Laws of thermodynamics???

HornedToad

Well-known member
I remember 20 years ago in El Paso we bought our 1st house with "Refrigerated Air", after living for 20 years before in houses that had "Swamp Coolers". The first summer in our new house Helen complained that the AC wasn't working right so I had my HVAC friend come out to take a look. He said everything was working just fine and the most a "Refrigerated Air" unit will lower the outside temperature is 30 degrees and for a "Swap Cooler" it was 20 degrees.

After all the recent AC post on the form (summers comin')...
I tried to research this on the internet with conflicting results, especially in regards to an RV AC. I think the correct term is "Temperature differential"?

So the next time we're at the lake and it's pushing 110 outside,
and Helen is upset because it's over 80 in the trailer...

Do I quote her the "Laws of thermodynamics"... even though I don't know which one???
OR
Should I take my trailer in to have the AC serviced???


"What if this is as good as it gets?" Jack Nicholson as Melvin Udall 1997
 
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jbeletti

Well-known member
I don't have any technical answers for you.

Only a smart aleck one - "Take her to Walmart" when it's 110 outside - it's always cool there in the summer :)

Sorry - don't mean to offend. The 30 degree differential sounds about right.
 

boatto5er

Founding VA Chap Ldr (Ret)
When we were in 107degrees in Delaware a couple of years ago, a trip to the casino worked wonders (but not for the pocketbook!).
 

Invizatu

Senior Road Warriors
Shade, insulation & A/C capacity will also be determining factors, so I tend not to agree with the theory of outside vs. inside temps.
If you are recirculating the inside air, then it will get cooler and cooler and cooler as you are cooling cooler air each time is recirculates.
I have been in 113 degree temps outside and gotten the inside temp down to as low as 68! (21 foot North Trail with 15k a/c unit in Amarillo, Tx. & full sun)
Of course it also depends on the capacity of the A/C vs. the cubic feet of the trailer. You are not going to cool down a 40 foot trailer with a 13.5 a/c in real summer heat! jmho
 

cookie

Administrator
Staff member
Might be a good time to tell her that you need to install a second AC unit.

Peace
Dave
 

HornedToad

Well-known member
I don't have any technical answers for you. Only a smart aleck one - "Take her to Walmart"

Yeah... I was kinda hopin' an HVAC guy would jump in with a technical answer in layman's terms.

Walmart is a great idea Jim, but from the lake it's over 60 miles round trip.
I figured this out right after I realized a Ford tire tool doesn’t fit the lugs on my boat trailer!!!

Thanks Invizatu for lettin' me know there is hope and boatto5er I stay outta casinos.

Dave, don't get me started on a 2nd AC, I'm workin' on gettin' a 3rd.
 

priorguy

Well-known member
I guess telling your DW to jump in the lake isn't an option either. I usually have it too cold for mine and she's looking for a sweater. Smaller trailer with a bit if shade, leave blinds down to keep the sun out too.


Sent from my iPhone using Tapatalk
 

rjr6150

Well-known member
Mike it's been over 20 years since I've figured heat loads and sizing. Though at that time it was generally accepted you would obtain 18-20 degree drop across the coil. For instance if one had 100 degree air coming through the return you would expect the air flowing across the coil would be somewhere between 82 & 80 degrees.
This is where sizing of the unit comes in figuring the heat or cooling load to determine correct BTU's for sizing.
 

evolvingpowercat

Well-known member
There are a LOT of factors that determine the temperature you can measure at the air outlet and how much the drop is between inlet and outlet. The more you can get the heat load off to begin with the better. In other words park in the shade if at all possible!



Some other things:

1. Try to minimize the moisture the AC is having to remove from the indoor air. The AC energy used to de-humidify is not able to be used to cool the air temperature, when its working at the max in hot humid conditions.

2. Make sure your ducts are as wide open as possible, look for any blockages you can eliminate. Make everything as smooth as you can. Some of that metal duct tape can be used for that. The easier the AC inside air blower can move the air the better it will work. Cold air is heavier than hot air, something to move that cool air down at the floor up can help make it "feel" better.

3. Try to minimize the air leaks between indoors and outdoors.
 

HornedToad

Well-known member
Last summer at the lake the heat was brutal, it was so bad one Saturday afternoon that Helen was about to make me pack up and head home, when I was saved by a passing thunderstorm that cooled things down. With the weird winter weather we've had I'm afraid this summer will be worse!!!

