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A/C air temps


hemsteadc

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I'm having a 'spirited' discussion on that other board regarding air conditioners and how they perform as to temperatures. I'm on one side, everyone else is on the other saying this: The temperature differential from incoming a/c air to outgoing is 15-20 degrees. That does not make sense to me, so I took my own measurements.


Here are the tests I did: I let the coach warm up to around 90, then turned on the air. Within a few minutes the air coming from the ducts was very cold, and the IR gun measurement of the plastic slats in the air vent is roughly 48F. The coach air temperature is just starting to drop at about 89F. After 11 minutes I made virtually the same measurements, with the coach cooling slightly. Those temperatures are well beyond the 20 degrees these other folks are insisting it's designed to do.


So what are the arguments?


1. My intake air must be much cooler than 90. That means it must be sucking in already cooled air from a nearby vent. Ok, I made sure there were no vents pointing at the air intake. I put my hand on the intake, it's warm. And I took an IR reading on the plastic grating, which measured very close to the temperature shown on the thermostat.

2. IR guns can't measure air temperature. No, but they can measure temperature of solid objects sitting in the path of moving air.

3. IR guns are not that accurate. Perhaps, but not 20 degrees inaccurate!

4. I am a pathetic individual who thinks I'm correct and everyone else is wrong. I've challenged them to take their own measurements, but so far no takers.


I've read before about how RV air conditioners are rated to cool the coach down to about 20 degrees below ambient, and that's how mine works. But, that does NOT mean the actual air temperature exiting the ducts is at 20 degrees below incoming air. If that were the case, it would take forever to cool down the coach.
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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.

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First, I don't see anywhere what make & model coach you have.

 

34' triple slide 5er, Okanagan, 2002. Roof mounted ducted air. Finally! Someone with the same experience!

 

 

Are you all talking about the same thing? ~20 deg. F would be what the average crappy A/C in an RV can cool the interior below outside ambient temperature.

I don't know if we are talking about the same thing. Nobody's really willing to talk, they're too busy telling me I'm crazy.

 

All you (not you specifically) have to do is hold your hand up to an a/c duct and feel how cold that is. It seems pretty simple to me.

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I have used an actual thermometer stuck in the intake and another in a vent at the end of the duct work. With 1 AC working, I have seen 24-26* temp difference. If I turn on the front AC with the main then I see 32-34* temp difference. When we had both Carrier AC's the temp difference was greater, this Dometic isn't as strong. 1 issues people need to account for is how well sealed is the duct work. If folks are cooling the attic area or basement, then their temp difference will be less.

That's the issue over on that other forum you mention. Everyone assumes that your an idiot with the bottom model from Forest River...

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.. other forum you mention. Everyone assumes that your an idiot with the bottom model from Forest River...

It's very possible that there are *some* a/cs in *some* coaches that operate like they say. But when it's said that "they all work that way", that's where I take issue.

 

I posted here because I think folks here use their brains, whereas the 'other forum' is a lot of emotion.

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I don't see where you used the IR on the incoming air to the AC.

If you are getting 48º out of the roof AC then you have a exceptional AC.

As 18-20º difference from intake to output is the normal of most.

 

Just checked mine with a IR and got the below.

Rear bedroom 13.5K Coleman PS

Intake 75º

Out 58º

Room Thermostat 74º

 

 

Front area 15K Coleman PS Heat Pump it was just turned on for the test a few minutes ago.

Intake 80º

Out 66º

Room Thermostat 84º

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I don't see where you used the IR on the incoming air to the AC. If you are getting 48º out of the roof AC then you have a exceptional AC.

#1.

 

Yeah, I think so too. Someone else said it was an "incorrect installation." We should all have an incorrect installation!!

 

My ducted air doesn't reach those temps right away, it takes a few minutes to cool down.

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It also depends on the humidity, how clean the coils are, if you're operating on high fan or low, how tight the duct work is, etc. I foil taped the divider and duct openings at the AC's. When I was doing all that I ran a mini camera snake into the duct work about 10' in either direction. I was surprised that there were no noticeable joints or openings.

