Interesting Temps Coming from A/C Vent - Are They Normal?
04-19-2009, 06:54 PM
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Interesting Temps Coming from A/C Vent - Are They Normal?
With warm weather fast approaching (Temps in the 80's today), I decided to do some experimenting with temperatures coming out of the A/C system on California. Last year I had replaced everything in the AC system except for the evaporator in the dash. The system has been charged now for a little over a year. I'd like for the A/C system to be working at factory spec for the coming summer so thought I'd measure temps and compare to the factory charts.
I used a digital A/C temperature probe during the tests. Yesterday afternoon when I took the car for a test drive with the temperature probe, I was getting temperature readings that varied by as much as 10 degrees depending on where I positioned the probe in the center vent. So, this morning I decide to remove the center console for my testing to make sure I could get consistent readings. Here's the test conditions:
Ambient temperature: 80 degrees F
Time of day: Mid-morning (so the car had not become heat soaked yet)
Operating conditions: Car at idle in driveway with A/C on - set on center vent, minimum cool (65 degrees on the slider)
Scenarios: Fan on "0" speed, "1" speed, "2" speed, "3" speed, and "4" speed.
With the center console out, I decided to clean the cabin temperature sensor with jets of electronics cleaner in the spray can. While the A/C system is operating, I confirmed the recirculation flap was open. I left the car doors and rear hatch open as well so the cabin temperature (intake temperature) would remain constant during the different tests.
Here's the results:
The first test was with fan speed on "0". After a minute or so, the temperature probe settled down to about 5 degrees F (-15 C)
On fan speed "1", the temperature settled down to about 28 degrees F (-2.2 C)
On fan speed "2", the temperature settled down to about 39 degrees F (3.9 C)
By now, you can see a pattern here.....on fan speed "3", the temperature settled down to about 50 degrees F (10 C)
And finally, on fan speed "4", the temperature coming out of the center vent was about 55 degrees F (12.8 C).
After running the tests with increasing fan speed, I repeated all the test scenarios in reverse order down to "0" speed with consistent results.
So the temperature seemed be heavily influenced by fan speed. Thinking about the physics of the test, I suppose that makes sense. At low fan speeds, the volume of warm air moving across the evaporator fins is low and slow - not much air mass to cool down and the air spends a longer time moving through the fins so has more time to absorb the cool. On high speed, there's a high volume of warm air and it's moving quickly over the evaporator fins so it doesn't have much time to absorb the cool. OK, so my newbie brain thinks this seems plausible. What do you think??
Now for the manual specs on temperature out the center vent as a function of intake air temperature. At 80 degrees F intake temp (26.67 C) The temp out the center vent should be between 40-45 degrees F (4.5-7.5 C near as I can tell from the graph). Since output temp seems to be heavily influenced by fan speed, what fan speed are they using for the chart?
If I go by my numbers and see where they fall on the factory chart, an output temperature of 40-45 degrees with input temp of 80 degrees was on fan speed "2" (or more like "2.5"). Given that fan speed is about the "middle" speed for the A/C system, does that mean my A/C system is performing at factory specifications??
The next test would be to try the same test scenarios while the car is driving at highway speeds to see if there's any difference.
Anyway, just thought I'd share these results and welcome comments, experiences or your own test results for comparison. THANKS for the help.
I used a digital A/C temperature probe during the tests. Yesterday afternoon when I took the car for a test drive with the temperature probe, I was getting temperature readings that varied by as much as 10 degrees depending on where I positioned the probe in the center vent. So, this morning I decide to remove the center console for my testing to make sure I could get consistent readings. Here's the test conditions:
Ambient temperature: 80 degrees F
Time of day: Mid-morning (so the car had not become heat soaked yet)
Operating conditions: Car at idle in driveway with A/C on - set on center vent, minimum cool (65 degrees on the slider)
Scenarios: Fan on "0" speed, "1" speed, "2" speed, "3" speed, and "4" speed.
