A/c Economy Mode...

[wwest]

The newer Prius models with the electric A/C compressor have an economy operation mode such that the A/C compressor is not run "full-out" as it is in the HH and RXh. In the Prius in economy mode an attempt is made to only keep the evaporator outflow just below the temperature setpoint. Say 68F for a 72F setpoint, instead of 32F as it would otherwise.

While Lexus has stated that this option will not be implemented on their hybrid models (HH??) you can manually manage the A/C operation and get even greater MPG returns than the Prius' A/C economy mode.

Simply turn off the A/C when the predominant outside temperature is in a range, say below 55F, wherein the use of the A/C for cooling is no longer beneficial. As of 2001 the RX300 had a c-best option wherein the A/C could be disabled indefinitely by the driver during any prolonged cool climate period. I leave mine off all winter. That option may exist on your RXh.

If you allow the A/C to operate it will run "full-out" in an attempt to SQUEEZE as much moisture, dehumidify, the already cool or even COLD incoming fresh airflow as much as possible. Now to maintain your personal comfort level most of that CHILLED airflow will have to be reheated via a trip through the heater core.

So, by disabling the A/C you will not only have the savings of no A/C compressor operation, but you will also be lowering the level of heat that must be extracted from the engine cooling water jacket resulting in the ICE running less often, or not as long, simply to keep the water jacket HOT.