From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: AC question
Message-ID: <t39dhu4fsplpcs9ie4gdorr7pof1f3kul7@4ax.com>
Date: Mon, 24 Jun 2002 01:05:52 -0400

On Sun, 23 Jun 2002 21:39:06 -0700, "Alan Robinson" <alr@bmi.net> wrote:

>
>"Pseymore814" <pseymore814@aol.com> wrote in message
>news:20020624000221.06749.00000884@mb-fm.aol.com...
>> Hello All,
>>
>> We have a "new to us" motorhome and I have a quesiton about the air
>> conditioning.  The air conditioner runs as normal with a great deal of
>> air coming out of the fins, after a few hours the air quits blowing and
>> you can feel some cold air "rolling" out but not blowing.  I know this
>> question isn't clear but I am hoping that some of you with experience
>> will know what I am talking about.  Any insights?
>>
>> Thanks,
>>
>> Paul
>
>It sounds as if the evaporator may be icing up and blocking the
>airflow - if you remove the inside shroud, you should be able to
>look and see the ice buildup on the coil. This is _usually_ due
>to not enough airflow through the coil for the thermostat
>setting - the coil may be partially blocked by dust/dirt/pet
>hair, etc (in which case, it needs cleaning), or you may be
>running the thermostat on full cold with the fan not on high
>speed. Some ac's have a separate thermostat which senses coil
>icing and shuts off the compressor until the ice melts, some
>don't. If yours has a coil icing tstat, it sounds like it isn't
>working.  If you can find the model number of your ac, someone
>may be able to give better advice.

All good advice, though I've never seen a defrost t'stat on a roof mount.

The OTHER major cause of icing is low refrigerant.  While this is initially
counter-intuitive (low refrigerant should equal less cooling, right?) it is in
fact the case.  Here's what happens.  When the refrigerant gets low the evap
gets starved for refrigerant and the pressure drops.  This lower pressure
causes what refrigerant that IS there to boil at a lower temperature, below
freezing.  The first part of the evaporator around where the refrigerant
enters freezes.  Ice is a fairly good insulator so the refrigerant travels
further down the evaporator before encountering exposed evaporator.  This then
freezes and the process repeats.  Gradually the entire evaporator freezes.

The clue is how the freezing starts.  If there is general freezing all over
the evaporator, then the cause is low air flow.  OTOH, if the freezing starts
at one corner and proceeds across, the cause is insufficient refrigerant.

If you don't want to wait on the system to freeze up to diagnose it, all you
really need is a pocket thermometer.  Get the system running normally.  Insert
the bulb of a pocket thermometer into the fins of the evaporator on the 4
corners, perhaps an inch or so in from the edges.  If the system is operating
normally, the temperature will be the lowest where the refrigerant enters the
evap from the capillary tube (small, wire looking tube(s)).  It will be a bit
warmer (couple of degrees) on the other side (assuming horizontal tubing runs)
and will gradually get warmer as you go down the evap.  The warmest place will
be at the refrigerant outlet.  The temperature should NOT be below freezing if
there is sufficient air flow.

If the temperature is cold near the refrigerant inlet, perhaps freezing, and
then rapidly rising, then the refrigerant charge is low.  If it is very low,
the outlet of the evaporator and the fins near it will be near room
temperature.

The most common cause of THIS is someone in the past installing a so-called
"saddle valve" or piercing valve on a refrigerant line.  This type of valve
just clamps around the line.  The difference in expansion between the copper
line and the aluminum or pot metal clamp means that the clamp invariably
loosens.  When it does it begins to leak.  Indeed, I've heard crooked service
techs refer to these things called "service call generators".

If access is needed to the refrigerant system, the proper method is to silver
solder on a service fitting.  With a hermetically sealed system like a roof
system, this is almost never necessary, as there are no leakage paths as long
as the system is intact.  For example, my 25+ year old system has never been
opened.

These saddle valves are easy to find - a white metal gadget clamped around the
suction (larger) line (usually).  The remedy is to recover the remaining
refrigerant, remove that dastardly device and solder in a proper access port.
Then recharge the system properly.  This is something that might take a good
service tech an hour to do.

This may be more info than you wanted but I'd rather err on the side of too
much info :-)

John