Dometic disappointing 15k a/c

[vcbice]

And also check the airflow baffle mounted below the AC. The baffle separates the incoming and outgoing air. Some people have found them not installed properly.

[TakerEasy]

Well, after 40 short years, you would definitely know most places short cut on duct work and most installers do not properly set the Fan CFM for the proper duct pressures. In fact, most of the time, the CFM of the duct work is to low for the proper deltaT of the evaporator.

Many new home builders try to take short cuts, but you have some that do over kill such as oversizing a unit to cover their behinds. That sounds good until you realize the system doesn't run long enough to remove the moisture and the humidity starts to climb, then you have a cool balmy atmosphere, oh, then you start to notice mold and wet duct work.

At any rate, the RV place is notorious for short cuts and not one I have ever seen had the duct work sized properly for the CFM required. !! This not only causes higher static pressures, lower flows, and higher deltaT's but causes lack of cooling and freeze ups!

Hum.... I'm glad I stayed at a Holiday Inn Express last night

The most famous words is "it's not suppose to cool more than 20F over outside ambient" because most folks don't understand that Ambient is filter inlet temperature, then you have to calculate wet bulb temp from that....

Originally Posted by jpolito

Just retired after just short of 40 years of ac repair work. I also noticed the same, ac units do not cool very well. To me it seems like low freon charge? can't tell about the charge without getting into the sealed system and I am not going to touch that until warranty is up. since the supply air is not cool enough even coming right out of the unit not going through the ceiling duct work makes me think that is the problem. But in hearing other peoples posts doesn't make sense that it would be that widespread. Will post if I check further.

[BJR]

Note, I am not the OP but the tips in here helped quite a bit. I can faithfully report that the camper is downright chilly right now in 81 degree outside temps. Two weeks ago we were lucky to get it to the *same* temperature as outside. Next week, I will post a thread with pics of everything we did.

[Cheaperrooter]

Originally Posted by TakerEasy

This simply is not true.

You mean to say, the AC duct temps should be 18-20 DegF cooler than the Filter Inlet temp. It has nothing to do with outside temp.

Outside temp has to do with the heat rejection on your structure and how well it is insulated. If your structure is insulated, you would drop initially 20 degrees from outside ambient temp, but now the air in your house is 20 deg cooler. As you recirculate air, the house and duct temp should continue to drop until you reach a certain point where the AC simply can't remove heat any faster than it's coming in (from outside or other heat sources).

It's 101F outside right now. My office is a cozy 68F. I also have very good insulation in my office and the AC is cycling!. I can drop it into the lower 60's if I wish.

Richard

This simply IS true. You’re talking about recirculating cold air over and over again and that has nothing to do with what I am talking about. I’m not trying to write an ecyclopedia here with all the possible variables. Trying to give everyone a basic understanding of the nature of the beast.

There’s always those that will disagree with the people that do this for a living. I stated the facts. Proceed forth how you wish with that information. Not going to debate.

I know what I’m talking about. But to make the statement that the outside air temperature has NOTHING to do with it when it has EVERTHING to do with it? LOL. Well...

[Cheaperrooter]

Originally Posted by TakerEasy

Well, after 40 short years, you would definitely know most places short cut on duct work and most installers do not properly set the Fan CFM for the proper duct pressures. In fact, most of the time, the CFM of the duct work is to low for the proper deltaT of the evaporator.

Many new home builders try to take short cuts, but you have some that do over kill such as oversizing a unit to cover their behinds. That sounds good until you realize the system doesn't run long enough to remove the moisture and the humidity starts to climb, then you have a cool balmy atmosphere, oh, then you start to notice mold and wet duct work.

At any rate, the RV place is notorious for short cuts and not one I have ever seen had the duct work sized properly for the CFM required. !! This not only causes higher static pressures, lower flows, and higher deltaT's but causes lack of cooling and freeze ups!

Hum.... I'm glad I stayed at a Holiday Inn Express last night

The most famous words is "it's not suppose to cool more than 20F over outside ambient" because most folks don't understand that Ambient is filter inlet temperature, then you have to calculate wet bulb temp from that....

You hit the nail on the head my friend!!!! Especially with oversize units. I can’t tell you how many times I have seen these contractors throw in a 5 or 6 ton unit thinking they have it “covered” if not to impress prospective homebuyers, not realizing that the unit doesn’t run long enough to pull the moisture out and you are absolutely correct about mold and wetness and etc.

