Question about Dometic Rooftop RV Air Conditioner

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Domedic duotherm 13.5, comp runs but no cool. looked at unit to find schrader valve... not there. does piercing tool work for adding freon. all else appears on good condx. I think it just needs gas. does this recharging method work?

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  • icemelts
    icemelts May 15, 2010

    The problem here is - 'if you're low on gas,' you will need to find the source of the leak - otherwise you will just continue to have to add gas. Using a self-piercing valve is a common practice but not one that I recommend because these type of valves have a history of 'leaking.' It is always best to 'braze/weld' a valve in the system (if needed). The fact that your unit 'does not already have a schrader valve' installed in it - makes me doubt whether or not you have a leak now. I would make 'certain' that you were low on refrigerant before - installing a valve. Are you 'certain' that the compressor is running? Can you feel (with your hand) the compressor running? Is it cold, cool, warm or hot to the touch? If you can feel it running - is the suction line 'coming out of the compressor' (the bigger one which is usually wrapped in black insulation) cold, cool, warm or hot to the touch? If youcan feel it (compressor) running - is the discharge line 'coming out of the compressor' (the smaller one which is bare of any insulation) cold, cool, warm, or hot to the touch?

  • jalewis765 May 16, 2010

    the compressor runs. I can feel it. It is warm/hot to the touch. The suction line is warm and the discharge line is hot. I couldn't find a schrader valve. It should be obvious! I haven't done a bubble test yet to detect any leaks.



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From what you've told me - I'm now thinking you're low on freon, and my advice (to you) would be to call a Service Tech who can (a) find the leak (b) fix the leak and (c) re-charge the unit with the proper charge.

Of course you can always put a 'tap valve' on the AC and try to charge it yourself - but unless you have experience with this kind of work, and the proper tools, i.e. gauges and welder - you will not be able to repair the leak (it will almost invariably need welding) - or properly re-charge the unit.

In any event - if a 'leak' is your problem - you should be able to get a Service Tech to fix it for you - for around $100-$150 without a problem.

good luck!

Posted on May 17, 2010

  • SouthrnCruzn Sep 24, 2010

    Is the "tap valve" called a "schrader valve"?


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If compressor is comin on an you r sure then you are low on freon,there is not a port to put freon in you would have purchase a tap valve on the bigger line then apply freon to it ,thats located inside of the system

Posted on May 16, 2010


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1 Answer

3 way valve charging procedure

On the data plate there is the amount of freon and it's type. Usually F22. There is a tube connected to the compressor that looks like it's going no where. You need a piercing valve to attach to this tube. Evacuate the system for several hours. Add the freon. Use a pinch off tool to close off the tube between the piercing valve and the compressor. Remove the valve and use an acetylene torch and silver solder to close the end of the tube, NO other type of solder besides silphos is acceptable. Remove the pinch off tool and turn on the refriferator

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some cars I've worked on, if you overcharge the system, the compressor will shut off automatically if the pressure is too high by means of two or more pressure switches. Theoretically you won't damage anything because the system just won't run for more than a few seconds before shutting off again. You won't get any cooling of course if this happens.
I just used a small screwdriver the open the schrader valve (like how you'd let air out of a tire) until it was back at the correct pressure. EPA won't like you.
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If there is a leak or block in the system this won't work.

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How do I apply refrigerant?

******This is for educational purposes only. Handling, recovery and installation of refrigerant should be done by experienced, licensed technicians.******

The first thing you want to do is very that the problem you are having is caused by failure in the sealed system or something else. There are a multitude of checks you can do but we'll take the fast track.
1) Confirm that the compressor is running. Usually when the machine is low on freon, the compressor will run but not generate a lot of heat.
2) Remove the rear wall of the freezer and check the evaporator coils. If the coils are mostly aluminum, with a small section of frost or ice, then you have a freon issue, if completely covered in frost or ice, stop this route and diagnose for defrost issues.
3) Verify all fans are running, coils are clean.
Okay, if your check list all check good, then move forward with the sealed system diagnosis. The tools you'll need for checking are as follows:
1) HVAC guages, not automotive.
2) A piercing valve, some machines have an access port already, but most don't. I use a bpv-31.
WIth the machine running, locate the suction line coming from the compressor, it will be the bigger of the two. The smaller line is called a discharge line and is part of the high side of the freon loop. DO NOT MESS WITH THIS LINE. Once located, install the piercing valve, pierce the line, attach your gauge using the blue line, and open the valve. You'll now be able to see the pressure in the machine. You'll notice the outside numbers on the blue gauge, most in blue, the lower portion in green going to 30. The green means a vacuum. Your freon level should be anywhere between 5 and 10 on the blue side.
If your number is in the blue but way past 10 with the compressor running you have a restriction and you should stop and call a tech.
If your number is in the blue but below 5, you may have a micro leak.
If your in the green, the machine is in a vacuum and has a significant leak.
********* Note that knowingly adding freon to a machine with a leak without first repairing the leak is a federal offense, and carries a major fine, like $25,000.*******

Now, lets say the machine is in a vacuum, like at -10, or 10 in the green, we'll go to the next steps. If your gauges read like 30 in the green stop because thats a major leak. The tools you'll need for the repair are as listed:
1) Gauges
2) valve (already installed)
3) vacuum/recovery pump
4) freon
5) Freon can piercing tool since your only going to be able to but the small cans without epa certification.
6) recovery bag

Okay, turn the machine off. Leave the gauges connected and connect the yellow line to the recovery pump, with the pump connected to recovery bag. Turn on and open manifold (gauges) valve and vacuum all the old Freon out of the machine. Let the pump run for about 30 minutes.
Next, close the manifold valve, then turn the pump off. The needle on the gauges should be at 30 in green and not move. If for any reason the needle moves upward toward the 0 then you have a major hole somewhere and it has to be sealed.
If the vacuum holds, take the yellow hose off the pump, pierce the Freon can with the clamp (tool) and connect yellow hose to it. You'll want to unscrew the yellow hose at the manifold for a brief second to vent the air out, leaving only Freon. You don't want any moisture in the sealed system.
Now this is the part where you should have Freon scales, or a graduated cylinder to measure the amount of Freon. On the sticker on the inside of the machine, you'll see the amount that goes in the machine. Most cans are 10 oz . Most machines don't use that much, and over charging a machine will cause it not to cool.
Add your Freon by opening the manifold valve, and be sure the piercing valve is open as well. Get as close to the amount as possible. When done, close the manifold and watch the pressure to make sure it stabilizes and doesn't drop down. With the machine off it will be pretty high.
Plug the machine in and leave the gauges connected and watch the pressure on the gauge, it will drop, your concern is that the needle finally rest between 5 -10 in the blue.
If this is the case, Close the piercing valve, remove the gauges and your done.
******* Remember this a short hand version of Freon installation. This is nothing compared to actually having the right equipment, training and experience.

If you need further help, I’m available over the phone at

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