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Date: Thu Apr 23 1992 04:28:39
Subject: 72 AC Freeze Up

>One of the guys that I work with has a son in the Naval Academy
>and has a 72 240Z, reportedly in good shape, and has been transfered
>to Florida.
>Problem is that the AC freezes up real bad.  Thinking back, I had
>the same problem with the AC in the 72 240Z that I had back in the 
>Mid 70's.
>Does anyone know how to fix it?  Is there any air door to shut off
>the outside air from flowing while the AC is on?  I know that this
>will bring in too much outside moisture and freeze up any AC
>(experiences from our full size van).

Several things.  First off, an AC on a 72 is an aftermarket unit
that mounts under the fan on the passenger side.  These things are
designed to fit tight against the normal air inlet to the heater
fan.  The evaporator is sucked through.  Suck-throgh evaporators are
generally bad news.  Especially so in Zs.  One of the major problems 
is the evaporator rarely fits tightly against the fan housing.  And
with age the weatherstripping seal deteriorates.  I have
fought this problem many times.  The most satisfactory solution 
is to pull the evaporator, gom the sealing surface with either 
duct putty or RTV and reassemble.  This will seal the interface.

About freezing.  Moisture is not the cause.  The cause is inadequate
airflow across the evaporator which allows the temperature to drop
below freezing.  There are two ways of controlling the situation.
The first is to put a thermostat bulb in the evaporator with the 
setpoint above freezing.  That is the way most Z A/Cs work.  The 
problem is the thermostat is capable of being set below freezing.
The solution is to back the temperature setpoint back to above

A more satisfactory solution is to install a POA (pilot operated absolute)
valve in the evaporator exit freon line.  These valves are found on 
older GM and Ford air conditioners - they are built into the suction
accumulator on later model units.  These valves work by throttling 
the suction line to keep the pressure in the evaporator high enough
that the boiling point of the freon is above freezing.  The Ford
valve, because it is longer and thinner, is easier to install.
This valve will absolutely prevent freezup under any condition.


From: John De Armond
Newsgroups: sci.engr.heat-vent-ac
Subject: Re: Central A/C Question about Iceing
Date: Thu, 16 Jun 2005 13:02:09 -0400
Message-ID: <>

On 14 Jun 2005 18:01:23 -0700, wrote:

>I'm not an A/C engineer, so pls forgive my ignorance....that's why I'm
>My home A/C unit is 4 yrs old.  I was walking out the back door this
>morning and noticed that the lawn maintenece guys had weed-whacked the
>isulation off of the cold line of the outside unit.  I told my wife to
>go to Lowes and buy a new one.  She installed the new one and asked if
>it was OK that she wrapped the (what looks like a valve) from the
>compressor to the beginning of the insulated cold line.  She remembers
>that the cold line was very frosty while she was wrapping it.  She
>later called to tell me that the A/C was not working.  I came home to
>find severe restricted duct air-flow.  I opened the indoor unit and
>blew the fan only to get the ice from the indoor coils.  Once that was
>done, replaced everything and it appeared to cool the place, but never
>got below 81-82 degrees in the house.  (it was a 91-95 degree day here)
>The temp from the vent is about 65-66 degrees.  I removed the high
>perferformance filter from the system and the temp is now down to 80 in
>the house and the vent temp is 67 (slightly higher) degrees.
>Are any of the following to blame?
>   1) Missing cold pipe insulation?
>   2) High Perf. Filter restricting Air-Flow?
>   3) Damage from wrapping that inlet thingie at the end of the cold

Probably not applicable in this case.

>   4) Damage from running too long w/frozen coils etc?

Probably.  In the last oh, 3 years I've seen a number of instances of
hard evaporator freeze-ups causing damage to the evaporator.  The ice
forms on the U elbows and forces the sweat joints apart.  The last one
I serviced I found every single U-tube sweat joint seeping

Check your cold duct temperature.  If it is higher than about 50
degrees and you have good airflow this is probably the problem.

