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Q: home central air conditioning units ( Answered 4 out of 5 stars,   0 Comments )
Subject: home central air conditioning units
Category: Family and Home > Home
Asked by: keltek-ga
List Price: $40.00
Posted: 17 Aug 2002 06:29 PDT
Expires: 16 Sep 2002 06:29 PDT
Question ID: 55573
The indoor part of my central heat & air where the air filter is installed 
froze up - literally. It is coated with ice crystals. Why does it freeze?
Our upstairs unit has been frozen now for over 24 hours. We tried a small 
ceramic heater blowing directly on some of the frozen bits and a floor fan 
blowing into the a/c closet and so far only more ice has formed. (It is "off".)
What's up? -and- How do we get rid of that ice?
Subject: Re: home central air conditioning units
Answered By: missy-ga on 17 Aug 2002 08:18 PDT
Rated:4 out of 5 stars
Hello keltek!

Our central air unit does this rather often during allergy season - we
all suffer ferocious allergies and end up having to run the air
conditioner constantly instead of opening the windows.  It's
frustrating when it freezes up, and the first time it happened, we
were similarly confused.

There are a couple reasons why this inconvenient event occurs.  Very
common in hot summer months is setting the temperature colder than is
necessary to cool things down.  It's almost automatic, on a sweltering
day, to want to cool your home down as quickly as possible.  Folks
often set the thermostat too low, with the intent of cooling things
off quickly.  This has the opposite effect, though - it causes the air
conditioner to work too hard, which in turn causes the air conditioner
to freeze:

"Setting the thermostat substantially above or below the desired
temperature will not heat or cool the home any faster. In fact, this
will place undue amounts of strain on the overall system. Running the
cooling system "colder" than is actually needed can damage the air
conditioning unit and may cause the coil to "freeze up."

Climate Control System

(We suffer from this form of freeze every time my friend comes to
visit.  He likes the room temperature to be somewhere between "frigid"
and "arctic", and immediately turns the thermostat way down, which
upsets the air conditioner and it goes on strike.)

To correct this problem, simply ensure that changes to the thermostat
are gradual, and that it isn't set to cooler than need be to cool the

Another common cause of this problem is dust in the air conditioner
unit.  In central air systems, both heating and cooling are handled
through the same ductwork.  These ducts can get very dusty - as air
travels through the ducts and is drawn through the air conditioning
unit, dust comes along with it and causes restricted airflow.  Check
your filters - it's possible that they are the culprit.  Change them
if necessary, and check them regularly.

You may also have a dusty evaporator coil.  If you remove the front
cover of your indoor unit, you can have a look at your evaporator coil
(the cover should just lift right off).  The evaporator coil is
covered by a finely woven "grate" that is the perfect home for dust. 
Give it a firm scrub-over with a long handled, firm (but not stiff!)
bristled brush to loosen the dust, then vacuum thoroughly. I'd
recommend using a shop vac, if you have access to one, because the
suction is usually better than a household vacuum. (We live in a dusty
area and the landlord isn't great about cleaning out the ducts, so we
get the dusty evaporator coil freeze at least once a year! This method
works very well.)

The Lennox heating and cooling FAQ says that this sort of cleaning
must only be handled by a trained repairman:

"Dirty evaporator coil:

Over time, the evaporator coil will become dirty. When this happens,
the results are similar to those of having a dirty filter. Gradually
you will lose airflow, slowly enough that you probably would not
realize it until it freezes up or is not cooling adequately. You will
need to contact your local Lennox Dealer to correct the problem."

Q: Why is my system freezing up? - The Lennox FAQ

Carrier, on the other hand, recommends you do it yourself first,
taking care to do this *only* when the coil is dry:

"Routine Maintenance

Check the air filter in your furnace or fan coil every 3 to 4 weeks. A
dirty filter will cause excessive strain on your furnace, air
conditioner or heat pump. Replace your filter when necessary, or clean
it if you have the reusable type. (If you have a reusable filter, make
sure it’s completely dry before you re-install it.) The prefilter and
collection cells of an electronic air cleaner should be cleaned at
least two or three times per year.

Clean dust off of your indoor coil. With a vacuum cleaner and
soft-brush attachment, you can remove any dust from the top and
underside of the coil. Make sure you only do this when the coil is
dry. If you can’t get the coil clean this way, call your dealer for

Carrier - Maintenance FAQ

If none of these methods corrects the problem, you most like are
dealing with a coolant leak.  This is not something you can fix
yourself, you must call a certified repair technician:

"Low refrigerant:
In some cases, freezing up is caused by a leak in the refrigerant
lines. Weak solder joints, friction from piping rubbing or vibrating
against an object, leaking valves or loose fittings can cause leaks.
The age of the system and the nature and location of the leak are the
determining factors on whether to have the system repaired or

Q: Why is my system freezing up? - The Lennox FAQ

There are several ways to get rid of the ice.  My own favored (and
relatively quick) method is to use a hairdryer, set to low, to melt
the ice off.  Be certain to have something to catch the drips!

The G&S Mechanical Services FAQ recommends switching the system to
"fan only" to allow the ice to melt off:

"If you keep running the system while it is frozen, you run the risk
damaging your compressor.  The best thing to do is to turn off the
and just run the fan to circulate room-temperature air over the coils
the ice melts and falls off.  Switch your thermostat from AUTO/COOL to
FAN to manually run the fan with the compressor turned off."

"What to do if your heat pump or air conditioner freezes up" 

Similarly, the Lennox FAQ suggests using the fan or, if you have a
split system (central heat and air combined) turning on the heat pump
for a while:

"Should you find that your system was freezing due to a dirty filter,
after replacing or cleaning the filter, you can speed up the thawing
process by turning the system off and turning on the fan. If you have
a heat pump system, you can try turning the system to heating mode
until the ice has melted. After the ice has melted, switch the system
settings back to normal. If the system refreezes, contact your local
Lennox Dealer to correct the problem."

Q: Why is my system freezing up? - The Lennox FAQ

I hope you find this helpful, and that you're back in air conditioned
comfort in no time!  If any part of my answer is unclear, please don't
hesitate to ask for clarification.  I'll be happy to be of further

Good luck!


Search terms:  [ "air conditioner" freeze troubleshooting ], [ "dirty
evaporator coil" freeze ]...and regular experience with this problem.
keltek-ga rated this answer:4 out of 5 stars
I appreciated how quickly there was a response to my inquiry. I fell
asleep at my computer for a few hours and when I awoke, there it was.
I also like the
fact that the answer consisted of personal experiences and researched
"professional" sources combined. However, after using the suggested
method, the ceramic heater and floor fan I mentioned, there is even
more ice, as I mentioned. We do have a call in to the repairman. What
he can do as
long as there is an icy coating, I'm  not sure. It may be that there
is a coolant leak. -?- The help/research is truly appreciated.

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