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 freezing. 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. John From: Neon John <no@never.com> 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: <lib3b1p3u6qm75pintilie1iq2k7ek6ajq@4ax.com> On 14 Jun 2005 18:01:23 -0700, myfriendhenry@yahoo.com wrote: >I'm not an A/C engineer, so pls forgive my ignorance....that's why I'm >here! >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 >pipe? 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 refrigerant. 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. John --- John De Armond See my website for my current email address http://www.johngsbbq.com Cleveland, Occupied TN |