RV Propane Leak Test

A 30-foot travel trailer carries somewhere near 60 feet of LP tubing, and a soap bubble test reaches about six inches of it.

Those six inches are the two pigtails and the regulator inlet, because they are the only joints you can see without pulling a cabinet apart. The other 59 feet run behind the refrigerator, under the shower, and down through the belly pan. Every one of those joints can leak for a season without a single bubble forming anywhere you were looking.

The Short Answer

The test that covers the whole system is a timed pressure drop test: charge the lines to 11 inches of water column, bleed down to 8, and hold for three minutes with every appliance valve closed. Any drop at all is a failure. A $40 manometer and a test spud at the stove manifold get you there in about twenty minutes. Bubbles are for finding the leak after the drop test tells you one exists. Spending $100 to $200 at a shop buys the same reading plus a paper trail.

The Soap Test Only Reaches Six Fittings

Count them on your own trailer. Two at the pigtail-to-regulator connections, two at the cylinder OPD valves, one at the changeover knob, and one where the supply line leaves the regulator. That is the visible half of the system, and it is also the half that gets exercised every time you swap a cylinder, which makes it the half most likely to be re-tightened by hand and the least likely to be the problem.

The joints that fail are the ones nobody looks at. A flare nut at the furnace, worked loose over four seasons of frame flex. A compression fitting behind the refrigerator access panel that was never quite tight from the factory. The tee feeding the water heater, buried under the wardrobe floor. None of these will ever see a spray bottle, and all of them are downstream of the regulator, which means all of them are inside the volume a drop test measures.

There is a second blind spot that surprises people. A regulator with a torn diaphragm vents through a screened port on its own body, outside the coach. No interior alarm sees it, because the gas is outdoors. No bubble sits on a vent screen, because the screen is designed to pass gas. You can bubble every fitting on the A-frame twice and still be towing a propane leak that only shows up when the furnace calls for heat.

Method	What it actually proves	What it cannot see	Time	Cost
Nose	That gas is escaping somewhere in this general area	Leaks that pool under the floor, and any leak after the odorant fades in an old cylinder	Instant	$0
Soap bubbles	This one joint, under this nozzle, is tight	Everything behind a cabinet, inside the belly, and the regulator vent	10 minutes	$5
Electronic sniffer	Gas is present within a few feet of the probe	Whether the piping holds pressure; false alarms from solvents and aerosols	15 minutes	$30 to $60
Manometer drop test	The whole run from pigtail to the last appliance valve holds, or does not	Where the leak is. It tells you the system failed, not which fitting	15 minutes	$40 to $80
Shop test	The same reading, plus a written record and someone else's liability	Nothing, if they hand you the numbers	1 hour	$100 to $200

The sniffer earns its place on the third row for one reason only: it finds gas where a bottle cannot reach. It cannot confirm a system is tight, because a tight system is a pressure claim, not a concentration claim.

Running the Drop Test

Five steps, one gauge, and a phone. The order matters more than the tools, because two of the five failures in a DIY test come from starting the clock at the wrong pressure.

1
Shut the cylinders, then burn the lines down
Close both cylinder valves, then open one stove burner and let it burn out. That empties the lines instead of leaving them charged while you take a fitting apart. Confirm every appliance valve is off, including the refrigerator, which will call for gas on its own schedule and ruin a test you are not watching.
2
Tee into the stove manifold, not the regulator
Pull the burner cover, usually two Phillips screws, and fit a test spud to the manifold with a hose to the manometer. The manifold is downstream of the regulator, so it reads the entire distribution run: the tee at the water heater, the drop to the furnace, the line behind the refrigerator. Tee at the regulator outlet and you have tested the regulator and nothing else.
3
Charge to 11, then bleed down to 8
Open a cylinder for about five seconds and close it. Crack a burner valve and the needle climbs to 11 or 12 inches. Now bleed it slowly to exactly 8. That step is not cosmetic: at 8 the second-stage diaphragm is fully seated below its lockup point, so the regulator stops adding gas mid-test and the gauge is reading your piping instead of your regulator.
4
Photograph the needle at zero and at three minutes
A phone photo of the gauge is the only baseline you cannot misremember. Three minutes, no sun on the gauge, door closed, no appliances cycling. NFPA 1192 allows not less than 8 and not more than 14 inches for the test and calls any drop in the three-minute window a failure. Two or three runs is normal practice when the first one is ambiguous.
5
Locate, repair, then run it again
A drop means the system failed, not where. Bubble every joint you can reach, then take a sniffer behind the refrigerator, at the furnace, and along the regulator body. After the repair the system gets charged and tested again from zero. A repair that is not followed by a passing drop test is not a repair, it is a guess with a wrench.

