How cold can you get the A/C in a humid climate???

[Duc Hunter]

I posted this on Reenlist and....there just does not seem to be much air cooled action over there so I thought I would try here (where I should have the first time around).

So I have just rebuilt my A/C in my 89 Carrera 3.2, and it is cold....meaning on the freeway I can cycle in the high 30's to low 40's vent temps when it is over 100F degrees outside and 60% humidity....on R134. I am very satisfied.....and yet being one who has a hard time leaving well enough alone...I have one curiosity I want to try to find an answer to.

My curiosity is this....around town I have to dial up the vent temps to mid 40's at the coldest. If I leave the thermostat set to its coldest possible vent temps that still let the compressor cycle on the freeway......around town the vent temps come down for a period of time. However with stoplights, 45mph and below speeds, etc. the compressor won't cycle nearly as often (if at all), and eventually the vent temps just start rising....up into the mid 60's sometimes. If I catch this, and turn the thermostat to a point where the compressor will cycle a few times regularly, often the condition clears. Usually when this happens I have frost on the output line of the evaporator. If there is only wet moisture on it, this never happens.

What I believe is happening is that, since the compressor is not cycling regularly, the evaporator is getting too cold because it does not warm up as it would when the compressor cycles off, and output line of the evaporator is building ice on it. This causes the Expansion Valve (EV) to close and stay closed, causing the temps to go up into the mid 60's (it is trying to fight its icing condition). Now keep in mind my EV and its sensor are open to the air, not covered in cork tape or tar (or what ever that gooey crap is ). I have been told by some that covering both will help prevent the EV's sensor from icing, and give me a little lower temps.

It is my belief that if I lived in Az, or another VERY dry climate, that evap temps well below freezing would be fine. This is because there is not enough ambient moisture to cause any icing. (Side note - when I live in CA I noticed my other cars A/C dripped very little water. When that same car came here to Florida, it was like a hose was pouring water out of it). Here in the lovely swamp of Florida though we have a lot of humidity, which means icing of the system is easier, so we would ideally need to alway run the system where the evaporator and it's output line are above 32F degrees.

• Do the experts here think insulating the EV sensor (and the valve?) would help this a bit? (Done)
• Do the experts here think that, all things being equal, colder evep temps...and therefor colder vent temps..... are possible in very dry climates vs. very humid climates?

Edit: I know my EV is working right (brand new). I know the EV sensor is placed right (brand new, camped at the factory). I know my thermostat is working (brand new, and can cycle normally, as long as I don't crank it too cold).My system is far from stock. I have almost the full RetroAir system, AND a 625cfm fan on the rear condenser. The RetoAir rear condenser is a great Multiflow condenser. I will insulate the pigtail sensor and report back. My belief is that the pigtail being insulted, and moving my thermostats sensor in the eval will help a lot.

Edited for ganum....not sure what happened.

[Ronnie's.930]

Chris, the pigtail sensor is indeed supposed to be insulated with sticky cork tape or strip calk (not the entire copper tube but the coiled portion that is in contact with the expansion valve body.

I live in a very hot and humid climate, not as humid as Florida, but I "think" a bit hotter (maybe?) . . . many months of upper 90s for highs (sometimes into the 100s) and lows in the lower 80s and with humidity levels above 60% most of the time and quite a bit higher much of the time, and I have bone chilling a/c regardless of highway cruising or low RPM puttering around town. I have never noticed a rise in vent temperatures if I keep the thermostat cranked, but the system produces such cold air, that I only have to have the thermostat on max in extremely hot weather, when puttering around making stops and starts, or if there is another person in the car adding body heat to the cabin; on the highway, especially after dark, I usually run the thermostat at 3/4 of max, but often switch to max for a while if I want to further clear humidity from the cabin or want additional cooling for whatever reason.

I've not noticed evaporator freezing even though my vent temps are downright arctic, but maybe I would when highway cruising after dark and the thermostat on max the entire time, but by then I would have icicles hanging off of all my parts, anyway!

A **** ton of water is always pouring out of the evaporator drain when I check it after parking.

My thermostat sensor is in the stock location - the evaporator is not stock, but I put the sensor in the stock location when I replaced the evaporator a while back.

[Duc Hunter]

Ronnie, great news......

