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Old 05-25-2013, 03:34 PM   #1
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AC question, it must be summer

What a beautiful spring we had here in the Houston TX area at least for a few weeks, mild days with low humidity. But summer is here now, 90+ degrees and high humidity. Here is my AC question concerning my 6 year old Dometic Penguin 13,500 BTU. At peak operating conditions or say when new with the thermostat set to max cold on a hot 92 degree humid day. What should the temperature range be coming out of the vent. Not the actual temp inside but the coldest air just as it comes from the vent.

Mine does not seem that cold, its cool around 74 degrees. But during the day the interior is not very cool

Thanks
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Old 05-25-2013, 03:43 PM   #2
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An air conditioner "professional" once told me that an air conditioner system would only drop the temperature by 20 degrees from outside temp.

A refrigeration system like is in your car is different and will go to almost freezing temp.

The difference is the type of refrigerant and the pressures used.
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Old 05-25-2013, 03:49 PM   #3
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A/C temp. diferential

I'm not an A/C tech. but...............

IIRC, an A/C should produce a temp. differential between 20 and 40 degrees from ambient inside temp. You should get a minimum of a 20 degree differential from smaller (BTU) or less efficient units.

So if it is 92 F, inside the trailer, in the shade, the cooled air coming out of the unit should be 72 F or less.

Less BTUs will require the unit to run longer to cool down the interior to a specific temp., if it ever can. Just depends on heat load coming into the trailer.........

Also - The temperature differential will be inversely proportional to the relative humidity. The condensation out of very humid air will build up on the evaporator and act as an insulator.

So the higher the humidity, the less the differential.

(EDIT) - beat to the reply by SteveH while editing!
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Old 05-25-2013, 04:09 PM   #4
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Originally Posted by SteveH View Post
An air conditioner "professional" once told me that an air conditioner system would only drop the temperature by 20 degrees from outside temp.

That is what I have heard as well, so at 92 outside you could expect the inside temp to settle in at around 72 to 75 degrees. But to achieve this the air just as it comes out the vent would need to be in mid 60 degree range since it starts to warm as soon as it passes into the interior space. I am at around 74 degrees when it comes out the vent and around 80 on the actual interior temp. At night with lower outside temp it cools fins as well as in the morning hours until the sun gets high. So what is the diagnosis of the unit, low coolant, dirty coils, thermostat, bad compressor, any thoughts.

Thanks
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Old 05-25-2013, 04:39 PM   #5
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A/C differential

The key is the temp. differential between the air on one side of the evaporator versus the other - not the "outside" temp.

The temp. differential of the air being drawn into the unit versus what is being discharged is what should be measured.

BTW - Most RV A/C units are "sealed" - no way to check the freon charge or add to it without adding valves - and don't go there unless you have a real A/C "Pro".

Most common problems:

Dirty filter
Dirty evaporator coils / fins
Return air mixing with "cooled" air (bad plenum sealing)
Electrical problems - either from a unit control standpoint or a unit supply aspect.
Unit not large enough (BTU rating) to handle the job.

PS - I am in Texas and have a 15k unit on a 27' 1966 Avion (still factory foam insulated!) and it gets COLD in there.........
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Old 05-25-2013, 05:28 PM   #6
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OK guys,

Here is the straight dope right from the manufacturers.....and NO, I didn't sleep in a Holiday Inn like some folks before they post their 'opinions'. This is from an A/C Professional who does this every day.

ANY roof air conditioning unit will show a delta T or temperature differential of 18F to 22F measured at the intake or return air grill INSIDE THE RV and the exhaust air coming from the evaporator INSIDE THE RV.

OUTSIDE TEMPS HAVE NO BEARING ON THE PERFORMANCE OF THE UNIT!

You get a maximum temperature drop of 22F from the inside air being sucked into the evaporator to the cool air coning back out of the evaporator. If it is 95F inside the trailer, the minimum temps you will see coming from the A/C unit will be in the neighborhood of 75F. As the trailer cools down, the temperature differential stays the same, but the intake air is gradually getting cooler, so the exhaust air temps will follow.

If your temperature differential is not in this range, you might have lost some refrigerant due to a leak, have dirty condenser of evaporator coils or perhaps the compressor is not even starting due to a burnt or defective start capacitor or PTCR (that's positive temperature coefficient resistor, also known as a motor starter).

