A/c Cooling Vs Mpg

[wwest]

This procedure is something of a nuisance but with fuel prices rising.....

I have already had my dealer disable the A/C system using C-best options (two) in "normal", automatic mode, and in defrost/defog/demist mode. Below 65F OAT A/C is a total waste of fuel as it only serves to dehumidify the incoming airflow and during the summer that capability is useless.

I am also in the process of installing an electrically, 12 volt, operated water flow control valve so that NO hot water flows to the climate control heat exchanger during cooling mode. Eliminating even the radant heating effects within the A/C plenum area.

The way our systems work is that once the cabin has reached the approximate temperature setpoint the incoming airflow is first chilled to as low as 34F and then via the reheat/remix servomotor and vane airflow routing system some of the airflow is reheated (190F engine water jacket) and then both portions are remixed such that the airflow reaching your body surfaces is not so cool as to be discomforting.

If you avoid the reheat/remix cycle then the A/C system becomes dramatically more efficient. Here is the way that can be done.

Turn the system to max cold, put the system in recirculate mode (some systems will do this automatically), and now use the blower speed manually to regulate your comfort level. You can also alleviate some of the discomfort of the now COLD dash outlet airflow by selecting combined floor and dash outlets. I would not recommend floor only in that the intake airflow for recirulate mode is too near the floor/footwell outlets.

Sometimes when the OAT is below 65F but the interior has been warmed by the sun I use the A/C to first cool the car down and then switch it off.

I NEVER use the A/C in defrost/defog/demist mode, especially in the wintertime as that will always lead to follow-on windshield fogging due to the design flaw of the Lexus climate control system. Besides which, although Lexus relies totally and completely on the dehumidification capabilities of the A/C for defogging and/or preventing fogging of the windshield/windows, the A/C will be automatically shut down if/when the outside temperature declines below 35F.

If your windshield begins to fog over then apply HEAT and then maintain the windshield surface temperature at an elevated level to prevent further instances of fogging.