Have you ever put a temp gauge in your vents to see the actually numbers? I think you did a big thread on your system at one point (been doing a ton of reading, so they all blend together). I would be interested. I cannot run mine fully cold, or the vent temps go down to high 30's a few time and then crawl up (frozen). I also know 45 degree vent temps feel pretty cold, esp when it is 100 outside.

[pavegeno]

33F driver left side vent temp on a 97F humid day in Germany. Believe you'll see coldest vent temps out of the center or bow tie vents. The humidity indicated is the inside humidity.

[ganun]

Chris, please proof read ...geez, I am sure I'm not the only one having trouble following.

[wwest]

From RennList:

Originally Posted by Duc Hunter

You were correct on both eh EV and the EV Sensor.

My system though is far from stock. I have almost the full RetroAir system, AND a 625cfm fan on the rear condenser. The RetoAir rear condenser is a great Multiflow condenser.

IIRC the engine lid PFC is the ONLY condenser in your system...

I will insulate the pigtail sensor and report back. My belief is that the pigtail being insulted, and moving my thermostats sensor in the eval will help a lot.

I believe the thermostat's capillary sensor is best placed nearest the outflow point in the case of a serpentine evaporator. Someone else may know for certain.

My main questions still remains....how cold can you get the evap in a humid climate? Am I correct that 32 is about it given the high Rh....vs in a place like Az (with single digit Rh) you could go below freezing?

Even in the driest climate an evaporator kept sub-freezing will ice over given enough time. Our systems are ALWAYS in recirculate mode, so human metabolism, breathing, perspiring, will play a large part in this in dry climates.

But, even in a high Rh climate the icing up period may be long since our cars have no "forced" fresh (HUMID!) air intake.

[Ronnie's.930]

Originally Posted by Duc Hunter

Ronnie, great news......

Have you ever put a temp gauge in your vents to see the actually numbers? I think you did a big thread on your system at one point (been doing a ton of reading, so they all blend together). I would be interested. I cannot run mine fully cold, or the vent temps go down to high 30's a few time and then crawl up (frozen). I also know 45 degree vent temps feel pretty cold, esp when it is 100 outside.

Chris, I use a heat gun to measure vent and cabin temperatures - vent temps are always 26-28 degrees when the compressor is running (even in upper 90s, etc.) - takes very little time to get there even when the cabin is heat soaked.

Yesterday it was 88 and overcast here, about 60% humidity, vent temps were 26 degrees, passenger's seat was 46 and headliner was 49 . . . this was idling under a carport for about 10-15 minutes with a barrel fan in front of the car aimed rearward. It was fuggen cold in thar = sweet!

[kuehl]

Duc,
Your issue reads that the evaporator is freezing up, based on your presentation.
The thermostat should be shutting off the compressor. To test this turn the thermostat
to max cold (CW) and wait for the vent temps to start to rise as you noted, then turn the thermostat to a warmer setting, a few degrees (CCW). If the issue goes away then you have to relocate your thermostat's probe.

[wwest]

CG, OP has said that "on the road" he "dials back" the cooling to maintain his cabin atmosphere comfort level (72-75?), but in stop and go traffic in an attempt to maintain that same level of comfort he turns the system to max cooling.

Basically he seems to asking if there is a way to prevent freeze-up, especially in high Rh climates, to prevent the inadvertent freeze-up that follows subsequent to dialing in max cooling

My advice was to somehow add condensing capacity/efficiency such that he does to have to use the maximum cooling setting.

[Duc Hunter]

Originally Posted by kuehl

Duc,
Your issue reads that the evaporator is freezing up, based on your presentation.
The thermostat should be shutting off the compressor. To test this turn the thermostat
to max cold (CW) and wait for the vent temps to start to rise as you noted, then turn the thermostat to a warmer setting, a few degrees (CCW). If the issue goes away then you have to relocate your thermostat's probe.

This will get in-progress tomorrow. Also, noticed today that any more than about 2:30 on the thermostat causes the behavior, which again points to your solution. I will report back.

Originally Posted by Ronnie's.930

Chris, I use a heat gun to measure vent and cabin temperatures - vent temps are always 26-28 degrees when the compressor is running (even in upper 90s, etc.) - takes very little time to get there even when the cabin is heat soaked.

