2 AC's, 2 different performance abilities?

[SargeW]

Well we are camped in a fairly warm campground, almost 100 degrees outside. Both airs cranked up, but the bedroom air felt like it was putting out colder air. So I did a little experiment. I got my digital thermometer, and put both AC units on straight downdraft.

The temp in the bedroom was about 40 degrees at the vent.  The living room AC was 60 degrees.  The LR AC is 15,000 BTU, the bedroom AC  is 13,500 BTU. Both units are working in full sun. Both are Dometic units installed on a new unit.

Any guesses why the big differences guys?

 

[carson]

OOH, I love riddles.
 
First, I assume both have the same return air intake in front.
2. The front A/C has multiple outlets and much more plenum to feed them
3. The rear A/C has 1 or 2 outlets and short plenums. i.e. ducts.

  In full sun the front plenums are more likely to get heated more than the rear causing reduced cold output.
  How is that for a stab in the dark ?

  A test on a cooler day may prove the point unless the front one definitely has a problem.

  Caveat: I've only had one cool one today  8)

Let's have 10 more replies.

 

[SargeW]

Interesting Carson. The LR AC is ducted, but I tested it with the down draft open, so little or nothing going into the vents. The BR AC is not ducted at all. There could be some temp differential, but that seemed like a lot.

 

[carson]

I understand, Sarge.  My A/Cs did not have downdrafts...  only ducts. (Coleman). Guess that blows my theory into hot air.

 

[COMer]

I'm way over my head here but I know that when you slow down the fan speed on your air, the temp of the air coming out is cooler.  Since it passes over the cooling coils slower and therefore is getting cooled longer.  Is the volume of air going through enough different than one is getting cooled more than the other?  I love riddles too. 

 

[Gary RV_Wizard]

You need to measure the temperature differential, inlet vs outlet, to get an idea of the a/c performance.  Was the living area warmer than the bedroom?

Typically the smaller area in the bedroom cools quicker, so the inlet temp quickly drops and the thus the output gets even colder, making it seem like it is working better. I would venture that your front a/c is cooling a much larger area, and probably has a grater sun-load on it as well..

 

[SargeW]

Yep, the living area was warmer Gary. The inlet is right next to the outlet, so getting an accurate reading between the two would be tough. At high speed there is a large volume of air passing over the condenser. I am going to check this morning to make sure the condenser is clean.

 

[tvman44]

I agree the true test is to measure the differential.  Try taking the input measurement a little further from the unit on both units, that would be a better comparison IMHO.   Also is the son hitting the end of the trailer with the warmer temp more?  I know on our 5er a lot of sites we use the front (BR) gets a lot more sun and causes the BR unit to run more.

 

[captsteve]

There may be an issue, but the l/r unit is cooling a much larger space and is only marginally larger itself. You may be asking a lot of it in those temps. Its working its butt off to exchange all that heat to 100 degree temps.

 

[carson]

You need to measure the temperature differential, inlet vs outlet, to get an idea of the a/c performance.  Was the living area warmer than the bedroom?

Typically the smaller area in the bedroom cools quicker, so the inlet temp quickly drops and the thus the output gets even colder, making it seem like it is working better. I would venture that your front a/c is cooling a much larger area, and probably has a grater sun-load on it as well..

  One more thought from me.  I suppose there is a fifference between the Dometic and the Coleman system.  My RV, albeit only 32', had only one inlet with filter in the LR and none in the BR.  Must have a duct to feed the blower to rear unit. So, all air feed temperature to both should be equal.

  Perhaps Sarge has an inlet in the BR as well. That would make difference when measuring the differential.

  Oh well, not trying the stir the puddin';  just curious.

 

[SargeW]

The way the RV is parked, the sun is hitting the side of the rv, so both units are getting about the same sun. I just checked the inlet/outlet temp in the BR, inlet is 70, outlet is 44.  The LR inlet is 75 and the outlet is 59.

And as another twist, this is in my 5ver, so the ceiling in the BR is higher. Since heat rises, you would figure that the ambient temps in the rig would be warmer on the high end.

I also sent an e mail to Dometic customer service. They responded that it sounded like the LR AC was not operating to specs, and to take it in for tests.

 

[tvman44]

You apparently have a issue with the LR unit, 16 to 20 differential is good, 26 is not very good.  Maybe as simple as dirt coils or bad as in needing a new unit.

 

[Charlie 5320]

Most air conditioning systems are designed with 75 degree return air, and a 40 degree coil. Results in a 55 degree discharge air temp. As the return air temp increases or decreases the discharge temp will vary. If the amount of air increases or decreases beyond design of the unit, it will also change the air temp leaving the unit.

Try running the fan on a lower speed and see if the air temp doesn't drop. If it does, then your unit has too much air going over the coil for the saturation temp of the coil. Two or three things could be wrong. 1 low on refrigerant, will cause bubbles in the capillary tubes or expansion valve, which ever it has. 2 condenser coils dirty, not being able to change the hot gas back to a liquid, has to do this to keep from having the bubbles. 3 the amount of air being pushed over the evaporator coils, bad design.

 

[M1894]

Sarge, have you been up on the roof and checked the coils for Arkansas dry snow [cotton wood] covering the coils like a felt mat? I have to clean out the coils at least twice a year here.


Lee

 

[SargeW]

Yes, I went up this morning and pulled the shroud off the AC to check the coils.  No obstructions that I can tell. All looked clean, and still looked new.  The shaft on the fan does have a noticeable wobble though. Don't know if that would contribute to the lack of cooling power or not.

 

[tvman44]

Not unless it was causing the fan to run slower than it is supposed to.

 

[Gary RV_Wizard]

You apparently have a issue with the LR unit, 16 to 20 differential is good, 26 is not very good.

tvman44: you are saying that a lesser differential is better?  I think the opposite, i.e. that a differential of 26 degrees is very good, while 16 is poor and 20 is marginally ok. SargeW's LR differential is only 16; the BR is 26.

Or did I misunderstand your statement?

 

[Gary RV_Wizard]

I suppose there is a difference between the Dometic and the Coleman system.

Ducting is supplied by the RV manufacturer, not Dometic or Coleman.  There is no substantive difference between the brands as far as air in/out - both are constrained (severely so) by the 14x14 opening to house both the inlet and the output into the ducting system.

One of my pet peeves is that no one, neither RV manufacturers nor a/c companies, has developed a new standard roof opening for a/c. They all cling to the old 14x14 vent opening that was established back in the day when a/c in an RV was rare and ducted systems non-existent. Interior a/c function could be improved dramatically, but they keep shoving out the same old crap, year after year. This is an area where the RVIA ought to be taking the standards lead, but instead they spend all their time polishing little details like which battery bank the slides get powered from.

 

[tvman44]

Gary, you are correct.  I was backwards.  Oh well.

 

[Gary RV_Wizard]

Wish I could say I've never done that, but...