With the center console out, I decided to clean the cabin temperature sensor with jets of electronics cleaner in the spray can. While the A/C system is operating, I confirmed the recirculation flap was open. I left the car doors and rear hatch open as well so the cabin temperature (intake temperature) would remain constant during the different tests.
Here's the results:
The first test was with fan speed on "0". After a minute or so, the temperature probe settled down to about 5 degrees F (-15 C)
On fan speed "1", the temperature settled down to about 28 degrees F (-2.2 C)
On fan speed "2", the temperature settled down to about 39 degrees F (3.9 C)
By now, you can see a pattern here.....on fan speed "3", the temperature settled down to about 50 degrees F (10 C)
And finally, on fan speed "4", the temperature coming out of the center vent was about 55 degrees F (12.8 C).
After running the tests with increasing fan speed, I repeated all the test scenarios in reverse order down to "0" speed with consistent results.
So the temperature seemed be heavily influenced by fan speed. Thinking about the physics of the test, I suppose that makes sense. At low fan speeds, the volume of warm air moving across the evaporator fins is low and slow - not much air mass to cool down and the air spends a longer time moving through the fins so has more time to absorb the cool. On high speed, there's a high volume of warm air and it's moving quickly over the evaporator fins so it doesn't have much time to absorb the cool. OK, so my newbie brain thinks this seems plausible. What do you think??
Now for the manual specs on temperature out the center vent as a function of intake air temperature. At 80 degrees F intake temp (26.67 C) The temp out the center vent should be between 40-45 degrees F (4.5-7.5 C near as I can tell from the graph). Since output temp seems to be heavily influenced by fan speed, what fan speed are they using for the chart?
If I go by my numbers and see where they fall on the factory chart, an output temperature of 40-45 degrees with input temp of 80 degrees was on fan speed "2" (or more like "2.5"). Given that fan speed is about the "middle" speed for the A/C system, does that mean my A/C system is performing at factory specifications??
The next test would be to try the same test scenarios while the car is driving at highway speeds to see if there's any difference.
Anyway, just thought I'd share these results and welcome comments, experiences or your own test results for comparison. THANKS for the help.
04-19-2009, 07:21 PM
#2
Race Car
Sounds like your freeze switch is not doing its job or is bypassed. You should not be seeing temps below 32 deg F from the vents regardless of fan speed.
I believe the manual specifications are for the engine at 2,000RPM and fan speed set to '2'.
Dan
'91 928GT S/C
475hp/460lb.ft
I believe the manual specifications are for the engine at 2,000RPM and fan speed set to '2'.
Dan
'91 928GT S/C
475hp/460lb.ft
04-19-2009, 07:34 PM
#3
Race Director
odd......I've found the center air vent temps are significantly higher staionary vs moving...but 80F isn't that hot really....... A couple ???
Which type of refrigerant? R12 or 134
Are you sure the recirculation flap works?
Does the heater valve hold vacuum and keep the hot water out of the heater core (common problem)..sometimes heater valves fail internally, so even though they hold vacuum and the arm moves...water still gets through (ask me how I know)....I even had a metal heater valve fail
I've found when running in recirculation (full cold "65"...if you move to 70 it runs fresh air) on hot days I can consistently get 50+ degrees under ambient....with output air typically close to freezing....Bill Ball has seen similar results...
Which type of refrigerant? R12 or 134
Are you sure the recirculation flap works?
Does the heater valve hold vacuum and keep the hot water out of the heater core (common problem)..sometimes heater valves fail internally, so even though they hold vacuum and the arm moves...water still gets through (ask me how I know)....I even had a metal heater valve fail
I've found when running in recirculation (full cold "65"...if you move to 70 it runs fresh air) on hot days I can consistently get 50+ degrees under ambient....with output air typically close to freezing....Bill Ball has seen similar results...