These units are hermetically sealed at the factory and have NO PROCESS VALVES to allow the use if pressure gauges to determine the state of charge remaining in the unit. It is not economically feasible to add valves to these systems to " just give it a shot of Freon". The process is as follows:

Remove unit to bench
Add piercing valves and vacuum out any refrigerant in the system (it is a Federal crime to knowingly allow the discharge of ANY refrigerant gas into the atmosphere. Fines are $10,000 per occurrence!)
Remove piercing valves and solder permanent Schrader valves in their place
Re-pressurize the system with nitrogen and dye to check for leaks
Find and repair leaks if possible (If not possible, like a leak inside one of the could, you are done at this point and need a new unit)
Evacuate any nitrogen and purge the system again with pure nitrogen and pressure test.
Remove nitrogen after a positive pressure test and re-charge with appropriate refrigerant (R-22 or R-410A) in the quantities listed on the data plate
Remove gauges and re-start the unit to test.
Re-install unit with new gasket seals

Note* All of this is happening on a test bench AFTER the unity has been removed from the trailer. It must be re-installed after completion.

As I said, it is no longer cost effective to do all of this when you can get a new factory fresh unit with a warranty for the same or less than completing the above precess.

The exception is the Armstrong units, which were made like tanks AND have the appropriate service valves on them.

CLASS OVER! Any questions?????????????
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Old 05-25-2013, 06:59 PM   #7
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OK guys,

Here is the straight dope right from the manufacturers.....and NO, I didn't sleep in a Holiday Inn like some folks before they post their 'opinions'. This is from an A/C Professional who does this every day.

ANY roof air conditioning unit will show a delta T or temperature differential of 18F to 22F measured at the intake or return air grill INSIDE THE RV and the exhaust air coming from the evaporator INSIDE THE RV.

OUTSIDE TEMPS HAVE NO BEARING ON THE PERFORMANCE OF THE UNIT!

You get a maximum temperature drop of 22F from the inside air being sucked into the evaporator to the cool air coning back out of the evaporator. If it is 95F inside the trailer, the minimum temps you will see coming from the A/C unit will be in the neighborhood of 75F. As the trailer cools down, the temperature differential stays the same, but the intake air is gradually getting cooler, so the exhaust air temps will follow.

If your temperature differential is not in this range, you might have lost some refrigerant due to a leak, have dirty condenser of evaporator coils or perhaps the compressor is not even starting due to a burnt or defective start capacitor or PTCR (that's positive temperature coefficient resistor, also known as a motor starter).

These units are hermetically sealed at the factory and have NO PROCESS VALVES to allow the use if pressure gauges to determine the state of charge remaining in the unit. It is not economically feasible to add valves to these systems to " just give it a shot of Freon". The process is as follows:

• Remove unit to bench
• Add piercing valves and vacuum out any refrigerant in the system (it is a Federal crime to knowingly allow the discharge of ANY refrigerant gas into the atmosphere. Fines are $10,000 per occurrence!)
• Remove piercing valves and solder permanent Schrader valves in their place
• Re-pressurize the system with nitrogen and dye to check for leaks
• Find and repair leaks if possible (If not possible, like a leak inside one of the could, you are done at this point and need a new unit)
• Evacuate any nitrogen and purge the system again with pure nitrogen and pressure test.
• Remove nitrogen after a positive pressure test and re-charge with appropriate refrigerant (R-22 or R-410A) in the quantities listed on the data plate
• Remove gauges and re-start the unit to test.
• Re-install unit with new gasket seals

Note* All of this is happening on a test bench AFTER the unity has been removed from the trailer. It must be re-installed after completion.

As I said, it is no longer cost effective to do all of this when you can get a new factory fresh unit with a warranty for the same or less than completing the above precess.

The exception is the Armstrong units, which were made like tanks AND have the appropriate service valves on them.

CLASS OVER! Any questions?????????????
Thanks Lew. I will check my temp differential as you outlined. My first course of action is the clean the condenser of evaporator coils. I did this on another unit years ago but I don't remember exactly what It did. What are the steps and any coil cleaner that you would recommend. Thanks again
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Old 05-25-2013, 07:17 PM   #8
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Hey, don't worry about the actual temp of the air coming out: it's working, right? Which is more than I can say for the third brand-new AC unit installed since last July. And you don't mention water dumps on your carpet when it is running, either. Count your blessings!

We're sitting here sweltering in the heat and humidity of College Station, TX, as our most recent (3 wks ago) installation of a brand-new Dometic 15,000 BTU AC unit has stopped running after thoroughly soaking our carpet for about a 2-ft radius.

We're leaving tomorrow to return to blessedly cool Idaho, so will have to pursue the warranty issue there. For our 9 mos here in TX, all three of our new AC inits have not given us a total of much running time. We noticed that although they carry the Dometic name, they are made in China. Any connection there? Ya think?

