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Old 06-12-2015, 02:25 AM   #1
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AC temperature

I have always heard that the AC can only cool to 15 to 22 degrees cooler than the outside ambient temperature. That being said, while in Kansas City yesterday, the weather was very hot. Here is the weather shot from there.



I could not get the temperature inside below 85 degrees. Is this to be expected and accepted? It was almost unbearable to drive while this hot.
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Old 06-12-2015, 02:53 AM   #2
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Not sure what the exact formula is but HVAC guys always told me that proper cooling is measured between the air temp entering the cold air return compared to the temp exiting a register.

I was told last year that when it is 105 outside do not expect it to get cooler than 80 inside. And that is in a house with a 2 zone heat pump system R30 insulation and dual pane, gas filled windows.
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Old 06-12-2015, 04:14 AM   #3
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Originally Posted by unw1red View Post
I have always heard that the AC can only cool to 15 to 22 degrees cooler than the outside ambient temperature. That being said, while in Kansas City yesterday, the weather was very hot. Here is the weather shot from there.

....cut.....

I could not get the temperature inside below 85 degrees. Is this to be expected and accepted? It was almost unbearable to drive while this hot.
I've never heard that limitation stated before, and in my experience doesn't make sense. Does it only apply to RVs?

How cold the inside temperature can be lowered relative to outside temperature depends on many factors, but mostly on cooling capacity relative to requirement.

In industry I've installed single-stage refrigeration (same as air conditioner) that kept product coolers at 30 to 35 F when outside temperature was around 100 degrees. Two-stage systems I've installed cooled freezers to -40 F with outside temperature in the same 100 F range.

Most RVs have an AC with a capacity of just over one ton of capacity. And that's not very much. I'm guessing that if you had two ACs in same RV there would be little problem making the inside temperature much cooler.
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Old 06-12-2015, 01:03 PM   #4
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unw1red:

Curious as to what size a/c you have in your coach? 15,000 BTU?
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Old 06-12-2015, 01:10 PM   #5
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15,000
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Old 06-12-2015, 01:16 PM   #6
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Originally Posted by Chance View Post
I've never heard that limitation stated before, and in my experience doesn't make sense. Does it only apply to RVs?

How cold the inside temperature can be lowered relative to outside temperature depends on many factors, but mostly on cooling capacity relative to requirement.

In industry I've installed single-stage refrigeration (same as air conditioner) that kept product coolers at 30 to 35 F when outside temperature was around 100 degrees. Two-stage systems I've installed cooled freezers to -40 F with outside temperature in the same 100 F range.

Most RVs have an AC with a capacity of just over one ton of capacity. And that's not very much. I'm guessing that if you had two ACs in same RV there would be little problem making the inside temperature much cooler.

This blurp from the link below:

Air conditioning systems have their limits. Each unit is designed for a given temperature drop in degrees. A typical system is designed to cool up to 20 degrees. The temperature differential (also known as Delta T) between the condensor's intake air and the evaporator's output air will be 20 degrees and no more. So, if you have 100 degree air entering your system the output temperature will never be any lower than 80 degrees. As the intake air cools down the output air temperature will also be lowered so the cooling will improve the longer it runs. They key is to start it early enough before the temperatures get too extreme.

RV Air Conditioning Service
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Old 06-12-2015, 01:27 PM   #7
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This blurp from the link below:

Air conditioning systems have their limits. Each unit is designed for a given temperature drop in degrees. A typical system is designed to cool up to 20 degrees. The temperature differential (also known as Delta T) between the condensor's intake air and the evaporator's output air will be 20 degrees and no more. So, if you have 100 degree air entering your system the output temperature will never be any lower than 80 degrees. As the intake air cools down the output air temperature will also be lowered so the cooling will improve the longer it runs. They key is to start it early enough before the temperatures get too extreme.

RV Air Conditioning Service
Thanks for sharing. I also find the impact of humidity on AC performance interesting, especially since I live in an area known for high humidity. In unwired's case, there appears to be a condition of high humidity (difference between temp and heat index (feels like). This would further reduce the cooling ability of the AC unit below the 20 degree maximum.
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Old 06-12-2015, 03:09 PM   #8
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Originally Posted by Tytlfamily View Post
This blurp from the link below:

Air conditioning systems have their limits. Each unit is designed for a given temperature drop in degrees. A typical system is designed to cool up to 20 degrees. The temperature differential (also known as Delta T) between the condensor's intake air and the evaporator's output air will be 20 degrees and no more. So, if you have 100 degree air entering your system the output temperature will never be any lower than 80 degrees. As the intake air cools down the output air temperature will also be lowered so the cooling will improve the longer it runs. They key is to start it early enough before the temperatures get too extreme.