The charge-and-bleed sequence is the step people skip, and it produces more false failures than any real leak. Charge the system and start the clock at 12 inches, and the second-stage diaphragm is still above lockup: it keeps feeding gas into your test volume, the needle sits high and then falls as the regulator finally seats, and you spend an afternoon hunting a leak that does not exist. Bleed to 8 and the regulator is out of the conversation before the timer starts.

What 8, 11, and 14 Inches Actually Mean

RV propane runs at a pressure so low that it is measured in inches of water, not psi. One inch of water column is about 0.036 psi. The whole system operates on roughly 0.4 psi at the appliance, which is why a pressure gauge from the hardware store is useless here and why a manometer, a U-tube or a diaphragm cell calibrated in inches, is the only instrument that reads the numbers in the code.

The figure that matters most is not 11. It is the spread between 8 and 14, because that band is what NFPA 1192 sets for the test, and 14 is also the ceiling on delivery pressure at the appliance connection. A regulator that will not settle below 14 with everything off has a seat that is not closing. It is not adjustable back to health. It gets replaced.

Reading	What it is	Where it comes from	If you are outside it
11 in. w.c.	Working pressure at the appliance inlet, gas flowing	NFPA 1192 5.2.19.3.12	Below 10 under full load means the regulator or the tubing is undersized
14 in. w.c.	Ceiling on delivery pressure at the appliance connection	NFPA 1192 5.2.19.3.12	A regulator that will not settle under this has a dirty or worn seat
8 to 14 in. w.c.	Test pressure band for the drop test with appliances connected	NFPA 1192 5.3.20.1	Charge, then bleed down to 8 before the clock starts
0.5 in. w.c.	Maximum pressure drop to any appliance inlet, everything at full fire	NFPA 1192 5.3.4.1	This is the sizing limit, not a leak allowance. A leak is zero drop
3 min, zero drop	Pass or fail window on the drop test	NFPA 1192 5.3.20.2	Any fall at all is a fail. There is no acceptable bleed rate
3 psi, 10 min	Rough-in test on the piping before appliances are connected	NFPA 1192 5.3.18.1	What a builder does. You cannot reach this test with appliances fitted
About 10 psi	First-stage outlet, between the two regulator stages	Regulator service data	Fixed by the first stage. Not adjustable at the appliance end

Clause numbers are from NFPA 1192, the standard RV builders are inspected against. Your trailer was tested to these before it left the plant, which is why a failure four years later is almost always a fitting rather than a design.

One more number that wrecks tests: propane expands about 1.5% for every 10°F. On an 8-inch reading, a 20°F swing between the start and the end of the test moves the needle roughly a quarter inch, which is enough to look like a slow leak on a gauge with no fine scale. NFPA 1192 requires the piping and the air to be at the same temperature before the test and held uniform through it. Practically: do not test a trailer whose copper run is in direct afternoon sun, and do not start at 80°F and finish at 60°F.

Two Full Cylinders, 74 Pounds, All of It Forward

This is the part that belongs on a towing site. The cylinders sit on the A-frame a few inches behind the coupler, which is the worst possible place for weight on a trailer: ahead of the axles, and at the end of the longest lever on the frame. Whatever the pair weighs lands on your ball almost undiminished.

The arithmetic is the same one that governs a water tank. The share that reaches the coupler is the distance from the axles to the load, divided by the distance from the axles to the coupler. Put the cylinders 6 inches aft of the coupler on a trailer with a 15-foot span and the arm is 174 of 180 inches, so 97% of the weight goes to the tongue. A 74-lb pair adds 71 lb of tongue weight. Nothing else you own does that.