This brings up several questions.  A 4 year old unit should have some
sort of evaporator freeze protection, either a low suction pressure
switch or a thermostat on the evaporator, to prevent this damage.

I'd think that a 4 year old unit should be covered under warranty.
Hope so, as replacing an evaporator can be expensive, depending on the
unit's configuration.

If your unit does not have evaporator freeze protection, I'd ask that
it be installed during the repair.  It involves a maybe $20 thermostat
that clips to one of the evaporator U tubes and is wired in series
with the compressor signal from the thermostat.  Or you can do it
yourself.  Anti-freeze kits are available containing the thermostat
and the necessary attachment parts.


From: John De Armond
Subject: Re: Home HVAC Question
Date: Fri, 27 Jun 2008 13:24:57 -0400
Message-ID: <>

On Fri, 27 Jun 2008 09:18:11 -0700 (PDT), wrote:

>Hey folks.  I have a 2200sqft home with a 30 year old bulletproof
>Lennox Landmark 4ton AC system installed.  It's about time to look
>into replacing this thing.  Our condenser is placed a very long
>distance away from the coil.  I am guestimating about 100 to 120 ft of
>pipe roundtrip.  I am curious if it can be calculated how much energy
>is being wasted in such a long run due to heat gain as well as
>compressor working harder and such?

If the suction line is insulated then there isn't enough heat gain to worry
about.  And if the installer followed the recommended tubing diameter vs
distance tables then the flow loss is too low to consider.

Be aware too, that the suction line can be TOO large.  R22 doesn't mix
particularly well with the oil in the system.  The mist of oil that the
compressor makes is entrained in the liquid flow.  It coalesces in the
evaporator and collects at the bottom.  It is returned to the compressor by
friction with the gas.  If the gas velocity is too low then the oil will
collect in the evaporator, reducing its efficiency.  Eventually the compressor
becomes oil-starved and fails.

The tube sizing tables have been developed to balance all these sometimes
conflicting requirements.

>I am trying to decide if I should put the condenser nearer to the
>coil.  Problem is, that then puts it into direct afternoon sunlight
>from noon to 5pm.  I've had people tell me that sunlight doesnt make
>any difference to condenser operation and efficiency but I dont
>believe that.

"People" will say all sorts of crazy things.  Think about this logically.
Sunlight raises the temperature of the condenser (just like it does, say, a
solar water heater).  The higher temperature does two things. One, it
increases the delta-P across the compressor, making it consume more energy.

Two, the liquid leaving the condenser is hotter.  When it is sprayed into the
evaporator, it must first cool itself before cooling the air.  That is does by
flashing to gas until its temperature is reduced to prevailing conditions in
the evaporator.  This flashed gas is lost to cooling your house.

Bottom line, keep the condensing unit away from sunlight and out of the
shrubbery and out from under porches or anywhere else heat is produced or

A significant improvement in efficiency can be made by installing a
de-superheater.  This can be nothing more than the liquid line running inside
a larger suction line.  The cold vapor returning to the condenser absorbs the
heat from the hot liquid, bringing its temperature down to about that in the
evaporator.  IOW, it removes the superheat from the liquid.  The liquid no
longer has to flash down to the evaporator's temperature and thus that
refrigeration (and energy) isn't wasted.

>In this long run scenario, I have thought about using a 5ton condenser
>replacement and keeping the 4 ton coil and blower in the attic.

Only if you like to waste energy.  Part of improving the EER of an AC unit is
carefully matching all the components.  The evaporator is sized to use all the
refrigeration available without causing excessive pressure drop in either the
refrigeration or the duct system.

Aside from energy wastage, another problem with too small an evaporator is
freeze-up.  The compressor can move enough refrigerant to bring the evaporator
down to below freezing.  What that happens, ice forms which insulates the
evaporator while blocking air flow.  This is a positive feedback loop and
ultimately the evaporator becomes a solid block of ice.  In the worst cases,
the ice exerts enough force on the silver soldered "U" tubes on the end of the
evaporator to jack them out, breaking the solder joint and creating lots and
lots of leaks.

