A/C air temps

[Kirk W]

I have had the same discussion many times, with measurements on mine up to 42 degrees different from intake & output (80 vs 38 in one case).

 

First, I don't see anywhere what make & model coach you have. I think that can make a difference. My Winnebago has a basement mounted dual compressor package unit. I have documented several times that with both compressors running, the temp differential will always be much more than 40 degrees. The naysayers tell me that is not possible, but it is the case. My argument is that they are not accounting for the second compressors, but so far I have found no one who believes that can make the difference.

My question would be, have you measured air temperatures on either side of one cooling coil? If you have two coils in the air path, one after the other and each one has a temperature drop of 22°, then measuring at the inlet of the system and again at the outlet of the system would appear to give you a 44° temperature differential, yet still fall withing the stated range for each of your cooling coils. The measurements that I quoted from Dometic and those given by Jimalberta are both for the differential across the cooling coils of one air conditioner compressor. What you are stating for a dual compressor unit still falls well within those limits unless you go inside of the unit and measure each coil individually.

[AFchap]

My question would be, have you measured air temperatures on either side of one cooling coil? If you have two coils in the air path, one after the other and each one has a temperature drop of 22°, then measuring at the inlet of the system and again at the outlet of the system would appear to give you a 44° temperature differential, yet still fall withing the stated range for each of your cooling coils. The measurements that I quoted from Dometic and those given by Jimalberta are both for the differential across the cooling coils of one air conditioner compressor. What you are stating for a dual compressor unit still falls well within those limits unless you go inside of the unit and measure each coil individually.

I don't believe there is an individual coil for each compressor. As I recall, each compressor has it's own path through the same cooling coil. Getting to either the system inlet or system outlet when the unit is installed is not possible. The best you could do would be to measure inlet air temp where it enters the duct going beneath the closet or bed (depending on floor plan), and to measure the outlet temp at the ceiling duct nearest the outlet, normally the closest one to the passenger side rear of the coach.

 

As I have my unit out right now repairing the outlet duct, I could try to test the inlet an outlet air temp on the unit itself before it goes back in. I will report back, but it will be 2-3 days before I can do that. With daytime temp yesterday reaching 97 with heat index of 111, it may be interesting.

[mscans]

Never measured the temps on my RV BUT. My home unit that was replaced 2 years ago works as follows:

 

Using a digital meat thermometer calibrated in ice water (32F.) and then boiling water (212F.) I read the input air at 90F. The system was only running for a few minutes when this was done so the house was still hot. The outlet air, measured in the duct just after the evaporator coil measured 46F. This did not surprise me as the system was new and the refrigerant was being throttled by a control valve to keep the evaporator from freezing. If the coil is maintained just above freezing then the air going through it will be very close to freezing, 46F. in this case. I could have lowered it even more by reducing the fan speed (airflow) which gives more contact time for the air.

 

I have heard the 20F. delta story also but see no sense with arguing with the facts or the "experts" who heard from "someone" that 20F. or so was gospel.. As Kirk and others have said, the devil is in the details and EXACTLY how and where the measurements are taken will affect the results.

 

If you have measured your system and are happy with the results and they pass YOUR common sense test then just smile at the naysayers and look for another windmill.

 

AF- the systems that I have seen with dual compressors only have one evaporator assembly but the two circuits run through the same assembly so there are 2 inputs and 2 outputs with NO mixing of refrigerants.

 

It is 93F. here right now with interior temperature at 78F. That is a delta of 15F. BUT the system is probably working on a 25% duty cycle, nice and cool and dry!

[AFchap]

I got to getting the unit back in before I expected to. Got the output elbow repaired, and got it reattached to the duct going upward behind the rear cap. Before sliding the unit back into the basement compartment I attempted to measure input and output air temps with both a meat thermometer and an infrared gun. I was very hot and sweaty from working outside for about 4 hours ...temp about the time I got through was 103 with heat index of 115.9. Fortunately I was working in the shade, but was still pretty wrung out.

 

Measuring directly at the unit, I was not terribly patient but held the meat thermometer in the airflow for apx 5 minutes. Intake air at the unit measured 100. Output air measured 78 with the meat thermometer and 69 with the infrared. I felt I was not getting good measurements due to the difficulty of isolating output air with the very warm outside air. Exhaust air coming out the bottom of the unit was quite uncomfortable on my sandal-clad feet and the sheet metal that end of the unit measures 120+.

