Fifth Wheel with Overpressure and Sooted Water Heater

RV appliance manifold pressure is nominally 11 in. WC, with a small tolerance. A flowing reading of 14.0 in. WC and a lockup of 15.0 in. WC is roughly 27–36% high, which is well outside acceptable. The manometer, not the flame, is the authority.

A two-stage regulator controls outlet pressure by balancing the outlet force against a spring and atmospheric pressure on the vent side. The vent must breathe. With a mud dauber nest plugging the vent, the chamber cannot reference atmosphere, so the regulator loses regulation and can creep high or become erratic. High pressure over-fires the burner orifices: the flame becomes lazy and yellow, lifts or floats, and deposits soot because the gas velocity and air entrainment no longer match the orifice rating. The same overpressure explains the tall soft stove flames.

The faint gas smell may be from the over-fired water heater, a slightly loose test port, or a pigtail connection disturbed by the previous adjustment. Do not assume the regulator alone; the smell and the soot mean the appliance must be inspected for a blocked flue, cracked burner, and carbon monoxide production in addition to the regulator repair.

Safety first. Shut both cylinder valves, extinguish all ignition sources, ventilate the coach, and tag the system out of service. Do not test with a flame. Wear eye protection and use an approved leak-detection solution.

Repair sequence.

1. Remove and replace the regulator with the correct capacity, orientation and vent direction. Do not attempt to clean and reuse a regulator with a blocked vent or a 2014 date code; the diaphragm is also aged.
2. Install the vent so it faces down and is screened, clear of the compartment wall and any mud dauber access.
3. Inspect and replace pigtails if cracked, and confirm the cylinder OPD valves and requalification dates.
4. Leak-test every joint with soap solution, then pressure-test the system at 11 in. WC for at least 3 minutes with no drop.
5. Run each appliance and set the regulator so flowing pressure is 11 in. WC with at least half the load on.
6. Check appliance manifold pressure at each test port, clean soot from the water heater flue and burner, verify a crisp blue flame with only slight yellow tips, and confirm the thermocouple or flame sensor holds.
7. Run a CO test in the coach with the water heater and furnace operating.

Document the before/after pressures and the parts replaced.

First, most RV two-stage regulators are sealed units with no published field-adjustment procedure. Adjusting one changes the spring set point without addressing the failed diaphragm, seat or blocked vent, so the system can drift again as soon as temperature or flow changes. Second, the vent blockage is the actual cause: no screw adjustment can restore atmospheric reference to a plugged vent chamber.

Third, the symptoms already show incomplete combustion: soot on the jacket and a yellow flame mean carbon monoxide is being produced. Turning the pressure down far enough to change the flame would still leave a contaminated flue, a possibly warped burner, and no proof of correct manifold pressure. The appliance has to be cleaned and checked, not just detuned.

Finally, there is a liability chain. A technician who adjusts a sealed regulator and signs off on a system that later produces CO or a fire has created a documented hazard with no manufacturer-approved procedure behind it. The defensible repair is replacement with the correct regulator, a documented leak test, verified manifold pressures, and a CO check. Explain to the customer that the regulator is a safety device, not a tuning screw.