RV Air Conditioning Service
I think the 20 F is being discussed out of context. It's not an inherent "maximum" at which point an air conditioning system no longer works. The article reads to me to say that a "typical" system is designed for cooling up to 20 F below ambient, but that's not limited by the equipment as much as by its sizing. Engineers have to size the equipment for some specific condition and it sounds like 20 F is typical.

What happens is that as the inside temperature drops, or to a lesser degree the outside temperature increases, the capacity of a given air conditioner is reduced. This effect limits how cold you can get the inside temperature very quickly. As the RV's temperature drops, the heat load increases and at the same time the AC's capacity is reduced. Basically what this means is that if you want to run 40 F below ambient instead of 20 F below ambient, it takes a lot more than doubling your "rated" AC capacity which is set under standard conditions. A 15,000 BTU/hr AC is not always capable of that rating because it changes with conditions.

If an RV has two identical ACs it will get cooler than if it only has one. The main limitation is capacity versus load. And for what it's worth, humidity just adds to load. It's just more BTU/hr that have to be pumped.


By the way, I did not realize the article you were quoting was mostly about the engine-driven AC system, not the roof-mounted AC I assumed you were interested in. Regardless, the principles are all the same with exception that engine-driven compressors spin at different speeds.
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Old 06-12-2015, 03:12 PM   #9
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Great explanation. Thank you.

I plan on getting a second 15,000 BTU unit added to my ACE 27.1 this fall.
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Old 06-12-2015, 03:26 PM   #10
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Page 32 out of the Dometic service manual explains it well......and when you take your AC for service, this is what they are going to tell you.


http://www.dometic.com/QBank/EPiServ...nual_17589.pdf
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Old 06-12-2015, 04:43 PM   #11
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AC Temperature Split

I recently (last fall) entered into an annual service agreement with an HVAC company for my residence and rental property systems.

After this thread got started I went to my file cabinet and pulled the agreements and supporting documents and included in the packet is a document labeled "Before requesting service"

In the section sub titled "System running" The first two items are:
1. Check for dirty filters
2. Measure the "temperature split" if possible

Item 2 goes on to explain how to measure the split and states that if the temperature split is 14 - 20 degrees no service is required. It also states that if the temperature split is greater than 20 degrees to look for obstructed or closed registers downstream from the air return. They also state that a temperature split less than 14 degrees is an indication of low refrigerant or obstructed or dirty evaporator coils or fins and they should be called for service.

They do provide a caution stating that "point and shoot thermometers" are not adequate for measuring the temperature spilt. They recommend a "clip on style probe style thermometer" with the probe in the registers for 3 - 5 minutes before recording the readings.

They also recommend things like turning the AC on early, making sure windows are closed and keeping doors between rooms open to maximize air flow.

It appears the concept of "temperature split or Delta T" is not just for automotive and RV AC units but for all types of AC systems.
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Old 06-12-2015, 07:04 PM   #12
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....cut....

"Before requesting service"

In the section sub titled "System running" The first two items are:
1. Check for dirty filters
2. Measure the "temperature split" if possible

Item 2 goes on to explain how to measure the split and states that if the temperature split is 14 - 20 degrees no service is required. It also states that if the temperature split is greater than 20 degrees to look for obstructed or closed registers downstream from the air return.

.....cut.....
What do they consider the temperature split? How do they suggest you measure it? Based on above wording it sounds like a different parameter than the inside versus outside temperature limit being discussed initially. If I misunderstood your comment I apologize. I'm just trying to make sure we are talking about the same thing.

Granted I'm reading between the lines, but if the split is above 20 F because of a blockage, it normally means the blockage is causing the air stream across the evaporator to move so slowly that the air can get too cool. This happens because the surface area of the evaporator remains the same, so when air moves slowly it has time to get colder. And although the air is getting colder, total system capacity goes down because of reduced air flow.

It may seem counterintuitive that air that is too cold coming out of an air conditioner is actually a sign of reduced capacity, but it can happen. The air may come out very cold but if there is too little of it the house will get hot.

In air conditioning the right air flow across the evaporator is also required to help control humidity at a comfortable level.
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Old 06-12-2015, 07:12 PM   #13
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Page 32 out of the Dometic service manual explains it well......and when you take your AC for service, this is what they are going to tell you.


http://www.dometic.com/QBank/EPiServ...nual_17589.pdf
Thanks for link. I was not aware that the capacity ratings were based on 80 F inside temperature. We normally like to set the thermostat in the low to mid 70s which means a significant capacity reduction.