 

After installing the unit I went inside and did the same measurements at the return air vent under the bed, and at a ceiling duct about the middle of the coach. Intake air was 88 degrees with both instruments. Output air after 5 minutes exposure measured 68 with the meat thermometer and 43 with the infrared. So I got a 20 degree differential with the meat thermometer, and a 45 degree differential with the infrared aimed at the Styrofoam inside of the air duct. Can anyone explain how the Styrofoam can be cooled to a temp 25 degrees colder than the air supposed is? Metal maybe, but Styrofoam??? I understand that the infrared measures a surface temp, but how does the passing air cool the surface temp to a level far below it's temp?

 

After I got back inside the stick house I checked the window unit that was running in the sun porch/DWs library. With a meat thermometer, input air was 75 and output air was 46 ...a 29 degree differential. With infrared I measured inside the output at 18 and the plastic vent ribs at 46 ...the ribs consistent with what the meat thermometer read.

 

How far does an infrared beam travel if there is no physical object in it's path?? Pointing the infrared gun horizontal outside I get 100, pretty much the ambient temp. But pointing it straight up with no visible obstructions I get 52. I don't know where/what it is measuring, but I think I want to go there!

[Kirk W]

Thanks for the efforts, AF Chap! Not sure what it proves but it sure was interesting info. Sounds like you picked the wrong day for that job! We only saw a high of 78 here today.

[mscans]

Infra red systems are affected by the emissivity (E) of the surface. Black vs. silver, reflective or absorptive etc. Odd surfaces such as highly reflective or insulating will read incorrectly. Some of the better infrared systems come with charts of these materials so that you can correct for the nature of the material. See this table for some idea of the problem http://www.omega.com/temperature/Z/pdf/z088-089.pdf Many of the standard "fixed emissivity" meters are preset for about .95-.98 which works for MOST materials.

 

Using an infrared reader on insulating material is an invitation to bad readings.

 

AF, pointing straight up will attempt to read deep space (2.3K.) buffered by high altitude clouds etc., god only knows what it is looking at.

 

using a standard thermometer designed to measure "stuff" is the real way to go.

 

Have fun.

[hemsteadc]

 

 

 

 

 

 

 

 

 

Do we still have a problem?

[Jimalberta]

Ok...need to clear something up here. The 20 degree rule of thumb is just that....a general rule of thumb that we hvac people use as a quick test to check performance of an air conditioner. THERE IS NO RULE STATING THAT THE AIR CANT BE COLDER. we consider a 20 degree drop in air temperature as a good indication that the air conditioner is performing satisfactorily. If it is colder so much the better. Many conditions will affect this test.

Just as Mike indicated if the return air is quite warm and the unit is pumping its little heart out the air can drop much colder than the 20 degrees we look for. However this is not the only test that we do. Most of our testing is done with our guages as well as amperage measurements and so on.

Now if you want cold disharge temperatures we can start talking about the output of large refrigeration sytems for walk in freezers.

 

Cheers. Jim

[Kirk W]

Ok...need to clear something up here. The 20 degree rule of thumb is just that....a general rule of thumb that we hvac people use as a quick test to check performance of an air conditioner. THERE IS NO RULE STATING THAT THE AIR CANT BE COLDER. we consider a 20 degree drop in air temperature as a good indication that the air conditioner is performing satisfactorily.

Jim, it is your call, but the point of this doesn't seem to be any valid service information. It seems to me that this is not a debate or discussion, but an argument.

[mscans]

Not to keep beating the dead horse but if you have a 20 degree delta it will take a L O N G time to cool the enclosure. With 90 F. input air to the system that would give you 70 F. output air. More importantly the dew point of the air would be very high and the enclosure would be uncomfortable.

 

The images that oldman shows are more in line with virtually all automotive, RV and home units that I have seen. Notice the high tech gauges being used, they work just fine.