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Could it be that the 15-20 measurements are made AFTER the A/C cycles/ after the thermostat clicks the compressor off? so it gets as cold as it can a certain temp. setting, and once it has cycled it maintains a 20 deg. differential? (it makes sense to me that a hot coach with the a/c just starting to cool would have the greatest differential @ the start, and would narrow as the coach cools).

The vents have to cool the coach air in order to lower intake temp, but the output is cooler at the vent cuz it passed thru the system and has not had a chance to mix with the coach air. so intake lags behind until coach has cooled, so i would check after cycling.

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Could it be that the 15-20 measurements are made AFTER the A/C cycles/ after the thermostat clicks the compressor off? so it gets as cold as it can a certain temp. setting, and once it has cycled it maintains a 20 deg. differential? (it makes sense to me that a hot coach with the a/c just starting to cool would have the greatest differential @ the start, and would narrow as the coach cools).

It's always been my understanding that the 20 degree thing means that the average RV a/c has the ability to cool the unit about 20 below ambient. That does not specifically mean the air temps on the a/c itself. That's a conclusion I cannot reach, nor does my data bear that out. Further, it makes no sense that if the coach is at 90, and the chilled air is at 70, the coach is going to have much chance of getting cool today.

 

And yes, the temperature delta would narrow, but that isn't because the chilled air temperature rises. That said, I have not tested that myself.

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First, if you use an IR gun to measure you will be measuring the temperature of what you point at and not the air temperature. The service manual from Dometic states that the proper way to measure is to use a quality dial thermometer, placing it first in the inlet are to measure that temperature of the air into the cooling coils. Once the dial has stabilized for a minute or two, then move it to the outlet side of the cooling coils to measure the differential. Temperatures should be taken as near the entry and exit of those coils as possible. They state that the differential temperature should be no less than 18° and that it should be no greater than 26 - 28°. The reason for this is that if that differential goes too high it will become probable that the condensation will begin to freeze and inhibit cooling.

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As a refrigeration mechanic for 47 years let me add my 2 cents.

 

The general rule of thumb we techs look for is 20 degrees of cooling across the evaporator. That means an actual air temperature reading of the return air versus the air leaving the evap coil. That is based on a proper air flow which I would consider high fan on an RV. By putting the fan on low you can drop the evap temperature somewhat and get it colder but you run the risk of freezing the evap. Low fan is designed and used for dehumidifying in humid climates and usually there is an evap temp switch that shuts the compressor off if the evap temp drops down to the freezing mark.

 

I would not consider a temp gun accurate enough for a proper test as it does not measure the actual air temp. I use stick thermometers designed for this test.

 

Also the general design parameters of an AC is to lower the air temp in the living space to 12 degrees below the ambient air temp.

However most people like their space to be similar to a meat cooler.

 

Another rule of thumb is 20 degrees of heat rejection on the condensor , so we look for the air coming off of the condensor to be 20 degrees hotter than the ambient air coming into the condensor.

 

As well as the above...what also adds to the mix is the total amount of heat that is being removed from the living space. Humid air has a lot more heat in it than dry air. So if you are sitting in the hot dry desert of Arizona your air conditioner is going to feel a lot colder than if you were sitting on south Florida on a hot humid day.

 

Cheers

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First, if you use an IR gun to measure you will be measuring the temperature of what you point at and not the air temperature. The service manual from Dometic states that the proper way to measure is to use a quality dial thermometer, placing it first in the inlet are to measure that temperature of the air into the cooling coils.

So you're saying that the temperature of the plastic fins in the outlet air, which feel as cold to the touch as the air passing over them, is not accurate? Just how inaccurate would it be?

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The general rule of thumb we techs look for is 20 degrees of cooling across the evaporator. That means an actual air temperature reading of the return air versus the air leaving the evap coil.

So, in a trailer whose interior temperature is 90F, I should be seeing chilled air of 70F?

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So you're saying that the temperature of the plastic fins in the outlet air, which feel as cold to the touch as the air passing over them, is not accurate? Just how inaccurate would it be?