"The information on the data tag is accurate at lab conditions only (50% relative humidity, 80 inside, 95 outside at unit and exactly 120 VAC). "
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Old 06-12-2015, 07:33 PM   #14
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What do they consider the temperature split? How do they suggest you measure it? Based on above wording it sounds like a different parameter than the inside versus outside temperature limit being discussed initially. If I misunderstood your comment I apologize. I'm just trying to make sure we are talking about the same thing.

Granted I'm reading between the lines, but if the split is above 20 F because of a blockage, it normally means the blockage is causing the air stream across the evaporator to move so slowly that the air can get too cool. This happens because the surface area of the evaporator remains the same, so when air moves slowly it has time to get colder. And although the air is getting colder, total system capacity goes down because of reduced air flow.

It may seem counterintuitive that air that is too cold coming out of an air conditioner is actually a sign of reduced capacity, but it can happen. The air may come out very cold but if there is too little of it the house will get hot.

In air conditioning the right air flow across the evaporator is also required to help control humidity at a comfortable level.
The split is measured at the cold air return vent and at a register, preferably the furthest away from the cold air return.

I would need to verify this, but I believe the blockage referred to a blockage of an air duct feeding a register which in turns causes more cold air to flow at one or more of the other registers and thereby causing an uneven cooling condition. That is the way I understood it.

I'm not a pro and with the exception of a five year period these folks have been servicing my systems since 1994 and they haven't steered me in a wrong direction yet.
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Old 06-12-2015, 08:09 PM   #15
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The split is measured at the cold air return vent and at a register, preferably the furthest away from the cold air return.

I would need to verify this, but I believe the blockage referred to a blockage of an air duct feeding a register which in turns causes more cold air to flow at one or more of the other registers and thereby causing an uneven cooling condition. That is the way I understood it.

I'm not a pro and with the exception of a five year period these folks have been servicing my systems since 1994 and they haven't steered me in a wrong direction yet.
Dave, that's what I expected, and makes sense. While also very important, it's a different issue (or limitation) from what the OP asked about, which is inside versus outside temperatures. In this case they just happen to both be in the 20 F range which could add to confusion.

Thinking back on original question, I live in very hot and humid southeast Texas and my AC at home can cool house to 75 F when it's 105 F outside. And my son's new home which is insulated much better and has an AC that is too large can do even better. The system just has to have enough cooling capacity to meet or exceed the actual load at whatever conditions one wants to operate.
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Old 06-12-2015, 08:59 PM   #16
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Dave, that's what I expected, and makes sense. While also very important, it's a different issue (or limitation) from what the OP asked about, which is inside versus outside temperatures. In this case they just happen to both be in the 20 F range which could add to confusion.

Thinking back on original question, I live in very hot and humid southeast Texas and my AC at home can cool house to 75 F when it's 105 F outside. And my son's new home which is insulated much better and has an AC that is too large can do even better. The system just has to have enough cooling capacity to meet or exceed the actual load at whatever conditions one wants to operate.
Chance,

I whole heartily agree. For me the majority of the threads provide a "take away" or a "lesson learned" and in thread my lesson learned is:

If the temperature split or Delta T is 15 - 20 degrees the coach will eventually cool down. The amount of time required for cool down is dependent upon how well the coach is insulated, the outside temperature, the outside humidity, and most importantly, the number of times someone opens and closes the door during the cooling cycle (especially if the cool air return is located close to the door).
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Old 06-12-2015, 10:58 PM   #17
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Yes, this turned out to be a very informative thread indeed!
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Old 06-12-2015, 11:35 PM   #18
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A big factor for my Vegas is sun exposure... the big windshield really heats up the front of the motorhome. I just installed sun screens that are supposed to reduce sun-induced heating by 94%... we'll see.
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Old 06-13-2015, 01:39 AM   #19
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A big factor for my Vegas is sun exposure... the big windshield really heats up the front of the motorhome. I just installed sun screens that are supposed to reduce sun-induced heating by 94%... we'll see.
Luckily the Challengers come with a motorized black out shade for the front window. We also have both solar and black out shades on most of the windows. This significantly reduces sun exposure to the inside.
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Old 06-13-2015, 03:10 AM   #20
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AC temperature

I'm glad I could post thought provoking questions from my own sweat trials.

I also purchased two small oscillating fans for insurance.
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