Auto vs. Home HVAC Test Temps?
Inspector
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?
Thanks for your time and consideration,
Aaron Miller
--
Aaron's Home Inspections
aa...@aaronsinspections.com
www.aaronsinspections.com
HvacTech2
Hi Inspector, hope you are having a nice day
On 03-Dec-01 At About 04:02:08, Inspector wrote to All
Subject: Auto vs. Home HVAC Test Temps?
I> From: "Inspector" <aa...@aaronsinspections.com>
I> Can someone in this group please explain to me why it is believed in
I> the home inspection business that a residential A/C compressor can
I> be damaged by operation at too low an outside temperature,
Because it can be damaged by doing this
I> but yet
I> an automobile A/C operates in conjuction with the defroster at any
I> temperature?
No it doesn't, auto A/C's have a temp switch which cuts them out at too low
an ambient temp.
-=> HvacTech2 <=-
.. "Whatever temperature a room is its always room temperature."- s.w.
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Greg O
"HvacTech2" <dmurp...@snet.net> wrote in message
news:00014fc5....@subvet.com...
> No it doesn't, auto A/C's have a temp switch which cuts them out at too
low
> an ambient temp.
>
>
>
> -=> HvacTech2 <=-
You better check again, all the Chevy's I have owned, the compressor ran on
defrost, regardless of the temp outside.
Greg
HvacTech2
Hi Greg, hope you are having a nice day
On 03-Dec-01 At About 06:01:06, Greg O wrote to All
Subject: Re: Auto vs. Home HVAC Test Temps?
GO> From: "Greg O" <goo...@SPAMemail.msn.com>
GO> "HvacTech2" <dmurp...@snet.net> wrote in message
GO> news:00014fc5....@subvet.com...
>> No it doesn't, auto A/C's have a temp switch which cuts them out at
>> too
GO> low
>> an ambient temp.
>> -=> HvacTech2 <=-
GO> You better check again, all the Chevy's I have owned, the compressor
GO> ran on defrost, regardless of the temp outside. Greg
I think you need to check. that would ruin the compressor if it was too cold
outside. I have never seen one that ran in cold weather. maybe it was a
factory defect if it did.
-=> HvacTech2 <=-
.. Anybody can win, unless there is a second entry.
bridge
> 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?
>
> Thanks for your time and consideration,
>
> Aaron Miller
Aaron,
The outdoor unit is subject to icing up when low ambient conditions
are prevalent. As ice builds up on the outdoor unit coil, its ability
to absorb heat and boil off refrigerant within the coil diminishes.
Consequently, liquid refrigerant which should normally be in a vapour
state enters the compressor, which is undesirable. The refrigerant in
a liquid state within the compressor washes oil off lubricating
surfaces and causes mechanical failure, in addition cold liquid
refrigerant hitting the hot motor windings in a sealed compressor can
break down motor insulation, causing burn outs.
Regards,
Bridge
HACRat
wouldn't the ambient temp around a hot engine keep the comp a bit
warmer....sort of like a poor man's crankcase heater?news:0001505d....@subvet.com...
Carl Porter
The only reason I can see for this is when the outdoor temps get extremely low, the condensor
pressure drops considerably. Generally I've found the condensing temp to be about 10 degrees above
outside ambient with a properly charged unit but then I've never had to mess with a unit in the middle
of the winter. If the outside temperature hits 20 degrees the condensing temp is going to be about 30
which will make the pressure about 60 psi on the high side. I'm not sure what the differential
pressure would be across the expansion device but it would have problems working right. The suction
pressure might drop and then we get into trouble because the indoor coil might ice up and liquid
refrigerant might get into the compressor.
On a car the engine compartment stays fairly warm so the condensor pressure/temperature would stay
higher. The heater also works in conjuction with the cooler to dehumidify so the suction pressure
might stay high enough to prevent liquid from getting into the compressor.