04-19-2009, 10:40 PM
#4
Chronic Tool Dropper
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Dwayne--
We generally look for vent temps to be no lower than about 40-45º lower that the temo of the air going in. With the fan at lowest (zero) speed, that would provide the most cooling since the inlet air flow is so low, and the amount of residence time in the evap is pretty high. Moving the fan speed higher means more cooling needed so obviously the temps were getting up to more realistic temps in the 40-45º below ambinet range with the fan set on speed 2.
That brings us to the next thing, and that's compressor speed. As Dan points out, a 'standard' test condition includes running the engine at 1500-2000 RPMs with the interior fan speed set on 2. Thtat would give you something close to driving conditions on the system, even though you are depending on the fans to take care of condenser loads. Sticking with a standard condition means you can better judge the system performance.
As others also mention, the freeze switch does not seem to be working. If it was working, those sub-36º readings would be impossible. On the other hand, if you use the car here in Cali, with the very low humidity, keeping the freeze switch closed means that you'll get much faster cool-down. If you start driving longer to the point where the condensate starts icing in the evaporator, you can replace the switch. Or just plan on giving the system a little break every once in a while by turning the compressor off. You'll know when it needs that-- airflow will start to fall off and things will get warmer. Don't worry about straining the system running with the evaporator too cold-- It's actually less strain on the system when it's suction-starved like that.
Good luck!!
We generally look for vent temps to be no lower than about 40-45º lower that the temo of the air going in. With the fan at lowest (zero) speed, that would provide the most cooling since the inlet air flow is so low, and the amount of residence time in the evap is pretty high. Moving the fan speed higher means more cooling needed so obviously the temps were getting up to more realistic temps in the 40-45º below ambinet range with the fan set on speed 2.
That brings us to the next thing, and that's compressor speed. As Dan points out, a 'standard' test condition includes running the engine at 1500-2000 RPMs with the interior fan speed set on 2. Thtat would give you something close to driving conditions on the system, even though you are depending on the fans to take care of condenser loads. Sticking with a standard condition means you can better judge the system performance.
As others also mention, the freeze switch does not seem to be working. If it was working, those sub-36º readings would be impossible. On the other hand, if you use the car here in Cali, with the very low humidity, keeping the freeze switch closed means that you'll get much faster cool-down. If you start driving longer to the point where the condensate starts icing in the evaporator, you can replace the switch. Or just plan on giving the system a little break every once in a while by turning the compressor off. You'll know when it needs that-- airflow will start to fall off and things will get warmer. Don't worry about straining the system running with the evaporator too cold-- It's actually less strain on the system when it's suction-starved like that.
Good luck!!
04-20-2009, 12:35 AM
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odd......I've found the center air vent temps are significantly higher staionary vs moving...but 80F isn't that hot really....... A couple ???
Which type of refrigerant? R12 or 134
Are you sure the recirculation flap works?
Does the heater valve hold vacuum and keep the hot water out of the heater core (common problem)..sometimes heater valves fail internally, so even though they hold vacuum and the arm moves...water still gets through (ask me how I know)....I even had a metal heater valve fail
I've found when running in recirculation (full cold "65"...if you move to 70 it runs fresh air) on hot days I can consistently get 50+ degrees under ambient....with output air typically close to freezing....Bill Ball has seen similar results...
Which type of refrigerant? R12 or 134
Are you sure the recirculation flap works?
Does the heater valve hold vacuum and keep the hot water out of the heater core (common problem)..sometimes heater valves fail internally, so even though they hold vacuum and the arm moves...water still gets through (ask me how I know)....I even had a metal heater valve fail
I've found when running in recirculation (full cold "65"...if you move to 70 it runs fresh air) on hot days I can consistently get 50+ degrees under ambient....with output air typically close to freezing....Bill Ball has seen similar results...
I'm using R-12 and the recirculation flap is working (visual confirmation). Although the test I was running had the doors and hatch open so it was still pulling 80 degree ambient air during all test scenarios. The heater water valve does hold vacuum but I don't recall if I checked it for leaks when I had the car apart about 18 months ago - I'll check for leaks when I'm in there replacing a couple of fuel lines this summer.