Yesterday it was 88 and overcast here, about 60% humidity, vent temps were 26 degrees, passenger's seat was 46 and headliner was 49 . . . this was idling under a carport for about 10-15 minutes with a barrel fan in front of the car aimed rearward. It was fuggen cold in thar = sweet!

Um...your driving a turbocharged meat locker there Ronnie!

Originally Posted by wwest

CG, OP has said that "on the road" he "dials back" the cooling to maintain his cabin atmosphere comfort level (72-75?), but in stop and go traffic in an attempt to maintain that same level of comfort he turns the system to max cooling.

Basically he seems to asking if there is a way to prevent freeze-up, especially in high Rh climates, to prevent the inadvertent freeze-up that follows subsequent to dialing in max cooling

My advice was to somehow add condensing capacity/efficiency such that he does to have to use the maximum cooling setting.

Close but no. I am saying if I drive on the freeway, and turn it down to where it is as cold as possible yet the system still cycles regularly I can get 38-42 minimum vent temps (obviously when the compressor cycles off the temps come up 10-12 degrees).....then get off the freeway and use surface streets.....the slower speed and stoplights prevent the compressor for cycling off (at a stoplight temps rise naturally as you sit). The vent temps can sit at 42-45 for a while (as I am moving)....but this eventually causes my icing issue where the temp then creeps up and up. So, to prevent this, I must turn the thermostat up (warmer) until I can get semi-regular cycling...which seams to be minimum vent temps above 44.

[kuehl]

Wwest,

My post starts with "Duc". It does not start with "wwest".

I read Duc's post.

Now, if Duc wants help let Duc talk for Duc.


Thank you.

[wwest]

In the below linked thread series you were struggling with an engine overheat problem resulting in engine pinging.

Your rear lid PFC is dramatically more efficient than the one supplied by Porsche, and likely even substantially more efficient than the standard aftermarket serpentine condenser. And now with the fan addition you have raised the condensing efficiency to an even higher level.

All the while adding HOTTER airflow for engine cooling.

Shooting myself in the foot here but that STELLAR condensing capacity was very likely the base causative factor for your engine heating/pinging episode.

There are a few naysayers that will now come forth, one in particular, but in stop and go traffic the IR heating of the rear lid condenser may well be playing a high percentage of you loss of A/C functionality.

The engine, exhaust, and especially the Cat-Con, will radiate a LOT of HEAT at all times. But the IR effect will clearly become more dominant as condenser cooling airflow declines.

You have ability to kill 2 birds with one "stone".

Remove some of the heat load from the engine, and then you can go back to the more aggressive DME ignition timing. Reduce the engine heat load by adding additional refrigerant condensing capacity downstream of the engine lid condenser.

ZIMs fender well condenser/fan kit, $500, would be an excellent solution.

Even better if you could talk RetroAire into supplying a ZIMs-like kit, but with PFC condenser

Plus remove the engine lid condenser cooling fan.

PS: Assumption: you have powered, directly or indirectly, the condenser cooling fan with the compressor clutch circuit. Install a Trinary pressure switch and us the third element to power the condenser cooling fan, that way the fan will continue to cool the condenser until the pressure level has sub-sided.

89 Carrera running hot and gurgling...

[wwest]

Originally Posted by Duc Hunter

This will get in-progress tomorrow. Also, noticed today that any more than about 2:30 on the thermostat causes the behavior, which again points to your solution. I will report back.



Um...your driving a turbocharged meat locker there Ronnie!



Close but no. I am saying if I drive on the freeway, and turn it down to where it is as cold as possible yet the system still cycles regularly I can get 38-42 minimum vent temps (obviously when the compressor cycles off the temps come up 10-12 degrees).....then get off the freeway and use surface streets.....the slower speed and stoplights prevent the compressor for cycling off (at a stoplight temps rise naturally as you sit). The vent temps can sit at 42-45 for a while (as I am moving)....but this eventually causes my icing issue where the temp then creeps up and up. So, to prevent this, I must turn the thermostat up (warmer) until I can get semi-regular cycling...which seams to be minimum vent temps above 44.

Shines a new light..

Check the refrigerant level. Quick check via the R/D sight glass, follow the instructions in your owner manual.

