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Topic: Dash A/C System Capacity and Pressures (Read 929 times) previous topic - next topic

Dash A/C System Capacity and Pressures

I see on BarryB's site Dash Air Refrigerant Charge Information - R134A Systems that dash A/C system capacity is 5# or 80 ounces for 1994 and newer coaches. A local outfit just filled mine (1995 U-300) after evacuating and flushing the system and claim it only took 3# and that putting in more raised the pressures too high. My questions: For those of you with older coaches, what is the capacity of your system and what are your high and low pressures? Also, what is your typical air temp on a hot day? Mine was blowing in the low 40s at the dash register. Thanks.
jor
93 225
95 300
97 270
99 320

Re: Dash A/C System Capacity and Pressures

Reply #1 –
That's a loaded question!

Refrigerant pressures in any system will depend on the ambient (outside) temp, room (inside) temp, condition of system components, amount of charge, and a few other variables. Here is a chart that will give you an idea of the relationship between R-134A and pressure:

Pressure - Temperature Chart for HFC-134a

If you KNOW the charge in your system should be 5# & it only took 3# before the pressure was "too high"  (what exactly was the pressure? Automotive systems can run 400# in some situations) here are some things to check:

Dirty Condenser- needs to physically and chemically cleaned

Low air flow across condenser- fan speed? clutch? belt(s)? I don't know what the setup is on a Foretravel

Failing Compressor- will put excessive heat in system

Plugged or failing metering device- You will need a professional to check that

Really high ambient and/or room temp. If it's 100F outside & inside, pressures are going to be near the top of specs until the room temp comes down.

There are other factors but you really need a pro to go much further.

HTH

TOM
SOB (Some Other Brand) division
1995 Wanderlodge WB40
8V92 :D

Re: Dash A/C System Capacity and Pressures

Reply #2 –
Tom,
  I'm not doing this work. A local shop just replaced the compressor, dryer and expansion valve. Also flushed the system. Condensor fans (2) work fine. So this is a fill from empty situation. The ambient temperature is about 100. They just called me with the above info and advised me it was blowing at about 53 degrees. That is warmer than it used to blow and I don't think the 3# is right. Anyhow, trying to arm myself with some info before I head down tomorrow to pick it up. Thanks for the chart.
jor
93 225
95 300
97 270
99 320

Re: Dash A/C System Capacity and Pressures

Reply #3 –
Add only enough to get pressures correct and forget the specifications.

Re: Dash A/C System Capacity and Pressures

Reply #4 –
Barry,
  I have added 134a to this coach and our last one. I just used the chart that comes with the can. e.g. about 50 psi for my current 100 degree ambient temp. But I have never had a set of real gauges and am ignorant of what the high side should be. The tech is telling me that putting in more than 3# produces too high pressure. I intend to ask him what pressures he is seeing and I will test the air temp at the register. I'd just like to know what would be ballpark appropriate high and low pressures so I can be sure I'm not getting BS'd. Also, from reading other posts it seems that many are seeing high 40s at the register in well working systems. Thanks.
jor
93 225
95 300
97 270
99 320

Re: Dash A/C System Capacity and Pressures

Reply #5 –
Jor,
I just replaced my compressor and drier after my compressor locked up.  I put 6 cans or 72 oz of 134 and the outside temp was 88 degrees/dash was blowing 50 degrees.  I added another 8 oz and at 87 degrees outside it is blowing 45 degrees from dash.  So I think the 80 oz is correct for my 2000 40 footer. 

James T confirmed a empty system was 4.5 to 5 lbs of 134 and that is why I started out with just 6 cans, but I wanted to see if another half pound would help and it did.  Pressures were still within range.  I use 2 times ambient plus 20 % for high and I was well below at 5 pounds of 134.

Hope you get a good outcome.

Brad
Brad

2000 U320 4000

Re: Dash A/C System Capacity and Pressures

Reply #6 –
Quote
Pressures were still within range.
Brad, what were the pressures?
jor
93 225
95 300
97 270
99 320

Re: Dash A/C System Capacity and Pressures

Reply #7 –
Jor, At 87 degree outside I had 18 pounds low side and 200 on the high side.  I never had my low side read any higher than 20 pounds and that is not what I get on my other vehicles.  I might replace my expansion valve and see if that helps, but it looks to be more work than I want to do at this time.