When you are getting 50+ under ambient, is that the temp at the center vent and is the fan speed on high?? THANKS for the reply!
04-20-2009, 12:45 AM
#7
Race Director
Hello Brian,
I'm using R-12 and the recirculation flap is working (visual confirmation). Although the test I was running had the doors and hatch open so it was still pulling 80 degree ambient air during all test scenarios. The heater water valve does hold vacuum but I don't recall if I checked it for leaks when I had the car apart about 18 months ago - I'll check for leaks when I'm in there replacing a couple of fuel lines this summer.
When you are getting 50+ under ambient, is that the temp at the center vent and is the fan speed on high?? THANKS for the reply!
I'm using R-12 and the recirculation flap is working (visual confirmation). Although the test I was running had the doors and hatch open so it was still pulling 80 degree ambient air during all test scenarios. The heater water valve does hold vacuum but I don't recall if I checked it for leaks when I had the car apart about 18 months ago - I'll check for leaks when I'm in there replacing a couple of fuel lines this summer.
When you are getting 50+ under ambient, is that the temp at the center vent and is the fan speed on high?? THANKS for the reply!
typically I have it on a medium fan setting....usually 2 sometimes 1!!!! Since anything more is just TOO COLD.... it will occasionally hit the freeze switch and shut down for a couple minutes....(which sucks since it gets hot quick)...
Another factor is I use an Infared sensor to get temps....I originally bought it for my nitro RC car to take engine temps....& I do get a bit of variance depending on where I point it.....IF I remember correctly (good luck) the coldest spot is usually the lower left (drivers) corner..... I'll have sharky out in the warmer weather shortly and will do some tests to see......
One thing you can do to verify the heater valve is 100% closed is to measure the temp out of the front AC as compared to the rear AC.....they should be close....but when the heater valve is open (even slightly) the rear ac will be MUCH colder!!
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04-20-2009, 12:51 AM
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Dwayne--
We generally look for vent temps to be no lower than about 40-45º lower that the temo of the air going in. With the fan at lowest (zero) speed, that would provide the most cooling since the inlet air flow is so low, and the amount of residence time in the evap is pretty high. Moving the fan speed higher means more cooling needed so obviously the temps were getting up to more realistic temps in the 40-45º below ambinet range with the fan set on speed 2.
That brings us to the next thing, and that's compressor speed. As Dan points out, a 'standard' test condition includes running the engine at 1500-2000 RPMs with the interior fan speed set on 2. Thtat would give you something close to driving conditions on the system, even though you are depending on the fans to take care of condenser loads. Sticking with a standard condition means you can better judge the system performance.
As others also mention, the freeze switch does not seem to be working. If it was working, those sub-36º readings would be impossible. On the other hand, if you use the car here in Cali, with the very low humidity, keeping the freeze switch closed means that you'll get much faster cool-down. If you start driving longer to the point where the condensate starts icing in the evaporator, you can replace the switch. Or just plan on giving the system a little break every once in a while by turning the compressor off. You'll know when it needs that-- airflow will start to fall off and things will get warmer. Don't worry about straining the system running with the evaporator too cold-- It's actually less strain on the system when it's suction-starved like that.
Good luck!!
We generally look for vent temps to be no lower than about 40-45º lower that the temo of the air going in. With the fan at lowest (zero) speed, that would provide the most cooling since the inlet air flow is so low, and the amount of residence time in the evap is pretty high. Moving the fan speed higher means more cooling needed so obviously the temps were getting up to more realistic temps in the 40-45º below ambinet range with the fan set on speed 2.
That brings us to the next thing, and that's compressor speed. As Dan points out, a 'standard' test condition includes running the engine at 1500-2000 RPMs with the interior fan speed set on 2. Thtat would give you something close to driving conditions on the system, even though you are depending on the fans to take care of condenser loads. Sticking with a standard condition means you can better judge the system performance.
