Air Conditioner Test

CavemanCharlie wrote:
I had to work on my car Air Conditioner today and did some more tests. I realize these test prove nothing and that they have nothing to do with RV's....

You still don't understand the principle since in this post you never once measured the temperature of the incoming air. The outgoing temperature continued to drop within the capabilities of the system because the incoming air temperature was dropping.

R12 and R134a are different than R22 and R410a- they will get larger split temps, with other tradeoffs. The lower temps are why refrigerators and freezers use R12/R134a.

Science.

Delta T of 15 to 20 degrees isn't an old wives tale, it's a well known design parameter of most residential type AC units used in Homes, RVs and Autos. It's not an indication of what the system is capable of but instead an airflow to Temperature relationships that results in maximum performance in the conditions the unit is designed to operate in. An HVAC mechanic will have access to technical data detailing the optimal temp changes to expect from the particular system being looked at, and take troubleshooting from there. Changes of more than 20 degrees in most systems and you start looking into the possibility of the unit being low on refrigerant, or clogged filters or other issues that might be reducing airflow. Temp changes that are to low could again be again low refrigerant, either through causing icing, or low enough to reduce cooling etc. etc.

As others have mentioned, your AC generally continually recycles the air within the home/rv and auto depending on setting. How effective it is at cooling that airspace is dependent on both temperature and volume of air processed. Designing the system to be the most efficient at doing that generally results in a system with an intake to output temperature differential across the cooling heat exchanger of approximately 20 degrees.

I had to work on my car Air Conditioner today and did some more tests. I realize these test prove nothing and that they have nothing to do with RV's.

My car is a 1984 Lincoln Mark VII. It originally had R-12 as a refrigerant but, you can no longer get that. Some years ago it was converted over to using R-134 for a refrigerant. The conversion was not done in the best possible way. The mechanic simply evacuated the system of refrigerant and oil, installed a new line dryer in the suction line and , refilled the system with R-134.

He actually put more ounces of R-134 in then what the system called for using R-12. A few months later while driving in stop and go traffic the high side pressure got to high, a check valve opened and some refrigerant bled off. The system worked better after that.

I have a slow leak in the system and every 5-7 years I have to add some more R-134. I did that today. According to the cheap gauge and refill system I bought at the local discount store the low side pressure in this system should be between 25 and 45 PSI. From experience I have found that the system works best when I stay to the low side of that range so I added refrigerant until the pressure read 25 PSI. I then shut the car off and let everything warm back up for about 3 hours to outside temps.

It was 90 degrees f when I started the test at 5:00 PM. I put a temperature probe in duct on the dash. At 5:02 the temp of air coming out of the duct had fallen to 62 degrees. At 5:03 it was 55. At 5:04 it was 48. At 5:05 it was 45. At 5:06 it was 42. At 5:07 it was 37 degrees. But,,,, as someone is sure to point out I had the car running on recirculation mode and it was cooling off inside the car. At 5:08 the temp of the air coming out of the duct was 35 degrees. I let it run for another 10 minutes and it never got any colder then that.

I also have for my primary vehicle a 2004 Chevy 1500 pickup. I did a test on it too. It also uses R-134 for a refrigerant. For some reason when I checked the low side pressure on the truck it read 50 PSI. That is higher then I thought it should be but, this system has never been touched and that is how it was designed by the factory.

It was 90 degrees when I started the test at 1:46. I also had the system on recirculate. At 1:47 the temp at the duct had fallen to 70 degrees. At 1:49 it was 66. At 1:51 it was 66. At 2:00 it was 65. I kept a eye on the temp coming out of the duct for the next 10 minutes and it never fell below 65.

Once again this proves, or means, nothing. These are just some observations I made today.

Have a nice Day

CavemanCharlie wrote:

beemerphile1 wrote:

dwayneb236 wrote:
Was just in Oklahoma where the top temp for the days was 99 to 100 I believe. The highest the camper got in the middle of the day was around 78 with both ac's running. Pretty close to 20 degree difference we expected....

Many people seem to misunderstand the 15 - 20 degrees. It is intake air and exhaust of the unit, outside temperature is irrelevant.

If the intake air is 95 the air coming out should be 75 to 80 degrees. As the room cools the intake air obviously is cooler, so when the intake air is 75 degrees, the output should be 55 to 60 degrees.

What the OP doesn't realize is that he has a problem with his unit resulting in the very low temperature output. What is happening is that due to defective separation of the intake and output, it is cooling the same air over and over again. The baffle between the intake and output needs fixed.

Ha Ha, If I have a problem with my unit I'm happy for the problem. It works well. See my second post.

I'm glad you are happy but the premise of this thread is that your A/C unit disproves the rule, it does not. In fact, your test proves that your unit is operating inefficiently due to inadequate separation.

fpresto wrote:
Old-Biscuit is correct. You totally misunderstood the rule of thumb.

Does that mean he's........all thumbs..:@

~

beemerphile1 wrote:

dwayneb236 wrote:
Was just in Oklahoma where the top temp for the days was 99 to 100 I believe. The highest the camper got in the middle of the day was around 78 with both ac's running. Pretty close to 20 degree difference we expected....

Many people seem to misunderstand the 15 - 20 degrees. It is intake air and exhaust of the unit, outside temperature is irrelevant.

If the intake air is 95 the air coming out should be 75 to 80 degrees. As the room cools the intake air obviously is cooler, so when the intake air is 75 degrees, the output should be 55 to 60 degrees.

What the OP doesn't realize is that he has a problem with his unit resulting in the very low temperature output. What is happening is that due to defective separation of the intake and output, it is cooling the same air over and over again. The baffle between the intake and output needs fixed.

Ha Ha, If I have a problem with my unit I'm happy for the problem. It works well. See my second post.

Rhigley wrote:
I just bought a 31 foot class C motorhome and it has a 13500 air (ducted).It dose not seem to cool down my RV very much.

Before you go spending the money (a lot) to replace the A/C in a brand new unit, I suggest that you do a few more test runs in some varying conditions to be sure that it isn't working as designed.

It would not be surprising for it to take up to two hours to make a significant dent in the temp if the inside is heat soaked at 95 F or more. In those conditions, having an additional 1,500 BTUs probably won't help much either.

dwayneb236 wrote:
Was just in Oklahoma where the top temp for the days was 99 to 100 I believe. The highest the camper got in the middle of the day was around 78 with both ac's running. Pretty close to 20 degree difference we expected....

Many people seem to misunderstand the 15 - 20 degrees. It is intake air and exhaust of the unit, outside temperature is irrelevant.

If the intake air is 95 the air coming out should be 75 to 80 degrees. As the room cools the intake air obviously is cooler, so when the intake air is 75 degrees, the output should be 55 to 60 degrees.

What the OP doesn't realize is that he has a problem with his unit resulting in the very low temperature output. What is happening is that due to defective separation of the intake and output, it is cooling the same air over and over again. The baffle between the intake and output needs fixed.

Was just in Oklahoma where the top temp for the days was 99 to 100 I believe. The highest the camper got in the middle of the day was around 78 with both ac's running. Pretty close to 20 degree difference we expected. We did use a fan on the floor to move the air around a bit. Was comfortable the whole time.

41ft fiver. Front bedroom got down to around middle 60's throughout the day if I closed the door. I loved it. Wife hated it.