Quote:
Originally Posted by Xist
Can someone tell me how much of a load the vent has on one, compared to the other settings? I imagine it makes a difference, but small.
While adjusting the numbers makes the cabin cooler or warmer, as far as I know, the AC is working just as hard.
I cannot drive 70 while using my AC with my current grill block, but it is 116° here in Phoenix.
Yeah, I opened my windows...
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I don't know anything about the AC control logic on these cars specifically, but generally, the AC compressor cycles between high and low pressure cutoff switches any time the AC button on the dash is engaged and thermostat is not satisfied and calls for cooling. It's job is to pressurize the refrigerant as needed so it condenses and can then evaporate so that need varies with thermostat position which varies its duty cycle. The evaporation rate is controlled by an expansion valve on older systems that varies the evaporator input flow rate based on the evaporator core temperature. Ideally, they open as much as possible without causing freezing. Newer systems use an orifice tube which has a fixed flow rate designed to never freeze and their sizes vary with some vehicles having multiple options to balance economy and cooling capacity.
If you're going to use the AC, in my experience the most efficient use of AC is to set the fan speed on high, set the vent for recirculate, and set the temperature as warm as you comfortably can. This will cycle the compressor as little as possible thanks to the high thermostat setting and will keep you as comfortable as possible thanks to the cooling effect of the fast moving high volume of air and will require less work to cool the same air over and over (which will gradually get colder and colder and drier and drier) than would cooling a new volume of hot, humid air constantly. Remember, the electric blower motor is more efficient to spin faster than the compressor is, so set it high to allow running the compressor less. Also keep in mind the blower motor speed control is a simple resistor circuit so any power you're not using to spin the fan is wasted on heating resistors. No real electrical gain to be had by setting the fan to 3 instead of 4, etc but the cooling efficiency of the AC varies directly with airflow.
Diverting coolant away from the heater core saves fuel too. It allows faster warm-ups thanks to the smaller volume of coolant absorbing heat and reduced heat loss thanks to the smaller volume of coolant and the reduced surface area of the cooling system thanks to removing the heater core itself from the loop. This also allows the AC to be more efficient and you to be more comfortable because the discharge air is not being cooled by the evaporator then heated by the heater core then blown onto you. I tested this last night in a new to me 98 Civic LX by turning the AC off and gradually setting the thermostat warmer and warmer until the air got warmer than ambient, then sliding it back just enough to back to achieve ambient discharge and turning the AC back on. I couldn't move the thermostat much at all before I started getting heated air but the compressor would still be running, and running more than really needed to keep me comfortable. I'll bypass the core as soon as I confirm whether or not that loop is required for proper coolant flow so I know whether I should couple the heater hoses together or block them entirely.