As others also mention, the freeze switch does not seem to be working. If it was working, those sub-36º readings would be impossible. On the other hand, if you use the car here in Cali, with the very low humidity, keeping the freeze switch closed means that you'll get much faster cool-down. If you start driving longer to the point where the condensate starts icing in the evaporator, you can replace the switch. Or just plan on giving the system a little break every once in a while by turning the compressor off. You'll know when it needs that-- airflow will start to fall off and things will get warmer. Don't worry about straining the system running with the evaporator too cold-- It's actually less strain on the system when it's suction-starved like that.
Good luck!!
This sounds about like what I'm getting out of my system (about 40 degrees below ambient on fan speed "2"). I was getting about 40 degrees out the center vent while input air was about 80. However, as you and Dan have mentioned, I need to run the test with rpms at about 2000. I'd like to try this test again with the higher rmps and fan speed on "2" and see what happens.
I agree about the freeze switch - seems to be NOT working. We do have long warm summers here in the High Desert and the humidity is very low (about 20%). I have not experienced a freeze up yet, but now I'll know what to suspect if the A/C starts getting warmer for no reason. Sounds like having a non-functioning freeze switch here in this climate may actually be a good thing! Newbie luck stricks again!! I think I'll leave it non-functioning for now.
THANKS for the info - very helpful!
04-20-2009, 01:03 AM
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Dwayne
typically I have it on a medium fan setting....usually 2 sometimes 1!!!! Since anything more is just TOO COLD.... it will occasionally hit the freeze switch and shut down for a couple minutes....(which sucks since it gets hot quick)...
...... One thing you can do to verify the heater valve is 100% closed is to measure the temp out of the front AC as compared to the rear AC.....they should be close....but when the heater valve is open (even slightly) the rear ac will be MUCH colder!!
typically I have it on a medium fan setting....usually 2 sometimes 1!!!! Since anything more is just TOO COLD.... it will occasionally hit the freeze switch and shut down for a couple minutes....(which sucks since it gets hot quick)...
...... One thing you can do to verify the heater valve is 100% closed is to measure the temp out of the front AC as compared to the rear AC.....they should be close....but when the heater valve is open (even slightly) the rear ac will be MUCH colder!!
Good test tip on the heater valve using the rear A/C - that makes sense. I should be able to run that test on Idaho. Unfortunately, California doesn't have a rear A/C.
So I'll probably have to remove the valve to check for leaks.According to the info from Dr. Bob and the manual, I'm getting very close to factory spec on the temps out the center vent on speed "2" but I need to re-run the test at 2000 rpms. I'll try it again when I can time the test to coincide with 80 degree ambient again.
THANKS for the tip, great info!
04-20-2009, 02:47 AM
#11
Race Director
Dr Bob is right about the 2000rpm..... I forgot about that.....I remember doing some testing at Bills on Kurts 86 and it got noticeably cooler at 2000rpm compared to idle...I want to say 15+ degree difference....
I do get my coolest readings on the freeway, so rpms are above 2000 and airflow is good.....
I do get my coolest readings on the freeway, so rpms are above 2000 and airflow is good.....
04-20-2009, 11:59 AM
#12
Chronic Tool Dropper
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My 'solution' to freeze switch operation in always-sunny SoCal is to leave it jumpered for normal local driving, and use the temp slider to set the target temps in the car. I always wondered what the numbers on that slider actually meant... Anyway, using the actual factory-intended mode of control, the car needs no assistance in avoiding the finger-freezing problems I have when the slider is full-left. That's usually enough to avoid evap freezing under most of our dry-climate circumstances. Note that this is not a solution for folks who have humidity or who use the AC while it's raining. If we had rain here, my evap would be an ice block in a fraction of an hour and there would be no airflow. Good news for the car is that it seldome rains, and I don't drive it in the rain anyway. For longer trips I usually pull the freeze switch jumper so I don't have to worry about the freeze-up with longer use of the compressor.
