RV Rooftop AC

John T

Well-known Member
Anyone know what temperature the "cold" air being discharged out of a 2006 Duo Therm by Dometic "Brisk Air" non ducted rooftop RV AC should be ??? Mine doesnt "seem" cold enough so I placed a thermometer direct in front of it, yet (Yes the compressor is engaged and running full time) after 30 minutes its still around 61 degrees which I "think" isn't cold enough. NOTE this isn't any exact or perfect or scientific test, simply a thermometer one inch out of the discharge direct in its air flow, but of course the RV interior ambient conditions still cause an influence I'm sure. If I recall in automotive AC systems they discharge air in the 40+ degree range.

Thanks in advance

John T
 
(quoted from post at 16:23:07 06/18/18) Anyone know what temperature the "cold" air being discharged out of a 2006 Duo Therm by Dometic "Brisk Air" non ducted rooftop RV AC should be ??? Mine doesnt "seem" cold enough so I placed a thermometer direct in front of it, yet (Yes the compressor is engaged and running full time) after 30 minutes its still around 61 degrees which I "think" isn't cold enough. NOTE this isn't any exact or perfect or scientific test, simply a thermometer one inch out of the discharge direct in its air flow, but of course the RV interior ambient conditions still cause an influence I'm sure. If I recall in automotive AC systems they discharge air in the 40+ degree range.

Thanks in advance

John T
ypically expect near 20F delta (return to discharge) across evaporator.. with your 61 out, I would expect return air to be about 80F
 
Especially if the unit recirculates interior air, it is to warm I look fro discharged air to be between 40 and 45 degrees on most systems. This includes automotive and window air conditioners, as well as rooftop devices. Too cold and evaporator freeze-up will happen. Jim
 
Discharge air temp is a poor indicator of performance, as the evaporator can only remove X BTU of heat, thus air out depends on temperature of air in, humidity, and air volume in a perfectly operating system. Therefore, for a 40F outlet air, given evaporator delta of 20F (at the very best of conditions, 35F) that would be a cool ambient (return air) of 60 to 75F. If it could hold the interior at 60-75, then it would be working real well. On the other hand, his air volume is OK & his outlet is 61 and if return is 81 to maybe 96 then it is probably fine. Delta is everything, not outlet temperature.
 
Hello John T,

Temperature drop across the evaporator is about 20* lower then the ambient temp for the first 1/2 hour, which is really not long enough for a proper test. If you have 61* still showing After it has run over night, then there is a problem. Thermometer tip should be inside the air duct. After an hour or two it should be in the fifty's if all is well. It may take three to four hours though to get to the 50's. Easy thing to checks would be the condensate drain. Also make sure the filter is clean. If the condensate flow slows or the through is partially clogged, it will short cycle the unit, reducing the unit efficiency,

Guido.
 
I guess then if its 90 outside and my discharge air is 60 (Delta 30) IF IT AINT BROKE MAYBE I BEST NOT FIX IT ????? Still seems in my car even if its 90 outside my discharge air is near 40+ ???????????

John T
 
Hello John T,

Automotive a/c is not a good comparison. The capacity of a full size car unit can cool a whole house,

Guido.
 
John your sort of comparing a house unit to a car unit. Cat A/C system are made to cool a much bigger area due to them being hotter due to lots of windows and metal roof so a car A/C has to be able to cool more then a house unit can.
 
(quoted from post at 17:46:10 06/18/18) I guess then if its 90 outside and my discharge air is 60 (Delta 30) IF IT AINT BROKE MAYBE I BEST NOT FIX IT ????? Still seems in my car even if its 90 outside my discharge air is near 40+ ???????????

John T
fter it has cooled down and you are on re-circulate, not outside air, maybe?
 

when in vegas, with dual roof a/c's at 106 outside... the motor coach would sit around 80 degrees... at best. And thats two 15k btu units running. But single pane windows did not help. Units ran full time till around 3 in the morning before they would finally cycle. Then around 10am, they were back to full time again. Everywhere else they worked well, so I'm blaming it on vegas. :D


Most units will pull down a 20 to 22 degree drop, no more. And if your IN THE SUN, and not under a tree, you can very well be lucky to get it to 80. Measure air at the input register and output to get the drop. If around 20, the unit is fine. Bigger coaches are ducted but still doable if you measure at the duct closest to the unit for output temp and at the filter for the input temp. Putting a cover over the windshield and letting the wipers hold it on, will help. But a huge shade tree is best. The quarter inch of insulation is a bit lacking in the walls. Also the unit is heat soaked if turned off so it takes a couple of days to cool down the interior walls. A small outside mini-fan on the fridge coils is a must for the ammonia fridge.
 
