By Paul Unmack, PE-CSE, ME #116483
In my September/October article, I discussed how the cooling unit on your Dometic- or Norcold-type refrigerator works. I gave the example of the Hallstatt salt mines in Austria using water pipe lines to transport salt from the mines to the transportation depot. A distillation process was used to separate the salt from the water. Your RV refrigerator uses heat to separate the ammonia from the water in your cooling unit by a distillation process. But what can possibly go wrong? This article will focus on the LP gas heater part of the cooling unit and discuss what could result in bad hot dogs and warm beer in your fridge.
Heat to Cool
In my previous article, I stated that heat drives the entire refrigeration process. It would follow that, if your refrigerator does not get the heat it needs, it will not work. There are two methods to heat your refrigerator’s cooling unit. One is the LP (liquid propane) gas which heats with a flame; the other is using electrical heating elements. Therefore, your refrigerator has a built-in method to test the cooling unit to see if one of the heat sources is not working.
Test Using Both Heating Modes
In order to test your refrigerator’s performance, you should run the refrigerator on either LP gas or the electric heating modes, one at a time. If the refrigerator performs properly on one heat source but not the other, we can deduce that one of the heat sources has a problem. Let’s say you plug into shore power and your refrigerator is cooling to 38°F. But when you turn on the LP gas mode the temperature rises to 60°F within a few hours. It is quite obvious something is wrong with your LP gas system. Following, we will discuss how the LP gas system works so that you, or your favorite RV tech, can get your refrigerator up and running. (My next article will cover the opposite scenario with the refrigerator working fine on LP gas, but the electric heat source does not function properly.
Overview of LP Gas System
The liquid propane (LP) gas system consists of a tank to hold the LP, a regulator to control the gas pressure and a burner to heat the refrigerator cooling unit. The boiler housing contains the flue (chimney) for the gas flame, and inside the flue is a part called the baffle. The purpose of the boiler housing is to contain insulation that keeps the heat on the boiler process tube.
LP Gas Flame
There are two states for the gas flame, off or on. If the gas flame does not light, one of the elements for the fire triangle is not present, that is either oxygen, fuel, or ignition is not present. We start off by hoping that oxygen is present; otherwise, you will have another set of problems not covered by this article. To test if gas is present, the first check would be to feel the gas on/off valve to see if it clicks when the refrigerator is turned on in the gas mode. If there is 12 volts present at the gas valve and there is not an audible click from the gas valve, the valve may need replacing. If there is a click from the gas valve, the igniter should start arcing with a “snap-snap-snap” sound every few seconds or so. The igniter should be adjusted according to the service manual in order to ignite the gas flame properly. As a rule of thumb, a 3/16-inch gap should be between the end of the igniter and the burner. If your gas valve is opening and you have an arc, the gas pressure should be tested next.
Troubleshoot LP Gas Regulator
Naturally, a good supply of fuel is critical for the function of your refrigerator. The LP gas regulator keeps the gas at a constant pressure throughout your RV. If the regulator has a problem, your fridge will not work on LP gas properly.
So how do you test your regulator? First, you have to have an instrument that measures the gas pressure. This instrument is called a manometer. If you Google the term “manometer,” you will see that they are easy to make out of a section of see-through vinyl tubing. Most RV techs attach their manometer to the RV stove-top burner jet to read the RV plumbing gas pressure. The specified pressure is 11 inches of water column, but I have seen a published acceptable pressure range from 9.5 to 12.5 inches of water column. I set my regulator to exactly 11 inches of water column. That is, the LP gas pressure in your LP gas plumbing should be able to push the water within a manometer to a total height of 11 inches. If you read 11 inches of water column, jump to the Regulator Load Test section.
Tip: Some LP gas regulators are affected by altitude. If your regulator is at one end of the acceptable pressure range given above, it may drift outside of this range when altitude change is taken into account. Thus, if you are camping at 10,000 feet of altitude and your fridge is not working, check your LP gas pressure.
If the pressure is not at 11 inches or within specification, most quality LP gas regulators may be adjusted. If there is not an adjustment, the regulator needs to be replaced. Referring to the figure above, a drawing shows a typical propane tank with a regulator and attached manometer. Generally (but not always), on the top of the regulator there is a plug that allows you to gain access to the regulator adjustment screw. Your U-shaped vinyl tube needs to be at least 20 inches tall and you need to mark the center of the tube at 10 inches. Fill water to the 10-inch mark and then attach the manometer to the stove jet. Turn on your propane tank, then go into your RV and turn on the burner that your manometer is on. You will see the water pushed down 5.5 inches on the regulator side and pushed up 5.5 inches on the side that is vented to the air (11 inches) if the regulator is adjusted correctly. Here is where the fun can start; it takes two people using this method to adjust the pressure, one reading the manometer and one turning the adjustment screw on the LP gas regulator. I turn the regulator adjustment screw while counting the turns and keep a piece of paper handy to show the direction you turned the screw and how many turns. This way, you can always back out of any adjustment you have made in order to start over. If you turn the adjusting screw and nothing happens, the regulator may need to be replaced.
