From: John De Armond
Newsgroups: sci.engr.heat-vent-ac
Subject: Re: Auto vs. Home HVAC Test Temps?
Date: Tue, 04 Dec 2001 15:56:06 -0500
Message-ID: <3C0D3866.1BE5E3A5@bellsouth.net>

Inspector wrote:
>
> Can someone in this group please explain to me why it is believed in the
> home inspection business that a residential A/C compressor can be damaged by
> operation at too low an outside temperature, but yet an automobile A/C
> operates in conjuction with the defroster at any temperature?

Like all rules of thumb, this one isn't universal.  The potentially
damaging condition can arise with split units that use long
refrigerant tube runs.  The refrigerant will condense in the coldest
spot in the system.  When outside is colder than inside, this spot
will be the condensing unit (compressor and condenser.)  A split
unit with long refrigerant tube runs may contain enough refrigerant
to fill the compressor can and/or condenser.  Normally the
compressor can will fill first because the cold compressor can will
reduce the pressure in the suction side and cause refrigerant gas to
flow from other parts of the system where it condenses on the can.
Many times a compressor will damage its valves or other structures
if it attempts to compress liquid.  If the compressor is (partially)
filled with liquid refrigerant/oil mix, it is likely that the
compressor will try to compress liquid when it starts.

The normal solution to this problem is for the compressor to be
equipped with some sort of crankcase (can) heat so that the
compressor is no longer the coldest spot in the system.  This may be
in the form of a wrap-around strap heater, a cartridge heater
inserted in a welded socket in the can or even a capacitor that
bypasses the contactor and meters some power to the compressor when
it is "off".  The coil heating does the trick.

Even the cheapest units manufactured today seem to have crankcase
heaters so this isn't much of a problem anymore. And this problem
normally never manifests itself with package units or window units
simply because the system does not contain enough refrigerant to
fill the compressor plus most of the system is in the cold space so
the refrigerant is spread out.

A car AC doesn't have this problem because a) the refrigerant charge
is relatively low, b) most of the system is in the cold space so the
refrigerant spreads out, c) the compressor, being on top of the
engine, will normally remain warm long after the condenser and
plumbing has cooled to ambient, d) the compressor is normally
turning fairly slowly at startup, when the problem would manifest
itself and e) the automotive compressors are fairly resistant to
damage from liquid pumping, particularly at low speed.

BTW, you are correct - many AC systems run the compressor on defrost
regardless of the temperature.  Older systems from all the Big Three
did that.  They used some sort of back pressure regulating valve
(POA in the case of GM) to keep the evaporator from freezing.  Newer
stuff (80s up) uses either an evaporator thermostat or low side head
switch to cycle the compressor.

John