(quoted from post at 21:15:57 06/18/18) 55 deg is the discharge temperature to shoot for.
K for a 75F return air. Just forget discharge temp....delta is the whole shebang!
 

You could add another unit if you have one unit its a poor boys AC in a motor home... I think you are doing good and about all you are going to get out of it BTDT...
 
The 40 degrees was 40 degrees below ambient. So, if outside is 100 the discharge air should be close to 60. That was the standard for R12.
 
Hello Janicholnson,

I have never measure 40 to 45* output temp on an A/C window or residential unit. Mid 50's is pretty normal. On residential units temp will also depend how far from the evaporator the it is taken. On a not so hot nor humid day, I have measured temps down to 52* on residential central A/C with R22 as the refrigerant,

Guido.
 
(quoted from post at 23:31:36 06/18/18) Hello Janicholnson,

I have never measure 40 to 45* output temp on an A/C window or residential unit. Mid 50's is pretty normal. On residential units temp will also depend how far from the evaporator the it is taken. On a not so hot nor humid day, I have measured temps down to 52* on residential central A/C with R22 as the refrigerant,

Guido.
top it with output temperatures! What is the matter with people? Don't know Jack Schitt about air conditioning or what? IT IS THE EVAPORATOR DELTA TEMP THAT MATTERS!!!!!
 
To deal with your question; a rooftop RV AC will not get colder than about 50 degree discharge air, it is the design more than temp differential. As long as the compressor is running the evaporator should be about 50 degrees. Same as a household AC. There is a frost sensor that will shut down the compressor if things go awry. The purpose is to keep the evaporator from icing up. The thermostat in the unit or on the wall has nothing to do with the evap temp other than to shut the compressor off when room temp meets setting

The new 15,000btu units exchange more heat but the big windshields need to be covered with that aluminum insulation stuff. The sun can put more heat BTU in an RV thru the windshield than can be removed.
 
(quoted from post at 07:43:01 06/19/18) Hello JMOR,

HA! you forgot to mention the evaporator's frost sensor...

Guido.
riginal poster said, " (Yes the compressor is engaged and running full time)". :wink:
 
Indeed. It was running all the time I was measuring temperature, it never satisfied the thermostat so it never dropped out leaving ONLY the fan like it does on days its not so hot and/or you changed the thermostat. T Stat works and compressor will drop out (fan still runs) once it gets cold enough, but it was running all the time I took temp readings.

John T
 

I don't remember it being referred to as DELTA TEMP but do understand measuring the temp difference across the evaporator. It is a VERY good indicator of system performance and state of charge... That is if you can get to the inlet and outlet at the evaporator...

I did google "EVAPORATOR DELTA TEMP " :D
 
(quoted from post at 15:04:26 06/19/18)
I don't remember it being referred to as DELTA TEMP but do understand measuring the temp difference across the evaporator. It is a VERY good indicator of system performance and state of charge... That is if you can get to the inlet and outlet at the evaporator...

I did google "EVAPORATOR DELTA TEMP " :D
Apparently "Greek" to many! :lol:
 
Hello Hobo,NC

I Nerver heard Delta Temp term used in the automotive industry, but have
in Air Conditioning tech talk. It is used to express the
difference between two measurements.
The family of HVAC INDUSTRY "deltas" in day to day slang
terms: Delta T ; Delta P ; Delta V; Delta H : and Delta C.
From air flow to pressure difference fast way to state the
two values,

GUIDO.
 
When I took the master course on Heating, AC, and Ref., the "delta T" between the contents of the tubes and the outside ambient was 30 deg. F. The evaporator coil output was 30 degrees cooler than the incoming ambient air and the condenser output was 30 degrees hotter than the temperature of the incoming ref. gas, aka evaporator coil temp. Being a closed system and essentially a "heat pump", you removed the rated amount of BTUs of heat from the incoming air per hour.....the cooler you cooled the room down, the cooler the inlet and outlet air...you can just do so much so fast.