Some good ideas to manage the high summer temps...

determine correct BTU's for sizing
1. Add BTU's = 3rd AC

Make sure your ducts are as wide open as possible
2. Open the quick cool vents on the ducted ceiling assembly.

move that cool air down at the floor
3. I always carry a box fan, I need to start using it.

leave blinds down to keep the sun out
4. Helen opens all the blinds, she likes the view of the lake


No shade trees to park under in West Texas,
but if everything else fails...
jump in the lake
 

Gary521

Well-known member
Opening all the blinds is part of the problem. There is so much heat gain from these windows that it is almost impossible to keep up with it.
 

Ray LeTourneau

Senior Member - Past Moderator
Even with dual pane windows, the sunny side of our trailer seemed to really increase the afternoon interior temps. I bought some cheap plastic foil insulating bubble stuff and cut pieces to cover the 3 windows that were most exposed to the afternoon sun. It was an amazing difference. I must add though that we don't see that extreme heat here in FL during the winter but, every little bit helps.
 

dave10a

Well-known member
Cooling and heating are all about R-value, A/C and heater capacity. Cooling is function of the compressor and expansion valve which is sized with the compressor. Expansion of gases is what cools things. Modern A/C's are capable of cooling to below freezing levels with proper sizing. I am sure there are many walk-in freezer in Los Vegas do just fine freezing food storage. I was able to obtain 4 deg kelvin (4 deg C above absolute zero) with helium compressors/cryo pumps used on MRI systems. So to make a long story short if your trailer is not getting cold enough increase the A/C capacity and if the heater is not getting hot enough increase the furnace capacity. Increasing R-value is not practical unless the manufacture changes the basic design and adds the appropriate components.
 

icechex

Well-known member
I'm an old retired AC guy and the best advice I can give you is "to go where it's cool." Seriously, these crates (RV's) are not made to be cool in hot places. Someone else said you should have a 18-20 degree drop across the evap coil which is generally correct - "in a normal house, well insulated, and with properly sized air conditioning working perfectly."

Of course.... the above description does not fit RV's.... sorry.

I've got a new RV (without double insulated windows which would help) and the best I could attain last year in Oklahoma during June (outside temp 100 degrees) was a measley 11 degree drop. In other words when it was a 100 outside it was 89 in the RV and this with 2 AC's going full blast. Of course one could 'tinker' with things and improve it marginally, but basically, the advice I gave above is the best I can do - and it bears repeating - "go where it's cool."
 

Bohemian

Well-known member
1) At what temperature and humidity are you comfortable.
2) What is the heat flow in/out of the system (RV). Heat flow is proportional to the temperature difference inside to outside and the surface area. Inversely proportional to the resistance to heat low (insulation R-values).
3) Air flow into/out of the system will further transfer heat
4) Solar flux will add heat.
5) Any air conditioning system has a maximum capacity, At 100% duty cycle no further cooling is available. An increase n cooling unit size or additional units will be required to increase cooling capacity.
6) Fans will move air contributing to more uniform temperature distributions. Also will increase evaporative cooling rate of human external surface at relatively low humidity of interior air.
7) Greatly excessive cooling capacities (too large an AC cooling unit) will excessively decease duty cycle and provide inadequate dehumidification leaving interior excessively humid and uncomfortable at commonly requested temperatures.

Solutions are not limited to but include:

1) Match AC Cooling capacity to required load
2) Move to Bali

:):)
 

icechex

Well-known member
"so I tend not to agree with the theory of outside vs. inside temps. If you are recirculating the inside air, then it will get cooler and cooler and cooler as you are cooling cooler air each time is recirculates. I have been in 113 degree temps outside and gotten the inside temp down to as low as 68! (21 foot North Trail with 15k a/c unit in Amarillo, Tx. & full sun)."

SORRY....I disagree with the above - because - 'the theory of "outside/inside differential temp" is true. I worked over 30 years repairing commercial/residential Air Conditioning and the "absolute best" one could expect a 'properly sized ac unit' to do was a 30 degree split (between outside/inside temp), - and that is when 'everything is working perfectly, house is well insulated, all the coils are clean and compressor and other components working perfectly. Most houses would get between 20-25 split. This is the law of physics...sorry.

