13.5K BTU vs 15.0K BTU Air Conditioners -- HOW????

[TenOC]

Question for my RV Air Condition Experts.

What is the difference in 13.5K BTU and 15.0k BTU? I know what a British Thermor Unit is. My question is HOW does the A/C achieve the difference in the RV? What is the real effect to the person inside the RV?

Given the same outside and inside temperatures.

1. Does the larger BTU unit blow more volume of air? More CFM?

or

2. Does the larger BTU unit blow colder air? I thought that under ideal conditions the A/C should cool the air only about 5 to 10 degrees.

[Chris_Bryant]

2oldman wrote:

beemerphile1 wrote:
The unit is internally recirculating the same air over and over to achieve those low temps.

Does that mean that if my a/c cools the room to 75F, it's now pumping out 35F air?

No, because the freeze sensor will shut the compressor down before it gets that cold- 35 degree air temp translates to below freezing coil temp- without the sensor, the coils would just freeze.

[Dirtpig]

TenOC wrote:
Question for my RV Air Condition Experts.

What is the difference in 13.5K BTU and 15.0k BTU? I know what a British Thermor Unit is. My question is HOW does the A/C achieve the difference in the RV? What is the real effect to the person inside the RV?

Given the same outside and inside temperatures.

1. Does the larger BTU unit blow more volume of air? More CFM?

or

2. Does the larger BTU unit blow colder air? I thought that under ideal conditions the A/C should cool the air only about 5 to 10 degrees.

The AC unit should blow out approx 20deg lower air then what it takes in. Example, your RV is at 100deg, and if your AC unit is working properly it should push out air at about 80deg. This will not happen instantaneously but in about 15min the AC unit will be working at 100% efficiency. Gradually your RV's ambient temperature will drop, and air out of the exhaust vents will continue to be approx 20deg colder then the RV ambient temp (or intake temp of AC unit to get technical). (80deg ambient, 60deg exhaust temps etc) Higher BTU units will push more air out (move more air over the coils) and keep that same ~20deg cooling capability which is the limit of the technology. You guys running in very hot climates are going to run into the uncomfortable range with the balance point equation. The thermal balance point is the temperature at which the amount of cooling provided by the AC equals the amount of heat gained inside the RV through inadequate insulation/direct sunlight etc from the RV. At this point, the AC SHOULD match the full cooling needs of the RV. Problem is, many RV's balance point is in the uncomfortable range because of poor insulation and workmanship in many RV's. Solving this is dual AC units, higher BTU units, or insulating the RV better.

This same reason is why a large home will have a higher btu AC unit or multiple units then a small home with a single lower btu unit. In an ideal scenario you want your AC to be running 100% of the time and still keep the perfect inside temperature. Oversizing would create increased cycling of the ac unit and less humidity removal.


1 : Yes

2 : No

[2oldman]

beemerphile1 wrote:
The unit is internally recirculating the same air over and over to achieve those low temps.

Does that mean that if my a/c cools the room to 75F, it's now pumping out 35F air?

[beemerphile1]

Chris Bryant wrote:
...I can go dig in 25 years of work orders to find some- I can guarantee that I never let an air conditioner leave here with split temperatures that large, because- for around the fifth or sixth time in the last week- the large split temperatures are because there is a leak between the supply and return, so the return air is actually much colder than the thermometer is reading.....

I've had this conversation with others. It seems to be a difficult concept to grasp that what one thinks is the incoming air - really isn't. The unit is internally recirculating the same air over and over to achieve those low temps.

People think because the temperature coming out is so low that the system is working great, it is actually the opposite. The internal mixing of air causes the unit to cool the room slower than it should.

[Chris_Bryant]

greenrvgreen wrote:
So far neither of those insulted experts have posted their own readings. While I'm sure they have better things to do, how is it that they found the time to hurl insults?

"Loose flap" would have been my guess also, and I'm no expert. But at the same time, NO ONE but Oldman has posted the temps going in and coming out of their ACs.

How many reading do you need? Do you want readings from only properly operating units or do you want them from defective units as well?
I can go dig in 25 years of work orders to find some- I can guarantee that I never let an air conditioner leave here with split temperatures that large, because- for around the fifth or sixth time in the last week- the large split temperatures are because there is a leak between the supply and return, so the return air is actually much colder than the thermometer is reading. Seal up the leaks, clean the filter and you will see the more normal temperatures, and the unit will cool the rig much better.