Baillie
> home inspection business that a residential A/C compressor can be damaged
by
> operation at too low an outside temperature,
Normally these come with crank case heaters or a heater that's around the
compressor. So unless this has been disconnected in cold weather (for some
reason) you shouldn't have a problem with the compressor.
Neon John
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
--
John De Armond
johngdDO...@bellsouth.net
http://personal.bellsouth.net/~johngd
Cleveland, occupied TN
HvacTech2
Hi Baillie, hope you are having a nice day
On 04-Dec-01 At About 06:09:06, Baillie wrote to All
Subject: Re: Auto vs. Home HVAC Test Temps?
B> From: "Baillie" <bai...@icon.co.za>
>> 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
B> by
>> operation at too low an outside temperature,
B> Normally these come with crank case heaters or a heater that's around
B> the compressor. So unless this has been disconnected in cold
B> weather (for some reason) you shouldn't have a problem with the
B> compressor.
This is not correct. There is a way to run an A/C in colder temps but it
takes a few expensive controls. without these controls you will eventually
kill the compressor running it at any temp below 60 degrees.
-=> HvacTech2 <=-
.. "Curiosity killed the cat, but for a while I was a suspect."- s.w.
HvacTech2
Hi Neon, hope you are having a nice day
On 04-Dec-01 At About 04:06:06, Neon John wrote to All
Subject: Re: Auto vs. Home HVAC Test Temps?
NJ> From: Neon John <joh...@bellsouth.net>
NJ> Like all rules of thumb, this one isn't universal. The potentially
NJ> damaging condition can arise with split units that use long
NJ> refrigerant tube runs. The refrigerant will condense in the coldest
NJ> spot in the system. When outside is colder than inside, this spot
NJ> will be the condensing unit (compressor and condenser.) A split
You have this totally wrong. it happens due to the lowered head pressure when
it is cold outside. the only way to run a unit safely below 60 degrees
outdoor air temp is to have the proper controls installed which includes a
crankcase heater, head pressure control ( fan speed type preferred ) and a
timed low pressure bypass if you have a low pressure control on your
particular unit. you cannot do this on a window air unit but package rooftops
are easy to do this on. and yes you cannot run a window air or rooftop in
cold weather either or you will ruin the compressor.
.. "Hermits have no peer pressure." - s.w.
Doug Fontaine
refrigerant systems.
To operate correctly a mechanical expansion valve has to
have sufficient pressure difference between the high and low
pressure sides.
If head pressure control is fitted, and the package has a
timed out low pressure switch which is operated for the
first two minutes at start up, then there is no reason why
you can't use residential AC's during low ambient
conditions.
Regards Doug
hvac man
> 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?
>
> Thanks for your time and consideration,
>
> Aaron Miller
Here's the answer to you question in layman's terms:
A direct expansion air conditioning system with no "extra" control
items is not designed to operate in the winter. If operated in the
winter, the condenser coil (the coil outside the house) will remove
too much heat from the refrigerant, resulting in excessive
sub-cooling. The lowered temperature and pressure caused by this
sub-cooling will cause the refrigerant to be left in a state
downstream of the expansion device, where the heat removed from the
conditioned space will not completely evaporate the refrigerant in the
evaporator coil. This will cause liquid to enter the compressor,
which can ruin it.
The "extra" control items that allow the A/C to operate in the winter
add cost to the unit. Manufacturers offer these control items on
large commercial equipment as accessories. As the size of the units
decrease the availability of these accessories also decrease. Typical
residential A/C systems are not installed with these accessories.
Although some residential A/C systems may have the "extra" controls
required to operate in the winter, the vast majority does not.
In an automobile it is advantageous to have the "extra" control items
installed. This will allow the defroster and the A/C to run
simultaneously. The reason for this becomes evident in late fall and
early spring when the windshield fogs up. Running the defroster alone
takes forever. However if the air to the defroster is allowed to go
through a functioning A/C evaporator coil (core), the air will
dehumidify as it cools. The drier air will then almost instantly
remove the fog.