When you are running your A/C and it's a little too cold for Grandma,
and you turn the temperature control up slightly, does that make the
compressor cycle less frequently, or does it only open the blend door to
mix warm air with the cold -- or both?
It is irrelevant if they use the blend door to modify the temperature.
When you move the blend door to increase the exit air temperature,
less air goes through the evaporator, so it cools down faster which
causes the compressor to cycle off sooner - well unless you have the
more efficient variable displacement type compressor. For these, it
results in a reduction in refrigerant flow by reducing the compressor
displacement - but that is equivalent to more frequent compressor
In the old days manufacturers tried to control air temperature solely
by cycling the compressor. This is not very effective becasue the
temperature of the air coming out of the vents is not consistent.
Today almost all A/C systems try to keep the evaporator at a constant
temperature and modify the temperature of the air coming from the
vents by adjusting the percentage of the air that goes through the
evaporator and mixing it with outside or heated air to acheive the
desired exit air temperrature. The refrigerant flow through the
evaporator is adjusted to maintain the evaporator at the desired
temperatue (somewhere in the 35 F to 40 F range). More air and/or
hotter air through the evaporator will casue the compressor controls
to increase the flow of refrigerant in order to maintain the
evaporator temperature. Some systems conrtol the flow by measuring the
low side pressure (pressure of refrigerant at the evaporator), others
by having a thermocouple in the evaporator, and some do both. The goal
is to keep the evaporator as cold as possible without allowing ice to
form in the evaporator.
Actually with most vehicles, the compressor would run *more* and cycle
on/off less. This is because you are adding an additional heat load, that
being the warm air added to the outflowing air.
If it is cold in the vehicle, the A/C compressor will cycle off so as not to
If it is warm in the vehicle, the A/C compressor will continue to run until
it approaches the freezing temperature.
An example of this is trying to charge an A/C system in the early spring
when the temperatures are cool. The A/C will cycle on/off very frequently.
You need the compressor to keep running so it will pump in the new
refrigerant. A trick to keep the compressor running is to turn on the A/C
full blast on recirculate, then move the temperature selector all the way to
warm. Then this keeps the compressor running fairly constantly.
This is with systems where you get full air flow through the evaporator and
the air is then heated further down the line. Then warm air enters the
evaporator as the air returns.
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