On 1/21/2013 1:39 PM, wrote:
On Jan 21, 1:20 pm, The Daring Dufas the-daring-du...@stinky-
finger.net wrote:
On 1/21/2013 11:31 AM, wrote:





On Jan 20, 5:50 pm, The Daring Dufas the-daring-du...@stinky-
finger.net wrote:
On 1/20/2013 1:14 PM, Ernie wrote:

" wrote:

The most inexpensive residential AC systems lack a factory
installed head pressure control which is why it's not prudent
to run them in cold weather.

Why couldn't a thermostat that can sense condenser coil temperature
be used to turn on the fan when the coils reach some pre-determined
temperature?

Because it's not needed and just one more thing to fail
and would then need to be serviced. As for the AC being
damaged by operating it lower than 60F, personally I
think it's BS.

I don't think the AC would dammaged by low-temperature operation, but
clearly the efficiency of heat-transfer would be affected if the
operation of the condenser fan resulted in insufficient line-pressure in
the evaporator line.

This would explain why most residential AC units don't seem to cool a
house very well when ambient outdoor temperatures reach "room
temperature".

Believe me, running an AC unit during periods of very low ambient
outdoor temperatures CAN and WILL cause damage to the compressor unless
steps are taken to control head pressure in the system.

And that is like 59F? The claim was made that it can't
be run below 60F outside temp.

The refrigerant
can migrate to the compressor crankcase causing liquid slugging of the
compressor which is supposed to pump vapor not liquid. That why most
systems have a crankcase heater to keep the compressor warm to prevent
the majority of the refrigerant charge from winding up in the
compressor.

A residential AC doesn't. That's what was being discussed.
And if it's such a threat to destruction, why don't they have
a lockout that prevents it from running at 50F? Would seem
there would be a lot of AC systems shot from say a kid
or someone accidently turning it on under those conditions,
no?

I'm sorry to disappoint you but most AC condensing units installed in
homes have crankcase heaters. Running a home AC system in cold weather
does not cause it to explode immediately. Damage accumulates over time
with liquid refrigerant instead of vapor being returned to the crankcase
diluting the oil and causing it to foam which will result in
a large amount of oil being picked up by the compressor and pumped into
the evaporator. The more expensive home AC condensing units not only
have a crankcase heater but will have a variable speed fan, high and low
pressure safety switches to shut the unit down when the pressures are
out of spec. Some units have a freeze sensor on the low pressure line to
detect frosting of the line which indicates a problem with the
operation of the unit and shuts the unit down. I don't recall any new
AC unit I've installed in the past ten years that didn't have at least
a crankcase heater. O_o

TDD- Hide quoted text -

- Show quoted text -

Well, Rheem, for example, must not know what they are doing then.
Because here is an example of their data sheet that covers 7 typical
models from 2 ton to 5 ton. Only the 5 ton has a crankcase heater
installed:

http://www.rheem.com/documents/rarl-...n-instructions

The installation instructions for all the models say:


FIELD INSTALLED ACCESSORIES
COMPRESSOR CRANKCASE HEAT (CCH)
While scroll compressors usually do not require crankcase heaters,
there are
instances when a heater should be added. Refrigerant migration during
the off cycle
can result in a noisy start up. Add a crankcase heater to minimize
refrigeration
migration, and to help eliminate any start up noise or bearing “wash
out.”


COMPRESSOR CRANKCASE HEATER (CCH)
The 5-ton (-)ARL is factory equipped with a crankcase heater.
Refrigerant migration
during the off cycle can result in a noisy start up. The crankcase
heater minimizes
refrigeration migration and helps reduce start up noise or bearing
“wash out.”
The heater is located on the lower half of the compressor shell. Its
purpose is to
drive refrigerant from the compressor shell during low outdoor ambient
conditions
(below 75°F), thus preventing damage to the compressor during start-
up. At initial
start-up or after extended shutdown periods during low outdoor ambient
conditions
(below 75°F), make sure the heater is energized for at least 12 hours
before the
compressor is started. (Disconnect switch on and wall thermostat off.)


Note that nowhere do they say you run the risk of
damaging the unit without a heater. Seems mighty
strange that they don't issue such a warning if
turning the thing on at 50F is gonna cream it.


I should have pointed out that reciprocating compressors are more prone
to damage than the scroll compressors which have fewer moving parts. If
you check out the options I would guess a crankcase heater is one of
them for all the units. I have had a scroll compressor fail because of
bearing failure due to low ambient operation but scroll compressors are
usually more tolerant of abuse than the reciprocating units. There is a
line of reciprocating compressors manufactured by Maneurop which are
basically immune to refrigerant flooding. The housing acts as an
accumulator and the compressors closely resemble those in Trane and
American Standard AC units. Johnstone Supply used to carry the Maneurop
compressors. I'm sure that the Rheem units that have/had reciprocating
compressors were equipped with crankcase heaters. I have also installed
low side accumulators on some systems to protect the compressor when it
had to be operated in low ambient conditions. There are some crankcase
heating methods that apply a low current to the compressor motor
windings to provide heat. ^_^

https://en.wikipedia.org/wiki/Crankcase_heater

https://www.youtube.com/watch?v=TRwJVY2OL-0

TDD