I've fixed a number of evaporators in which that has happened.  If I were
charging the industry labor rate (I don't, as I do this part time for
enjoyment), it would be cheaper to replace the evaporator.  "Repair" consists
of extracting the evaporator from the unit, standing it on end and using
several burners, heating each U-joint enough to silver solder it back into
place.  A major job.

Icing can be controlled using a "back pressure regulator valve" but that
results in the compressor running underloaded with a relatively low flow of
refrigerant at a very low pressure.  AC compressors rely on the refrigerant
flow for cooling so unless additional steps are taken such as suction line
liquid injection (energy wasteful), the compressor will fail early.

I like to slightly OVERSIZE the evaporator (or depending on how you look at
it, undersize the condensing unit).  I can then turn the fan speed down to
restore proper conditions in the evaporator.  The result is a LOT less air
flow noise and a little less energy use.

A complete new unit could pay for itself in as little as a year or two.  New
ones are that much more efficient.  Some high EER units draw as little as a
third of the power that a 20 or 30 year old unit of the same capacity did.

Get rid of that dinosaur and enjoy your "raise". :-)


From: John De Armond
Subject: Re: Home HVAC Question
Date: Sat, 28 Jun 2008 10:04:01 -0400
Message-ID: <>

On Fri, 27 Jun 2008 13:31:18 -0700 (PDT), wrote:

>What would you say to installing a ten year old "second hand" 15 SEER
>5 ton unit with dual stage pump, 5 ton coil, but leaving the current 4
>ton blower.  In this scenario, how would one go about allowing for
>dual stage control when the blower is only a single speed?

I'd have to see the details of the unit and the installation to give any
advice.  Did the original installation have a dual speed blower?  It's easy to
convert a single speed blower to dual speed - just install a dual speed motor.

Do you know what the original blower looks like?  Are the dimensions similar?
If so, it might not matter about the blower since capacity tends to scale with

In any event, you'd need to do some careful data gathering after installation.
This is always true whenever you deviate from a factory-designed setup.  In
particular, I'd measure the evaporator suction line temperature and the air
temperature.  This is best done on a cool evening after a hot day where the
house is heat soaked and presents a good load but ambient is cool which lets
the condensing unit be most efficient.

If either temperature drops below about 35 deg then you'll have to do
something to protect against freeze up.  A suction line pressure switch made
for the purpose will do the job, as will a clip-on Klixon thermostat, also
designed for the job.  Also, depending on the control system, the dual speed
setpoint might can be adjusted to reduce the capacity before freezing occurs.

Like I said, I'd have to look at the specific unit to know for sure.


From: John De Armond
Subject: Re: Home HVAC Question
Date: Wed, 02 Jul 2008 10:47:11 -0400
Message-ID: <>

On Tue, 1 Jul 2008 21:09:18 -0700 (PDT), wrote:

>So how about a Lennox-5 ton 2 Speed Power Saver Condenser Unit, Model

Sorry, I don't have an encyclopedic knowledge of manufacturer's model number
and specs.  I envy those tech who do.

>The blower unit is not available.  I am not sure where the two speed
>control magic is done in a typical system.

I'd have to see the schematic to know for sure.

>Would I be able to change the motor in my current 4ton attic unit to
>simply blow faster to perhaps mate better with a 5 ton coil?

Possibly.  Or possibly you might not have to do anything.  Lacking prior
experience with that particular combo, the only way I could know would be to
test using the methods I outlined earlier.

The problem that you run into is that in past years when power was almost too
cheap to meter, low EER was acceptable.  I've been chatting with a friend via
email concerning a 30 year old Lenox.  Major brand name.  Actual measured EER
is below 6!  Just looking at the photo of the unit to gauge the condenser size
and the nameplate, I guessed about 7.

The major EER improvements have come from the condensing unit but still,
there's no way to know about the evaporator without actually looking at it and
testing.  Hooking a high efficiency condensing unit to a low efficiency
evaporator will bring down the system's efficiency.


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