[acampingwewillgo]

OK....I like this discussion! I'm been called a bold face liar when I tell the tale of what the Temp difference was when relating this . Two Carrier 15,000 BTU air's(newer at the time), a well insulated 96 Vogue motorhome.....the outside temp was roughly 115 degree's in Pahrump, Nevada in the summer.....the inside when measured, was 58 degree's! No shade outside to speak of, awning's deployed shades in MH closed. I can't speak to how this happened but with several thermometer's in the MH, the Temp's are correct and it was a routine happening while in Pahrump. Same AC's today in Texas heat and humadity(105), I'm seeing 75 degree temps in MH.....I know these things because I keep a close eye on temp's, we have always traveled with dogs.

[hemsteadc]

Notice the high tech gauges being used, they work just fine.

he he. Nobody trusted the IR gun, so I had to blow $1.94 on those hi-tech units.

[Jimalberta]

Jim, it is your call, but the point of this doesn't seem to be any valid service information. It seems to me that this is not a debate or discussion, but an argument.

p

 

I agree Kirk....its now just a bunch of recycled ignorance.

[AFchap]

p

 

I agree Kirk....its now just a bunch of recycled ignorance.

Ignorance = lack of knowledge, education, or awareness.

 

Which individual is ignorant? ...the one who knows all the text book answers but rejects practical experience? ...or the one who has learned in the school of hard knocks what works but doesn't know all the scholarly theory and reasons behind it?

 

Relying on text book knowledge and education only, and rejecting awareness of verifiable facts has led many into "ignorant" actions. It takes a balance of them all to get the job done. For example, when looking for a mechanic to work on my vehicles I ALWAYS prefer one with hands on experience and a keen sense of awareness to one who only knows the text book answers.

[Kirk W]

IRelying on text book knowledge and education only, and rejecting awareness of verifiable facts has led many into "ignorant" actions. It takes a balance of them all to get the job done. For example, when looking for a mechanic to work on my vehicles I ALWAYS prefer one with hands on experience and a keen sense of awareness to one who only knows the text book answers.

So, how many years in a related career filed does one need in order to have that balance?

[AFchap]

So, how many years in a related career filed does one need in order to have that balance?

It depends less on the number of years than it does on the individual's thought process. Some get it, and some never will. Some see only what is in front of them, while others see, hear, and think of how the individual parts of the unit work together and how they can be affected by one another. After 20 years working, some have 20 years experience and some have 1 year's experience repeated 20 times. The ac guy who sees a 20 degree differential and then tells the customer, "There is nothing wrong with it" when the customer insists it isn't cooling as well as it has been has totally missed the boat. As Jim said,

 

The 20 degree rule of thumb is just that....a general rule of thumb ... Most of our testing is done with our guages as well as amperage measurements and so on

.

 

I had that argument with a car dealer service manager once on a relatively new car. After a quick check of the output air he told me that was as good as it could do. He asked where I lived (Arizona), and told me it was just hotter in Texas. He didn't want to work on it and told me I would have to pay for them to find nothing wrong. I told him to check it out anyway, and they did find a malfunctioning valve. I have had the same experience more than once with the basement air system in my RV, sometimes resulting in finding an output air leak and other times a malfunction in the unit. I know from my familiarity with the unit and how it has worked in the past that if all it is managing is a 20 degree differential, yes it is working, but it is not working as well as it should be ...time to check something other than just the output temp.

[hemsteadc]

I just wish a few more folks would have the innate curiosity to take a simple temperature measurement of their own. Would help me judge if mine's malfunctioning.

[mptjelgin]

It depends less on the number of years than it does on the individual's thought process. Some get it, and some never will.

 

I agree with this completely. I've worked as a registered professional engineer for 35 years. In that time I have worked with engineers who "got it" within the first year and demonstrated the ability to work through difficult problems effectively. I worked with engineers with the same degrees (in some cases a Masters or PhD) who never did demonstrate the ability to solve new problems. They could cookbook it if they had an identical example to reference, but they just weren't good problem solvers. I imagine that this range of aptitude exists in every profession, including HVAC.

[Ray,IN]

The most informative air conditioning website I have been able to locate is very detailed and explains everything so a layman can understand the article.

http://www.inspectapedia.com/aircond/Air_Conditioner_Temperatures.php

It states the same thing several other such websites state. When we see more temperature differential than 17-20*F. there is likely something restricting air flow over the expansion coil.

