1998 Chevrolet Astro Owner's Manual - Page 158

If your vehicle has rearair conditioning, setting it on LOW may enhance the front A/C performance by allowing trapped refrigerant in the rear lines to circulate. The rear air conditioning system willonly send cooled air if the front system ison. It can still be used to circulate air, even if the front system is off. When the air conditioning, DEFROST or BLEND is on, you may notice a slight increase or decrease in engine speed, due to compressor operation. This is normal because the system is designed to cycle the compressor on and off to keep the desired temperature. Defrosting Use DEFROST to remove fog or ice from the windshield quickly in extremely cold conditions. The temperature knob should be in the red area and the fan control toward high. The air conditioning compressor may operate in this setting to dehumidify the air. The BLEND setting is usefulfor cold weather witha large number of passengers or very humid conditionsto help keep the windshield clear. Ventilation System For mild outside temperatures when little heating or cooling is needed, use VENTto direct outside air through your vehicle. Air will flow through the instrument panel vents. Your vehicle's flow-through ventilation system supplies outside air to the inside of your vehicle when is it moving. With the side windows closed,air will flow into the front air inlet grilles at the base of the windshield, through the vehicle andout the rear air exhaust valve. or Outside air will also enter the vehicle when the heater the air conditioning fan is running. Heating On cold days, use HEATER with the temperature knob in the red area. Outside air will be brought in through the floor outlets. The heater works bestif you keep your windows closed while usingit. If you use the optionalengine coolant heater before starting your engine, your heating system produce will warmer air faster to heat the passenger compartmentin cold weather. See "Engine Coolant Heater" in the Index. 3-6