What’s REALLY funny is watching them put in an air handler that’s rated for much lower

[TakerEasy]

I wasn't trying to start a debate either and I never said the outside temp had nothing to do with it. I said it had nothing to do with the 20 degree split except for the initial cycle. (read the second paragraph I typed). The 18-20F split is based in air temp at the filter. That is a fact.

The outside temp determines heat load/gain and other operating characteristics of the AC such as calculating proper superheat. Also subcooling since outside temperature affects discharge pressure, which is converted back to the temperature for calculating subcooling.

*IF* you always use outside air for your home or car, then YES, you only expect to see an 18-20 drop from outside temperatures. This is like running your car in normal mode or testing performance and vent temps with the doors and windows open. However, when you put your car in MAX mode, you recirculate inside air and achieve much more than 20 degree split.

Now, since RV's always recirculate, you ideally would achieve more than 20 degrees there also, especially if they had decent insulation. The main reason that they perform poorly is because of the poor insulation factor (thin walls, missing insulation or sagging insulation, cheap insulation, so on).

I've also built many walk in coolers (hanging meat in to cure) from standard AC units so if they never could cool more than 20F then how are we keeping them in the mid to upper 30's? (insulation and recirculation).

So to summarize. Yes, AC's design performance is 18-20F in a Single Pass. (which is what I think you were trying to say) However, rarely is it ever designed to be used in a single pass application.

Originally Posted by Cheaperrooter

This simply IS true. You’re talking about recirculating cold air over and over again and that has nothing to do with what I am talking about. I’m not trying to write an ecyclopedia here with all the possible variables. Trying to give everyone a basic understanding of the nature of the beast.

There’s always those that will disagree with the people that do this for a living. I stated the facts. Proceed forth how you wish with that information. Not going to debate.

I know what I’m talking about. But to make the statement that the outside air temperature has NOTHING to do with it when it has EVERTHING to do with it? LOL. Well...

[Cheaperrooter]

My bad for trying to incorporate the RV and auto in the same statement as a home residential sysyem. I was just trying to dumb it down to give everybody a basic or core understanding of how it works. It’s a bit of information that people are totally unaware of.

But I assumed that everyone knew they could then take that same air that had been cooled down by 20° and recirculate it over and over again. I knew I was in good shape until another qualified air-conditioner guy came along...JERK!!! LOL

And yes I was trying to save homeowners from it being a very hot day, 90 plus, and stroking the thermostat down to 60 thinking that’s as cold as it would get.

Unless you live in a state of the art well-built home, it’s just not going to happen. There is the theory behind the design and then the actuality out in the field and from my personal experience, 90% of homes out there are just simply not ever going to get that cold on a very hot day and unit will run 24 hours a day trying to achieve it, of course until night time when it gets cooler. People would be absolutely shocked to understand how vital insulation is when it comes to cooling the house lower.

Everyone thinks in terms of insulation as waste, how fast the air escapes and how long before it cycles again which is part of it. But a house that is not properly insulated will ALSO NEVER allow it to get to a lower temperature PERIOD.

RV and auto, which of course is what this thread is about, is a different beast and that’s why you will find the vast majority of qualified heating and air guys will not work on an automobile air-conditioning system and vice versa.

The air “can” recirculate over and over getting colder, assuming that is the setting that you choose on most models. Which in a car is normally max ac or just put on the recirculate button.

And I wasn't referring to you when I said there are always people who will not believe those to do this for a living, I was actually referring to hardheaded homeowners who insist other

[BrentB]

Originally Posted by firerescue712

When we lived in Albuquerque, the humidity was usually in the 20's or lower. We did not have AC on our vehicles. The summer daytime temps would run in the 90's. It was still comfortable. When we returned to Arkansas, we HAD to add AC to our vehicles due to the higher humidity. Hear of the term heat index? This relates to the moisture content of the air. When the humidity is low, so is the heat index - if there is one at all. We use dehumidifiers in our house. Keeps the air more comfortable at higher thermostat settings. In the RV, a small fan to move the air around, the RV AC unit, and the dehumidifier is all we need to stay cool. I used to work in a 65 degree environment. I cannot take the heat anymore from firefighting heat stress issues. I cannot stand the RV without the dehumidifier on these hot summer days. With it, I am comfortable all day long. The biggest issue is the 30-amp service. WE upgraded to 50-amp to allow dedicated circuits for electric heater, dehumidifier, and electrical cooking appliances. LOVE IT! And yes, the AC cools the RV just fine for our needs.

What dehumidifier do you use in the RV?