Your 'idea' that the AC continues to cool the air each time it recirculates is also false. If this was true you would eventualy be "freezing" the plumbing in the house, come on.

Actually, the AC conditions the air - by removing heat/humidity from the conditioned space (inside air), hence the name "Air Conditioning."

All AC systems "must be" sized correctly otherwise you would have big problems. Here's an example: Let's say you have a 1800 sq ft house. The 'rule of thumb' for sizing says 1 ton (12000 btu) for every 600 sq ft of properly insulated space. Therefore - you would need a 3 ton (36000 btu) AC for the house.

Note: actually 'sizing' a house for Air Conditioning is somewhat more complicated than this - this is just as I said a 'rule of thumb' example.

Anyway.. back to the 1800 sq ft house. So.. "Joe Homeowner" in an attempt to get 'extra cooling' decides he's going to install a "4 ton (48000 btu) AC on his house. Note; I've actually seen this done!

What he (Joe Homeowner) gets is an AC unit (because it's so oversized) that "does not run long enough" to pull the humidity out of the air and the insided temp (while indeed cold) is drafty and uncomfortable. Remember- "humidity is the chief reason" you feel uncomfortable when the temp is warm/hot.

Let's say "Joe Homeowner" tried to 'cheap his way out' and installed an 'undersized unit' say a 2 ton (24000 btu) AC, then the 'undersized' unit would run all the time and still not get the humidity and temp down inside.

"Sizing" is everything when it comes to Air Conditioning.

A short AC course follows:

Freon is reduced in temp (causing about a 40 degree coil) and into a liquid (by a metering device) and as it passes through the evap (inside) coil, it picks up heat from the air inside the cooling area (due to evap fan motor drawing warm air across the coil)

This air is warmer than the liquid in the evap coil and therefore a 'heat transfer' takes place. Think of a 'coke bottle' sitting on an outside table on a hot day. It will 'sweat'.... that is because it is absorbing the warm air. This is a 'heat transfer.' Your inside coil does the same thing - 'it will sweat as the warm inside air is absorbed into the coil.

This 'heat laden' freon is then pumped (by the compressor) to the outside condenser coil and cooled by the condenser fan motor and distributed outside - and then the cycle starts all over.

This is a 'basic' refrigeration cycle. It 'will' continue to reduce the inside air temp as it recirculates the air, "to a certain point." But... this all depends on "outside temp," and will only reduce inside temp to a certain point - which brings us back to the outside/inside temp differential. i.e. 90 degree outside temp - equals - approx 60-65 inside temp " if " everything is working perfectly.

This 'differential split' does not 'go away,' it will always be there. Therefore, inside temp will be "limited" in how far it drops by the temp of the outside air, as well as the working operation of the AC equipment.

Note: it is 'possible' to overload a conditioned space with excessive equipment but that is not "conditioning the air," and the resulting problems you would have - would be (often) worse than having no air conditioning.

Even with that said, ..... as to the claim of a 45 degree differential between inside/outside temp .... uh, I'd have to see that.
 
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Bob&Patty

Founders of SoCal Chapter
Mike, if your trailer is parked facing north and south it will be hotter. 30-35* is about all you will get in difference between inside and outside. Cars and trucks are about that also. We dont have 6" walls in our coaches like a home does. You could also get some spray foam and fill every void in the floors that have plumbing going though them. If the frame work under the BR floor is not insulated....you could add some 2" foam insulation there also....make a big difference. We have a tower fan we use when it gets very hot outside to move the air from the BR to the LR and it helps. If DW wants all the shades up....then you might have the windows tinted with "reflective tint".
 

HornedToad

Well-known member
Yeah... I was kinda hopin' an HVAC guy would jump in with a technical answer in layman's terms.

Thanks icechex... and everyone else.

At the lake a couple of weeks ago, pushing 100 degrees outside, implemented all the tips and advice and I was able to get down into the mid seventies, which was comfortable.

Something I tried that seemed to make a difference was to close all the ducts in the bedroom & bath forcing the ducted air into the main living area and setting the thermostat on the bedroom AC way below the main so it stayed on.

We broke 100 degrees the first week in May and have had several 95+ days since...
still wouldn't want to trade for the white stuff!!!
 
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