Cylinder	Propane in it	Empty weight	Full weight	A pair	On the tongue
20 lb, refilled	20 lb	17 lb	37 lb	74 lb	71 lb
20 lb, exchange	15 lb	17 lb	32 lb	64 lb	62 lb
30 lb, refilled	30 lb	25 lb	55 lb	110 lb	106 lb

Empty weights are the tare stamped on the collar, 17 lb and 25 lb on the common steel cylinders. The last column applies the 174/180 lever ratio above. Aluminum cylinders cut 10 to 15 lb off the pair and are the one swap that improves the number.

The exchange cage deserves its own line. A 20-lb cylinder holds 4.7 gallons of propane, and the OPD valve stops the fill at 80% of the water capacity, which is exactly where the 20 lb figure comes from. Exchange programs pre-fill to 15 lb, about 3.5 gallons, and the cylinder is the same size. So a swap does not change what fits in the rack, it changes what the rack weighs by 10 lb for a pair. You will not feel that on a 7,000-lb trailer. On a 3,500-lb single-axle trailer already sitting at 12% tongue weight, that is the difference between a stable rig and one that wants to wander.

Note that the cargo carrying capacity sticker already assumes both cylinders full, so none of this is a payload surprise you need to re-budget. What it changes is the reading on a tongue scale, which is a number you should know before you buy the trailer rather than after. Run the pair through the tongue weight calculator with and without a full fill and see what it does to your payload margin. If the answer is tight, the CCC breakdown shows where the rest of the weight went.

Buy a manometer and a test spud, test at the stove every spring and again before the first cold trip, and photograph the gauge both times. Twenty minutes twice a year is the whole cost, and it is the only maintenance on this trailer that can kill you by being skipped. The trade-off is that the gauge tells you the system failed without telling you where, so keep a bottle of leak detector in the same drawer for the day it does.

Weight You Forgot You Were Carrying

Two full cylinders are 74 lb on the ball before you load a single bag. Run your own numbers, free and independent.

Tongue Weight Calculator Payload Calculator GVWR Calculator

Frequently Asked Questions

How often should you do an RV propane leak test?

Twice a year is the working interval: once in spring when the appliances come back into use, and once before the first trip that will run the furnace. Add a test any time a fitting is disturbed, an appliance is pulled for service, a pigtail is replaced, or a cylinder is swapped at an exchange cage. Rough pavement counts too. The system on a travel trailer rides on a frame that flexes for a living, and every flare nut on it is being worked by that flex a few thousand times a season.

Can you use dish soap to check for propane leaks?

Yes, diluted, and rinse it off afterward. Roughly one part soap to three or four parts water in a spray bottle, applied to a pressurized joint, and watch that spot for a full minute before moving on. Skip anything with chlorine or ammonia in it, since both attack brass and copper and will pit a flare nut you were trying to protect. The limit is coverage: soap proves the joint under the nozzle, not the system, which is why it belongs after a failed drop test rather than instead of one.

Why does my propane detector alarm with no smell of gas?

Three common causes, and only one is a leak. Low supply voltage makes most RV detectors chirp or alarm falsely, so check the battery and the converter output first. Aerosol sprays, cleaning solvents, and some new upholstery off-gas enough to trip the sensor for an hour. The third is real: propane is heavier than air, so it pools at floor level where the detector sits and your nose does not. A detector samples the air in one room. It cannot tell you whether the piping behind the refrigerator holds pressure.

What does a shop charge for a propane leak test?

One hour of labor, typically $100 to $200, and you should leave with a written gauge reading rather than a verbal all clear. Ask for the starting pressure, the ending pressure, and the hold time. A shop that cannot produce those three numbers did a bubble test and called it a leak test. The test itself is not the expensive part; the access is. Getting a manometer onto a stove manifold means pulling a burner cover, and on some floorplans that is most of the hour.

Will a leak test catch a failing regulator?

Sometimes, and never the way people expect. A regulator with a torn diaphragm vents gas through a screened port on the body, outdoors, so the interior alarm never sees it and no bubble will sit on a vent screen long enough to notice. What the drop test shows is a system that will not hold, with every fitting downstream still tight. A regulator that creeps upward past 14 inches of water column with all appliances off has a seat that is not closing, and regulators are replaced rather than repaired.

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