As far as I am aware, no research on that sort of thing has been done, and to what purpose would anyone do it? I doubt that it would be consistent, but why would we care? The issue is air cooling ability so would it not make more sense to measure air temperature? But that is only one of many factors in just how much you can cool the interior of your RV. Just as important to that would be amount and quality of insulation, the amount of outside air penetration, the surface area of the windows and their RV value, and a very long list of other factors. The only thing that the air conditioner can control is the cooling of the air which passes through it or ΔΤ.

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I doubt that it would be consistent, but why would we care?

Right now, I care. I would expect a thin piece of plastic sitting in a stream of cooled air to assume the temperature of said air, and I think it would be valuable information to be able to use that as a 'reasonably accurate' indication of the air temperature when used with a laser thermometer.

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It would seem that you now have two forums who do not agree with your theory. :)

 

As I read your measurements, there are several problems with what you state. You mention the cooling below ambient, but does that mean the temperature in the room you are cooling, because if so that is the first problem with what you are stating. The only accurate way to measure the starting temperature for the cooling capacity is to do so at the point where air enters the intake, usually done at the grill of the intake side of the unit. It also seems that you are measuring the air out at the ceiling vents, rather than at the discharge from the air conditioner cooling coils. By measuring in the wrong locations, you are introducing many factors that are completely beyond the control of the a/c cooling system, and so it is anyone's guess what sort of information you might find. The only way to know what your air conditioner is really doing is to isolate the measurements to as narrow an influence as possible. That means that you must measure in the proper locations and use the proper measuring device.

 

What you are using is a device intended to measure surface temperatures to determine air temperatures out. As far as I can tell you have not said how or if you measured the air into the cooling coils? There simply is no way to respond with any degree of accuracy to what you are asking, unless we first agree with how and where to measure for our data. While I happen to have a laboratory grade dial thermometer which typically cost a bit less then $20, you can find reasonably accurate ones for this type of measurement from Amazon or some stores for around$5 or so. Unless we can agree to use some consistent form & location for measurements, there is no way to discuss this meaningfully.

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Hello oldman first we need to know where the measurements were taken by these claimers of the 20 deg. diff., If in back of and in front of the coils you need to get into your a/c unit and test there.

I read a Lowes a/c article online. many factors come into play one example, that a temp diff. greater than 20 degs. could also be a problem in the ducting like an air flow restriction that causes the air temp to drop in the ducting, yes the article was about a home a/c unit but still the same principle.

so in order to get a reading we must start the same place as the temp readers.

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As a refrigeration mechanic for 47 years let me add my 2 cents.

 

The general rule of thumb we techs look for is 20 degrees of cooling across the evaporator. That means an actual air temperature reading of the return air versus the air leaving the evap coil. That is based on a proper air flow which I would consider high fan on an RV. By putting the fan on low you can drop the evap temperature somewhat and get it colder but you run the risk of freezing the evap. Low fan is designed and used for dehumidifying in humid climates and usually there is an evap temp switch that shuts the compressor off if the evap temp drops down to the freezing mark.

 

I would not consider a temp gun accurate enough for a proper test as it does not measure the actual air temp. I use stick thermometers designed for this test.

 

Also the general design parameters of an AC is to lower the air temp in the living space to 12 degrees below the ambient air temp.

However most people like their space to be similar to a meat cooler.

 

Another rule of thumb is 20 degrees of heat rejection on the condensor , so we look for the air coming off of the condensor to be 20 degrees hotter than the ambient air coming into the condensor.

 

As well as the above...what also adds to the mix is the total amount of heat that is being removed from the living space. Humid air has a lot more heat in it than dry air. So if you are sitting in the hot dry desert of Arizona your air conditioner is going to feel a lot colder than if you were sitting on south Florida on a hot humid day.

 

Cheers

We don't need no stinking facts ...LOL..

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I'll call in a Forensics team.

Is there some reason why you will NOT take stick thermometer, a standard meat cooking thermometer works fine, and test the way the air conditioning industry checks the operation of air conditioner? After all you have heard from an experienced A/C tech about the way they test. Also every home a/c inspection I have ever had, uses the stick thermometer.

 

Or do you just want to argue about whether or not the IR gun is the right way to test?

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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.

I have exactly the same basement unit and have documented (with a digital thermometer) the same results.

I, like oldman, have been derided for stating these measurements. Our last 5er with roof units performed much the same.

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