Helpful tip: If you keep smelling propane in your LP tank compartment, and you have bubble tested all the plumbing, check the regulator vent. I hold my finger over the vent and apply the bubble solution around the regulator and my finger. If you see bubbles at the diaphragm vent hole, the regulator is leaking and it needs to be replaced.
Regulator Load Test
Now that you have confirmed that your regulator is set at 11 inches of water column, it is time to see if the regulator can handle a load. Simply turn on your stove-top burners and watch the water level in the manometer. If the water level drops by more than an inch or so, the regulator is not maintaining the pressure properly. Now, turn off the burners. Did the water level return to 11 inches?
As a rule of thumb (not cast in stone), if the pressure changes by more than one inch when there is a load on the regulator, it needs to be replaced. If, when the load is taken off of the regulator, the pressure does not return to a value of around 11 inches of water column consistently, the regulator may have a lot of wear on it and it needs to be replaced.
LP Gas Safety
If you do not feel at ease working with flammable gas, then hire a qualified technician. Here is a partial list of things I do whenever working on the LP gas system:
• Be sure to turn off the main gas valve at the propane tank when working on gas fittings.
• When turning on the propane, only turn the valve on ¼ turn. This will help you to turn off the gas quickly if things get exciting.
• Get one of Mac the Fire Guy’s fire extinguishers nearby, and know how to use it.
• After the LP gas is turned off at the propane bottle, turn on the stove-top burners and bleed off the gas pressure in the pipe lines by burning off the gas rather than allowing flammable raw gas to escape.
• Whenever an LP gas line is being worked on, always bubble test the fittings to see if there are leaks. Soapy water and an old toothbrush work for this purpose.
LP Fuel Filter
Now that the system LP pressure has been tested, you’ll need to see if the refrigerator LP fuel filter is plugged or not. (Yes, there is a fuel filter on most refrigerators.) The fuel filter is located between the LP gas inlet and the gas on/off valve. There is a test port on the side of the gas valve assembly, and the plug can be hard to remove. If you can remove the test port plug, connect your manometer and see if you have your 11 inches of water column when the flame is lit. If the system pressure is at 11 inches and the reading at the test port is below this value, the filter is plugged. Turn off your gas at the tank and remove the tubing connector at the gas inlet to the valve.
For many Norcolds, there is a foam filter just inside the valve assembly. The foam can be fished out with a sharp instrument and then use 98% isopropyl alcohol to clean the filter. On Dometic refrigerators, the filter is a ceramic filter. I flood the gas valve with the alcohol repeatedly to clean out the filter because it cannot be removed. Reassemble the gas valve, bubble test for gas leaks and then recheck the pressure to confirm that you have fixed the problem.
Flue Tube and Baffle
Finally, the fridge manufacturers recommend cleaning the flue tube annually. The purpose is to clean out any rust, which would prevent heat transfer into the process tube. This job is best done with two people, one to clean the flue tube, and the other to use a vacuum cleaner to suck up any rust that may happen to drop down the tube, thereby keeping the rust out of the burner assembly.
In order to clean the flue tube, you have to gain access to the top of the boiler housing. There is usually a sheet metal cap on the top of the flue tube that can be pulled off by hand. Once the cap is off, there should be a flue baffle attached to a wire. There should be a baffle in the flue tube. Because it can be left out by someone working on the fridge, if there’s not a baffle in the flue tube, you’ve found a problem. The refrigerator will not operate correctly on LP without this part.
Remove the baffle so that a brass or bottle brush can be used to clean the inside of the flue tube. I am told that a 12-gauge shotgun cleaning brush works great for this task. A word of caution; It is with the brush that most folks bend the igniter electrode described above, so be careful. If you do bend the igniter, please readjust the arc gap. This is another important reason to use two people to clean the flue. The person on the bottom cannot only suck out the rust that drops down the flue tube, they can also tell you that the brush is at the bottom end of the flue tube before it hits the flame ignition electrode.
As always, feel free to contact my wife, Mao, and me with questions. Further, the ARP Control can be used to determine if there is a problem with the electric or the LP gas heaters. (See ad on this page.) This is because the ARP Control measures the boiler temperature. By using the boiler temperature reading from the ARP Control, you can immediately determine if the proper amount of heat needed to drive the refrigeration process is available.
Paul and Mao Unmack are mechanical engineers. Paul ran an automotive repair business in Red Lodge, Montana, for 20 years before receiving his engineering degree. He has practiced nuclear, fire suppression and industrial process control systems design. Mao designed pressure vessels for ammonia plants in China for 12 years, then came to the U.S. to get a master’s of welding engineering. She designed biodiesel plants and worked for a government-funded research and development organization. Paul and Mao run the entire ARP Control business while taking on engineering consulting gigs.