5 minutes. Then turn back on and wait 5 minutes until controller turned on fully. In the settings of the remote controller, in "Initial Settings", hit next page, click "R.C. Sensor" (temperature sensore of controller), click "used". Then in default view of controller you should see a temperature symbol pop up at bottom of screen. Using the air handler temperature sensore will cause short cycling of your heat pump and this lowers efficency.
Before I set my Fujitsu heat pump to use the wall thermostat's temperature sensor, I could not keep my home’s climate precisely where I wanted it. There was too much variation in what the air handler’s temperature sensor would measure. That makes sense as the sensor is very near or even in an unconditioned space (though the air handler intake is insulated, the sensor is somewhere at the margin). After switching to the wall thermostat’s temperature sensor, the inconsistently measured temperatures went away. Now my home is always within a degree of where I want it to be.
It’s strange that Fujitsu’s air handler’s temperature sensor is the default even with a wired RCU (with sensor) connected. It’s even stranger that you must power cycle the entire system to use the wall thermostat’s sensor. I had actually tried the above settings long ago but without the power cycle.
Other than the wrong sensor being used, this heat pump has been amazing and it’s even better now. It’s so much more efficient & safer than nature gas and it is ridiculously quiet.