FWIW, Doug and I have records giving outside temperature, inside temperature, ac voltage, current draw of the compressor, current draw of the fan motor and split temp after 30 minutes of operation (I do 30 because of the extreme humidity- it takes a lot of btus to dry the air, so I normally get a very small split for the first 15 minutes or so). These figures are all required for warranty work, so it's not like it is something we never have done before.

Does this satisfy your requirements?

[westend]

Better pull over, I see the Physics Police have their lights flashing, lol.

[Bob_Landry]

They are not "contaminating" each other as they are spaced out. Cold air is likely being drawn directly back to the return side in the discharge plenum(lower unit)usually the result of a poorly done installation. Drop the cover on the AC and stick your hand up into the plenum. If you feel any cold air at all, it needs to be sealed off/re-taped.

"Since I've only used my hand to gauge delta-T I have no idea what a ~20 degree drop should feel like."

And you will not be able to gauge it.

[2oldman]

greenrvgreen wrote:
..the pics seem to show the large AC ceiling unit as the intake and a small, remote ceiling vent as the outflow. Doesn't seem likely to me that one is contaminating the other.

No, I made sure of that. The vents are pointing away from the intake.

A long-time refrigeration tech from the 'other' board said this:

"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."

[greenrvgreen]

Good point, Bob, but the pics seem to show the large AC ceiling unit as the intake and a small, remote ceiling vent as the outflow. Doesn't seem likely to me that one is contaminating the other.

Good point also, RLS-something. Since I've only used my hand to gauge delta-T I have no idea what a ~20 degree drop should feel like.

[Bob_Landry]

Unless I'm reading the thermometers incorrectly, I'm seeing around 48 degrees on the discharge side and 85 degrees on the return grill. That's a far cry from the 18-20 Delta a properly sized, charged, and ducted AC should provide. We don't know that the unit is properly charged without knowing the amp draw of the compressor and the outside ambient temp, but I would say the lower discharge temp is likely from some kind of air flow restriction or cold air being drawn back into the evaporator and the high return is because the unit is undersized for the space it's trying to cool and it just can't keep up. At any rate, it's operation is far from acceptable.

As far as air feeling extremely cold from an air restriction, with less air moving less heat transfer is taking place and the air may feel extremely cold at the vent, but if you move away from it, you don't feel much air movement. That's the reason people are fooled into thinking their AC is really pouring out super cold air. It is, but only for a few inches from the vent. If you aren't moving air, you aren't cooling it.

[RLS7201]

I also thought I was getting more than a 22 degree drop. So I went in to my 81 degree coach and started my 1995 13.5 Coleman. At 15 minutes I had 68 degrees at the return duct and 46 degrees at the cold outlet. Color me wrong! Thanks all for your educated/experienced responses.

Richard

[greenrvgreen]

So far neither of those insulted experts have posted their own readings. While I'm sure they have better things to do, how is it that they found the time to hurl insults?

"Loose flap" would have been my guess also, and I'm no expert. But at the same time, NO ONE but Oldman has posted the temps going in and coming out of their ACs.

[westend]

In 2oldman's case the split temperature reflects a leak between the intake of the evaporative coil and the standard from-room intake area. I'm guessing there is a flap loose in that area that allows the cold air flow to cycle back through. It is well established what an air conditioner's Delta T can be and the information posted about latent heat and wet bulb is also accurate.

To ask an A/C technician if he has used a temperature measurement device is kind of insulting to the Tech. It is one tool that is commonly used. The actuality is that Delta T will always fall close to 20f.

[RayJayco]

https://www.swtc.edu/ag_power/air_conditioning/lecture/basic_cycle.htm
This explains basically how it works, same principles for refrigeration and air conditioning.
To heat, one must produce heat. To cool, one must remove heat. They are not called air coolers, they are air conditioners as removing moisture is a part of the cooling effect. The laws of the gases involved are what governs the amount of heat that can be removed.

With that said, what they are trying to explain is that the return air (the air inside the RV that is going into the air conditioner) is limited by science, physics, thermal laws, etc., as to how much heat can be removed before the conditioned or cooled air can be blown back out into the room.

If there is a problem such as what appears to be, then the air from the room is not being drawn in as it should be, it is sucking in the air that has just been cooled instead. So the temperature coming out of the vent is not actually the air from the room, it is the air that should be blowing into the room.
IOW, the already cooled air is being sucked back in and re-cooled thus it appears that the air conditioner is defying science...
I hope this helps!