"Abeness" <news@nada.x> wrote in message
news:VOidnRkII9T3glXcRVn-2A@rcn.net...
> Philip wrote:
> > In fact, A/C systems start cycling the compressor at no less than 38
degrees
> > F. SO... when the air coming into the evaporator reaches this low
> > temperature, the compressor is cycled or shut OFF if the air temperature
> > entering the evaporator continues to drop. Otherwise the evaporator
would
> > become a block of ice, which would prevent any air from reaching the
heater
> > core (in nearly all cases).
>
> Now THIS makes perfect sense. (I'd have to look up the temp at which
> refrigerants go liquid to confirm or deny Steve G's belief.) I'm almost
> certain that the A/C on my old 85 Pontiac would cycle at any temp,
> though. I'll check up with the Honda soon.
>
> Anyway, I doubt it would be that difficult (in the original design) to
> construct an airflow system that would shunt warmer air around the evap
> unit when the temp dropped in winter, in order to preserve the
> dehumidifying characteristics of an A/C-based system.

You would also have to direct warm air to the entire low pressure side of
the ac system to prevent the condensing of refrigerant gas back into liquid.
I live in a climate of 10 months of winter and 2 months poor sledding, have
been a licensed mechanic since about '72 and have never seen an ac comp
cycle in ambient temps colder than somewhere around that 40 degree mark.
and remember from my days in tech school that is the reason.
Don't forget when researching boiling point of refrigerant to factor in the
pressure. Adding pressure also raises the boiling point. As I'm typing
this I'm also recalling that there are cautions that when servicing/charging
the system to not allow low side pressure to drop too low for fear of
damaging the compressor. Go to K-mart and look at the labeling/cans of 134a
they market to the consumers and read the instructions. When charging you
override the low side cutout so you can keep the compressor running. In
this situation, temp above ambient cut-out and low pressure limit overidden,
a drop in low side pressure can take it below it's boiling point and risk
liquid refrigerant in the compressor. The warning on the charging kit
doesn't get into the explanation, but does give the caution about low side
pressure drops and damage to the compressor.
Steve