I now think that's why we who have basement air conditioners are seeing such a large differential, ie; the long run of ductwork involved is restricting air flow.

That, at least for me, is not a problem. I routinely see 46*F air measured at a ceiling ductwork register, BUT I have not waited the recommended 24 hrs. operating time before taking air temperature measurements.

I've learned a lot from this discussion, my thanks to oldman for beginning this thread.

[Jimalberta]

You make good points Ray......you can spend a long time in a career just getting by and not being very profficient at it. I have seen it over and over. Some people simply are not meant to be working in their field.

 

I have to respect the knowledge that is displayed on this forum. There are very knowledgable electricians, solar experts, RV repair experts. We are all tire experts...lol.

 

My years in the field were all hands on even when I owned my own business for 23 years. After I sold my business I worked for Carrier doing tech support on the phone as well as traveling the country training carrier service men and women . I do know what I'm talking about both from a theoretical perspective as well as hands on practical experience.

 

Getting back to the air conditioning field one thing that hasnt been mentioned is enthalpy. Enthalpy is the total heat present in the air. There is sensible heat which is dry heat and then there is latent heat which is the " extra" heat that is present in moist air. Air in a high humidity environment holds quite a bit more heat than dry air. That is why an air conditioner can put out better cooling in Arizona than Texas or Florida . This all adds to the mix when working on air conditioning. In the high humidity climated its necessary to measure both the welb bulb temperature of the air as well as the dry bulb temperature to determine the total heat present in the air. Then we have a picture of how well an air conditioner is performing. I mention all of this to reiterate that the 20 degree rule of thumb is at best a very crude test.

So for you guys arguing that you can get much colder air than the afforementiond 20 degrees...yes you can. I agree. In some conditions you can...in some conditions its hard for the air conditioner to even reach that rule of thumb.

[Kirk W]

 

It depends less on the number of years than it does on the individual's thought process. Some get it, and some never will.

While I can't disagree with the comment, the context in which it is dropped into the mix does make one wonder who the message is for. I suppose that even though I had a 40 year career in doing service and repair work on elector-mechanical equipment, and more than 30 years of things RV related for myself and others around me, it really isn't my place to judge if I'm qualified or not. I'll only say that I'm happy to help those who want my assistance, but am not offended when others choose to ignore or disagree.

[AFchap]

the context in which it is dropped into the mix does make one wonder who the message is for.

The context was as a direct response to a specific comment (that was not made by you) declaring the discussion of common experiences by several non-tech types as "just a bunch of recycled ignorance." When the text book answer does not fit what I can personally observe, I want to know why. That may in fact mean I AM ignorant, but the only way to overcome ignorance is to study the situation and to figure it out. We finally are achieving that here to some extent as Ray's info above suggests a very plausible reason for what many of us see in temp differential ...restricted air flow could very well be a factor. And looking at all the factors is all the more difficult as measuring the intake and output temps directly at the unit is simply not normally possible with a basement mounted unit.

 

Over and over again on this and other RV forums (and when I take a vehicle in for ac service) I have seen the 20 degree rule stated as law, and anyone who questions that related to their own experience is ridiculed/called ignorant/etc, etc. I accept the 20 degree differential as a general principle in theory, but at the same time I know from MY experience that if my RV house OR dash air is only managing a 20 degree difference from intake to output, then something is wrong. The service manager who tells me I don't know what I am talking about loses a customer and a service fee. The forum what declares the discussion of the conflict between theory and common experience to be "just a bunch of recycled ignorance" looses the opportunity to teach me and others like me something (and perhaps to learn something at the same time). The only one who is truly ignorant is the one who refuses to consider ALL the available info ...the one who outright rejects any info that does not agree with what he/she already "knows." The rest of us, at various levels of age and experience, are just not-quite-totally-wise-yet learners.

[hemsteadc]

I'll high-five that AFChap!

[Jimalberta]

Paul that was a tongue in cheek comment.....dont get your panties in a twist about it.

[AFchap]

Paul that was a tongue in cheek comment.....dont get your panties in a twist about it.

Perhaps so... My panties are just fine, but it did have a definite negative impact on the discussion as I received several PMs about it after that.