Good luck,

Brad
Brad

2000 U320 4000

Re: Dash A/C System Capacity and Pressures

Reply #8 –
John, So maybe the tech is just afraid to add more, maybe based upon his auto experience. These coaches do take more than a car, probably because of the long hoses and maybe larger condenser. I agree 53 is too warm at the evaporator.

We did take a indoor outdoor thermometer with the hard wired outside probe, and put the probe down into the dash evaporator so it is near the coil. When driving down the road we can check the working temp to see if all is running well. We have seen temps in the 30's

The thermometer is handy when it is very warm and the coach does not feel cold enough, but if the temp reading in the evap coil is 40's or so, we know it is working as hard as it can.

Re: Dash A/C System Capacity and Pressures

Reply #9 –
Quote
I agree 53 is too warm at the evaporator.
I'll have to see where they are getting the 53. I have been measuring the temp at the vents with a digital thermometer and getting low 40s.

So I'm thinking I could look for maybe low 30s at the evap and 250psi on the high side. Oldmansax, does 250 seem OK? I guess what I really need to know is what reading would be obviously too high. Thanks.
jor
93 225
95 300
97 270
99 320

Re: Dash A/C System Capacity and Pressures

Reply #10 –
Sorry to break into this thread but - - who can work on our a/c units?  Local auto shop?  Where can I find a drawing outlining parts and part specifications for our 2002?  Not getting cold air out of dash vents.

Thanks in advance.
Scott & Carol Seibert
2001 42' double slide U320 - Sold
Previous - 2002 36' U320

Re: Dash A/C System Capacity and Pressures

Reply #11 –
Quote from: jor –
So I'm thinking I could look for maybe low 30s at the evap and 250psi on the high side. Oldmansax, does 250 seem OK? I guess what I really need to know is what reading would be obviously too high. Thanks.
jor

250# is OK to a little low for 134A. Check the chart again. 250# = 147F...... not real high on a hot day. The first thing I would look at would be dirty condenser. Did your system work fine before this problem? If so, what has changed?

Low side (suction) should run about 30#. That would be a 34F degree evaporator. Much lower than that & the evaporator ices up (water freezes at 32F  :o ) The TD (temperature differential - the difference between the evaporator temp & the air flow through the evaporator) should be about 11F to 15F. A 34F evaporator + 11F = 45F or the correct discharge air.  NOTE: none is this is set in stone. 45F to 55F discharge is fine, particularly on an older system.  All systems will be a little different but these figures should be close.

On stationary systems the manufacture furnishes a charging chart with a scale of temperatures and pressures. You measure every thing & adjust the charge to accordingly. Transport systems are done by weight. The manufacturer designs the system & figures out exactly how much refrigerant is required and the tech weighs it in. If the system will not take the correct charge, SOMETHING has been modified or is malfunctioning. It's up to the tech to find out what's happening.

I feel you guys' pain with AC systems. I started out in life owning & operating refrigerated trucks. The reason I ended up in the HVAC business was because I could not find reliable refrigeration techs. After 40 years in HVAC, I still learn something most every week.

TOM
SOB (Some Other Brand) division
1995 Wanderlodge WB40
8V92 :D

Re: Dash A/C System Capacity and Pressures

Reply #12 –
Quote
The first thing I would look at would be dirty condenser. Did your system work fine before this problem? If so, what has changed?
Worked fine until it didn't! I'll be down there today to pick it up and I'll post whatever I find out. Still wonder why the guy says he can only get 3# into a 5# system.As far as I know, the system has never been modified. Had hoped to be on the road by now but I'm not leavin' until that thing is blowing cold!
jor
93 225
95 300
97 270
99 320

Re: Dash A/C System Capacity and Pressures

Reply #13 –
Quote from: Carol & Scott –
Sorry to break into this thread but - - who can work on our a/c units?  Local auto shop?  Where can I find a drawing outlining parts and part specifications for our 2002?  Not getting cold air out of dash vents.