If you shot the condenser coil (the one outside) with water, heat of evaporation would lower the temperature of the output liquid going back to the evaporator coil such that the capacity for a given equipment was increased (efficiency) and the cooling more effective. That's the reason for the slinger ring on window units. The condensate from the evaporator coil is deliberately slung on the condenser coil to increase the efficiency. That, the long freon runs between the condenser and evaporator coils, and the high cost of central systems and their maintenance is why I built my house around window units. Nothing wrong with them. My last one had a 11.6 eer as I recall. They got a bad name back when first introduced as being expensive to run. The real problem back then was that the houses (down here anyway) weren't insulated. Yes they weren't as efficient as today but the real problem was lack of insulation. My last bill for a 2700 sq ft house (all the modern conveniences) was 646 kWH @ $00.113/kWH + $25 connection fee for $98.50 and May was a hot month.

My rule of thumb has always been sweat. If there is adequate humidity in the air, if the suction line is sweating (after the system has been running long enough to have cooled the premises, either at the compressor input on window units, or at the input to the compressor unit for central AC, you have an adequate charge of refrigerant and there was no reason to get the gauges out and check pressures/temperatures. If you can park under a tree, do so. If you can park under a shed even better.
 
(quoted from post at 08:55:23 06/20/18) When I took the master course on Heating, AC, and Ref., the "delta T" between the contents of the tubes and the outside ambient was 30 deg. F. The evaporator coil output was 30 degrees cooler than the incoming ambient air and the condenser output was 30 degrees hotter than the temperature of the incoming ref. gas, aka evaporator coil temp. Being a closed system and essentially a "heat pump", you removed the rated amount of BTUs of heat from the incoming air per hour.....the cooler you cooled the room down, the cooler the inlet and outlet air...you can just do so much so fast.

If you shot the condenser coil (the one outside) with water, heat of evaporation would lower the temperature of the output liquid going back to the evaporator coil such that the capacity for a given equipment was increased (efficiency) and the cooling more effective. That's the reason for the slinger ring on window units. The condensate from the evaporator coil is deliberately slung on the condenser coil to increase the efficiency. That, the long freon runs between the condenser and evaporator coils, and the high cost of central systems and their maintenance is why I built my house around window units. Nothing wrong with them. My last one had a 11.6 eer as I recall. They got a bad name back when first introduced as being expensive to run. The real problem back then was that the houses (down here anyway) weren't insulated. Yes they weren't as efficient as today but the real problem was lack of insulation. My last bill for a 2700 sq ft house (all the modern conveniences) was 646 kWH @ $00.113/kWH + $25 connection fee for $98.50 and May was a hot month.

My rule of thumb has always been sweat. If there is adequate humidity in the air, if the suction line is sweating (after the system has been running long enough to have cooled the premises, either at the compressor input on window units, or at the input to the compressor unit for central AC, you have an adequate charge of refrigerant and there was no reason to get the gauges out and check pressures/temperatures. If you can park under a tree, do so. If you can park under a shed even better.
efinitely some merit in your words. The refrigerant lines loss in central air systems are minor, however, compared to the supply & return duct losses that we suffer here in the south & west where most duct work and even the units themselves are installed in the attic. The really bad is the flex duct with R-4.2, R-6, R-8 insulation! Builders of course opt for cheap & almost always us R-4.2 even in million dollar homes. Crazy, when the losses are so great that the cost difference to move up to R-8 could be recovered in one month of electricity savings in July or August. It is not uncommon in such large homes to see 30, 40, even 60 foot duct runs and when attic temps reach 115-130 degrees, sometimes 50% to as much as 70% of unit capacity is lost in supply & return duct losses. Just let that sink in for a moment..........you pay for 36, 000 BTU but you get 18,000 BTU (or less) delivered to you living space! In the best of cases in smaller homes, it can be a 20% loss. Rather than paying extra for a couple of SEER points on a unit, money would be better spent on better insulated duct work. Of course, you duct runs are short in your home! :)
 
Hello Texasmark1,

The was one time this SMART guy decided that spraying water on the condenser would boost its performance. It only worked for awhile though. Minerals in the water ate the coil fins.........OOPS! Just one of the story,

Guido.
 
A quick dip in anticorrosive materials (Alodine for one) by the mfgr. easily solves that problem. Question on any of these systems, since with any refrigerated AC system the evaporator coil sweats profusely removing the humidity from the incoming air regardless of what the condenser coil is subject to..........is....do they or don't they (as it can be obtained in a clear coating).......apply the corrosion inhibitor? Even without it, systems endure many years of "abuse" and still work adequately.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top