LP Gas System and RV Furnace Service

Regulators, pressure, and why appliances misbehave together

RV appliances are designed to run on 11 inches of water column, which is a little under half a pound per square inch. A two-stage regulator drops tank pressure, which varies with temperature from roughly 40 to 200 psi, down to that figure in two controlled steps. When the second stage drifts or the internal diaphragm hardens, delivery pressure sags under load, and the symptom is not one broken appliance. It is the furnace, the water heater, and the range all behaving badly at once, usually worse when two of them run together.

Regulators are wear items with a service life around ten to twelve years, and nobody replaces them on schedule because they have no moving parts an owner can see. The vent on the underside is the tell: if it is corroded, packed with mud dauber nest, or pointed upward because the regulator was mounted rotated, it cannot breathe and the diaphragm cannot regulate. We set pressure with a manometer to the appliance manufacturer specification, not by feel, and we verify it again with the largest burner running.

Automatic changeover regulators on twin cylinder trailers add another failure point. The changeover mechanism is supposed to draw from the service cylinder until it empties and then switch to the reserve while flipping an indicator to red. When the internal seat wears, it drafts from both cylinders at once and the indicator never changes, which is how owners end up with two empty tanks and no warning on a cold night. Testing that behavior takes a few minutes and is not part of any routine service most shops perform.

What a real leak test involves

A proper test isolates the system, pressurizes it to a known value, and watches it hold for a timed interval on an instrument. We use a manometer on a test port, bring the system to specification, shut the supply valve, and time the decay. Any measurable drop over the interval means gas is leaving the system somewhere, and the test tells you that before you have any idea where. Soap solution then finds it. Soap solution alone, without the timed test first, only finds leaks large enough to bubble.

The connections that fail are predictable. Flare fittings on copper lines work harden and crack at the bend from years of vibration. Rubber pigtail hoses on trailers dry out and check at the crimp, and they carry a date stamp that most owners have never looked at. Threaded joints sealed with the wrong compound, or with tape on a flare that never needed it, weep slowly for years. Appliance connections behind the refrigerator and under the cooktop are the least accessible and therefore the least often checked.

If the test fails and we cannot find the leak at accessible joints, the line gets sectioned. Gas plumbing on a coach runs through chassis rails, along the belly, and up into cabinets, and a rub-through where a line crosses a frame member is a genuine and common find. We do not release a vehicle with a failing test on the theory that it is probably a small one. Either it holds pressure or the job is not finished.

How a forced-air furnace actually sequences

A Suburban or Atwood forced-air furnace runs a fixed startup sequence, and knowing the order turns a vague complaint into a specific part. The thermostat calls, the blower spins up, and once it reaches speed the airflow closes a sail switch. Only then does the control board open the gas valve and fire the igniter. If the flame is not sensed within the trial period, the board locks out and the blower purges. Nearly every no-heat complaint maps to one identifiable step in that chain.

Blower runs and nothing lights is the classic sail switch or airflow problem. Lint and dog hair pack the intake, ducts crush behind a slide or under a bed, and the wheel loses speed until the switch never closes. Owners hear the fan and assume the fan is fine. It is turning, but it is not moving enough air to satisfy a safety device that exists specifically to prevent firing without ventilation. Cleaning the wheel and clearing the return path fixes this more often than any part replacement does.

Ignition without sustained flame points at the gas side or the sense circuit. A partly blocked burner orifice, a spider web in the venturi tube, a weak igniter gap, or a corroded flame sense electrode will all produce a light-off followed by a lockout a few seconds later. Combustion air intake and exhaust share a concentric wall vent, and a wasp nest in it will choke combustion in a way that looks electrical. We pull and clean the burner assembly as a matter of course rather than reaching for a control board.

The limit switch is the last one and the one worth respecting. If the heat exchanger reaches an unsafe temperature because airflow is restricted, the limit opens and shuts off gas. A furnace that heats for a few minutes, quits, and then restarts after cooling is telling you it is overheating, and the correct answer is to restore airflow. Bypassing a limit switch to stop the cycling is something we occasionally find on used units, and it is the single most dangerous shortcut in this trade.

Combustion products, detectors, and the paperwork

A cracked heat exchanger lets combustion products into the cabin air stream, and on a sealed-combustion RV furnace it is uncommon but not impossible after decades of thermal cycling. Sooting at the exhaust vent, a yellow lazy flame instead of a crisp blue one, and a carbon monoxide detector that alarms only when the furnace runs are all signals we take at face value. Any of them stops the job and turns it into an inspection before the unit goes back to a customer.

Detectors themselves expire. Carbon monoxide and propane detectors carry a sensor life around five to seven years and a printed date, and an expired unit is worse than none because it produces confidence without protection. The propane detector belongs low in the cabin since the gas is heavier than air, and the carbon monoxide detector belongs at breathing height. We check placement and date as part of every gas job and replace what has aged out.

The regulatory side is straightforward and it matters. The shop holds California Bureau of Automotive Repair license ARD00288521 and EPA identifier CAL000367879, and gas work is documented on the invoice with the pressures recorded. If you are selling the unit, buying one, or satisfying a park or storage facility requirement, that written record is the thing they actually want to see. Nothing here happens mobile. Gas testing needs a controlled environment and a bay with ventilation, and we do it all at the Yorba Linda facility.

What is included

Timed instrument leak test

System pressurized to specification on a manometer and held for a timed interval, with the reading recorded.

Regulator inspection and adjustment

Delivery pressure set under load, vent condition checked, and changeover function verified on twin cylinder setups.

Burner and venturi service

Orifice, burner tube, and concentric vent cleared of debris, nesting, and combustion deposits.

Furnace sequence testing

Sail switch, limit switch, igniter, gas valve, and flame sense checked in the order the board runs them.

Duct and return path clearance

Crushed runs and blocked returns corrected, since restricted airflow trips the safeties that stop the heat.

Detector placement and date check

Propane and carbon monoxide detectors verified for position and replaced when the sensor has aged out.

Signs you need this work

• The furnace blower runs continuously but the burner never lights
• Heat for a few minutes, then a shutdown, then a restart on the same thermostat call
• A rotten egg odor near the cylinders or inside a bay after the unit sits
• Yellow tipped or lazy flames on the cooktop instead of crisp blue cones
• Two full cylinders that both go empty at the same time with no changeover indication
• A carbon monoxide alarm that sounds only while the furnace is running