Thanks in advance.

Scott, I would probably look for a reputable truck shop. That said, there are still no guarantees the tech will actually find & fix your problem. Most mechanics need to be knowledgeable many areas to work in a shop. I don't know of any shops that have a dedicated HVAC guy. Someone who works in A/C & refrigeration will see 2 to 5 jobs a day, year round. An automotive AC guy may see 2 or 3 jobs a week during summer only. Experience does help!

Since I am semi-retired, I have considered setting up a shop to service RV HVAC systems. I just don't know if the outlay for a shop that big would be justified. Maybe if I was in the southwest.....  but not so much in southern Delaware.

Then again, it IS a nice place to vacation! I could setup some RV sites..... the Atlantic Ocean is only 1/2 hour away......  lots of good seafood...... WHOA!! This is beginning to sound like WORK!  ;D ;D

TOM
SOB (Some Other Brand) division
1995 Wanderlodge WB40
8V92 :D

Re: Dash A/C System Capacity and Pressures

Reply #14 –
Tom sounds more like you need to become a mobile RV HVAC guy. You know, travel around the country and all.......



Larry
Larry Warren
1996  U320 36' SBID "Lola" sold 2020
Build #4970
Motorcade #18318

Re: Dash A/C System Capacity and Pressures

Reply #15 –
Quote from: ohsonew –
Tom sounds more like you need to become a mobile RV HVAC guy. You know, travel around the country and all.......
Larry

Larry, I have seriously considered that but I can't see how to make it work. First, where would I work? I am not real experienced in campground regulations as we mostly boon dock, but I think most campgrounds don't want folks working on their rigs. How would I get parts? Ship to care of the campground? Don't think so. I am a big believer in the right tool for the job & I have lots of them. I would have to haul a BIG trailer full of tools. Also, I like comfort! Lazy, I guess! ;D  Hard to beat working in a shop, where everything is at hand and neat, heated or cooled, with room to work on things correctly. How would I let folks know I work on HVAC? Big sign on the coach? Then I have a commercial vehicle and all the hassles that go with it.

If anyone has done anything like this and has ideas, I would like to hear them.

I didn't mean to hijack the thread. Maybe I should start another one????

TOM
SOB (Some Other Brand) division
1995 Wanderlodge WB40
8V92 :D

Re: Dash A/C System Capacity and Pressures

Reply #16 –
Another thing that has not been mentioned. Engine rpm. Compressor's don't produce full capacity at idle. Needs to be at least 70% of road rpm. All of these numbers take a good 10 to 15 min to settle after adding gas. Also a txv or expansion valve won't work right with out full column of liquid. Txv are very for giving for over charge. It needs to have enough 134 to fill the last portion of the condenser and solid liquid all the way to the txv. My 2 cents
99 U320 40
97 Jeep wrangler toad
Now full time 8 months
Starting a new brick and wood home
Chattanooga, Tn

Re: Dash A/C System Capacity and Pressures

Reply #17 –
As far as I know, my dash A/C system has only suffered the 12VDC CONDENSER FAN failure about 2 years ago, had it checked Thursday, the tech pronounced it well, recommended not touching it due to the 49deg discharge temp at dash and the sweat at compressor & filter-dryer. Never hooked up the gauges, so either he knows A/C or is stupid, think he has a clue ;D
Dave M
 

Re: Dash A/C System Capacity and Pressures

Reply #18 –
Quote from: alan1958 –
Another thing that has not been mentioned. Engine rpm. Compressor's don't produce full capacity at idle. Needs to be at least 70% of road rpm. All of these numbers take a good 10 to 15 min to settle after adding gas. Also a txv or expansion valve won't work right with out full column of liquid. Txv are very for giving for over charge. It needs to have enough 134 to fill the last portion of the condenser and solid liquid all the way to the txv. My 2 cents
SOB (Some Other Brand) division
1995 Wanderlodge WB40
8V92 :D