[Duc Hunter]

Westy,

Well thank you for you thorough interest. My overheating and pinging has been largely solved (one more piece to the puzzle is going in this week). Once I know I have it nailed I will go back to that thread and post my solution. The A/C has nothing to do with it though, that I do know. I just turned the AC off for a few days, and the problem and oil temps did not change.

I looked at the ZIM fender kits, and chose not to go that route as I believe the RetroAir condenser to be plenty efficient....not my pinging issue....and enables me to easily take my 1 of 240 built 911's back to stock imply by swapping the condenser for the stock on that I kept when it was removed. While I do not disagree that, in principle, removing the condenser from the deckled, and moving it to the fender with the ZIM kit will help engine cooling. You could go crazy and then add an oil cooler in the condensers place. I think that is over kill though.

As for refrigerant levels, they are great. Pressures are great both high and low side. I have been through the A/C several times now changing challenges to get it to work right, so I know refrigerant is not my challenge.

[Duc Hunter]

Originally Posted by kuehl

Wwest,

My post starts with "Duc". It does not start with "wwest".

Thank you.

And I thank you for your help sir....it is MUCH appreciated.

[wwest]

Duc, didn't realize that you were still struggling with some aspect of the engine overheat problem.

This is a bit of a stretch, but..

The HIGHLY efficient PFC rear lid condenser may be a problem even with the A/C off.

The PFC has several unique design aspects that result in that stellar performance.

The number of fins/vanes/sq. in. is substantially greater than the nearest competitive, the serpentine condenser, and probably twice that of OEM condenser.

In addition to that, in order to have the greatest # of air molecules make "contact" with the fin/vane surfaces, the fins/vanes have a "dimpled", wavy, pattern, creating a greater level of turbulent air flow within the PFC.

Enough restriction to airflow to adversely affect engine cooling.?

Maybe.

[Bob Kontak]

Originally Posted by Duc Hunter

And I thank you for your help sir....it is MUCH appreciated.

He puts the rubber to the road.

[Bob Kontak]

Originally Posted by wwest

In addition to that, in order to have the greatest # of air molecules make "contact" with the fin/vane surfaces, the fins/vanes have a "dimpled", wavy, pattern, creating a greater level of turbulent air flow within the PFC.

Enough restriction to airflow to adversely affect engine cooling.?

See me below after reading. Sorry, did not stick to the script.



Who pays you to type this crap? Come on, Willy. Nut up and tell the truth. This is your life? Are you kidding me?

Is it fixed fee or per post?

[Duc Hunter]

Originally Posted by wwest

Duc, didn't realize that you were still struggling with some aspect of the engine overheat problem.

This is a bit of a stretch, but..

The HIGHLY efficient PFC rear lid condenser may be a problem even with the A/C off.

The PFC has several unique design aspects that result in that stellar performance.

The number of fins/vanes/sq. in. is substantially greater than the nearest competitive, the serpentine condenser, and probably twice that of OEM condenser.

In addition to that, in order to have the greatest # of air molecules make "contact" with the fin/vane surfaces, the fins/vanes have a "dimpled", wavy, pattern, creating a greater level of turbulent air flow within the PFC.

Enough restriction to airflow to adversely affect engine cooling.?

Maybe.

Westy, I really appreciate the advise.....all I can say is you are way off base in my particular case. My cooling issue was timing related (Thanks to a ****ty chip), and oil cooling related (thanks for various seals, a fan, 2 thermostats, etc). Again....I have that licked, just need to make the last change to finish it. That will solve my issue, A/C regardless.

Originally Posted by Bob Kontak

See me below after reading. Sorry, did not stick to the script.



Who pays you to type this crap? Come on, Willy. Nut up and tell the truth. This is your life? Are you kidding me?

Is it fixed fee or per post?

Bob....ok I cannot stop laughing.....and I SO needed a laugh today (personal reasons). You are a true gem! Thank you! Keep it coming.

[Bob Kontak]

Originally Posted by Duc Hunter

Bob....ok I cannot stop laughing.....and I SO needed a laugh today (personal reasons). You are a true gem! Thank you! Keep it coming.

Thanks.

Bizarre thing is I am kind of ok with Willy. Would love to choke him out, for like a long minute, but that's it. Blue face is the cut off. Never purple.