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Old 05-25-2013, 09:09 PM   #9
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Hey, don't worry about the actual temp of the air coming out: it's working, right? Which is more than I can say for the third brand-new AC unit installed since last July. And you don't mention water dumps on your carpet when it is running, either. Count your blessings!

We're sitting here sweltering in the heat and humidity of College Station, TX, as our most recent (3 wks ago) installation of a brand-new Dometic 15,000 BTU AC unit has stopped running after thoroughly soaking our carpet for about a 2-ft radius.

We're leaving tomorrow to return to blessedly cool Idaho, so will have to pursue the warranty issue there. For our 9 mos here in TX, all three of our new AC inits have not given us a total of much running time. We noticed that although they carry the Dometic name, they are made in China. Any connection there? Ya think?

Vivian
A bit of Dometic history:

Several years ago, all of the A/C units Dometic sold in the US were made in Mexico in a big, company owned plant. They had a devastating fire that totaled the place, and moved production to the US.....Indiana to be precise.

I had experienced few manufacturing defects when the units were made in Mexico, but these defects seemed to rise considerably when production shifted to the USA. This is NOT something that I expected. I am also sad to say that the defect rate on new installations has again dropped, now that the units are assembled in China. As Dometic is a large, world wide conglomerate, they source their parts from around the globe for their products.

Believe me, I surely am no fan of anything Chinese (except perhaps the chinese-style food that we eat) , and can substantiate their shady business practices and downright unethical dealings with many US companies from friends that do business there. The thing is.....these units have been better out of the box than their US counterparts. I have no explanation other than perhaps that the US workers don't care. But then RVP Coleman makes their A/C units in Kansas and they are......for the most part.....well built and good running units.

SO you are on your 3rd unit. Doesn't sound right, but then again, I'm not defending Dometic or their products. I would surely look at the installation process to be certain that they are operating correctly and within specifications.

As for the water dumps, I would inspect the connection from the drain pan to the interior drain hose and then the hose itself. It is a simple, thin wall non-reinforced piece of light weight tubing that is very susceptible to kinking and clogging, and you should already know my opinion of the drain pan!
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Old 05-27-2013, 02:48 PM   #10
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I was able to take a look under the outside AC covers today. The coils seems to be fairly clean nothing caked on them just dusty. I brushed them off a little but did not want to drive over to the wash area here in storage that has a water hose so I put the outside covers back on and took off the inside ceiling covers. What I saw was just a little bit of duck tape sealing a few place but really not doing much good. Air was passing in holes and cracks from the supply air side to the return side. I cleaned everything up and insulated with rubber foam strips and Reflectix and then used the thin silver duck tape to seal everything around the edges and duck openings. The filter is not very good and there may be more dirt and stuff in the Evaporator coils than what I was able to see. The thin filter material that is there now is sucked almost up the coils when the AC is on so it is not doing anything to filter the return air. I was only able to finish the front AC today but I did notice much better air flow out of the vents and was able to feel cooler air flow in the front portion of the coach. Based on Lew's instructions I took my temp readings and had 85 degrees at the return grill and around 69 degrees at the supply vent, 16 degrees difference. After awhile the coach did cool down to below 85 degrees.
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Old 06-05-2013, 09:36 AM   #11
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Originally Posted by 7GenTex View Post
The temperature differential will be inversely proportional to the relative humidity. The condensation out of very humid air will build up on the evaporator and act as an insulator. So the higher the humidity, the less the differential.
If high humidity effects the cooling because the condensation builds up on the evaporator coils would a product such as Ultra Ever Dry when applied to the coils prevent this from happening. It seems to be amazing stuff, watch the video.


Ultra Ever Dry - Waterproofing Coating - Industrial - Tools - Clothing

The Official Ultra-Ever Dry Product Video - Superhydrophobic and oleophobic coating - YouTube
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Old 06-05-2013, 12:41 PM   #12
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If high humidity effects the cooling because the condensation builds up on the evaporator coils would a product such as Ultra Ever Dry when applied to the coils prevent this from happening. It seems to be amazing stuff, watch the video.


Ultra Ever Dry - Waterproofing Coating - Industrial - Tools - Clothing

The Official Ultra-Ever Dry Product Video - Superhydrophobic and oleophobic coating - YouTube
It does look amazing.

Coating the evaporator coils with anything would insulate them and adding a layer of insulation on the evaporator coils might not be helpful, I think it would be the same problem as the condensation.
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