RV furnace and carbon monoxide: The warning signs the manuals actually give

The signs the makers publish

Suburban's furnace operation guide ties condition to risk in its opening warning: "If the user of this appliance fails to maintain it in the condition in which it was shipped from the factory, or if the appliance is not used solely for its intended purpose, or if appliance is not maintained properly, then the risk of a fire and/or the production of carbon monoxide increases which can cause personal injury, property damage, or death."

Then it tells you what to look at. On venting and condition:

• "For the furnace to work efficiently and safely, the furnace venting for air flow must be free of any obstructions."
• "Periodically inspect the vent for obstructions or presence of soot." with the warning above.
• "Inspect return air inlet openings to the furnace. Remove any restrictions to assure adequate air flow."
• "Immediately shut furnace down and call a dealer or qualified service technician if furnace cycles erratically or delays on ignition."

Dometic's own operating instructions for their furnace series give the same instruction in a different order, and name the part:

• "Inspect the chamber and vent system for holes or excessive corrosion."
• Owner checks monthly, and "Annual inspections should be performed by a qualified service technician."
• And the hazard line: "This Furnace can produce carbon monoxide, which has no odor and can be life-threatening. The burner and vent assembly system MUST be kept clean."

If the furnace failed alongside another appliance rather than alone, that is usually one fault rather than two, and if the fault is on the gas side, the propane guide carries the pressure test. So the manufacturer-documented watch list is short: soot, holes or corrosion in the chamber, restricted airflow, and short cycling or delayed ignition. Three of those four are things you can see or hear.

The recall that shows the mechanism

The clearest published example of how a furnace puts carbon monoxide into a living space comes from a recall, and it is worth reading because the failure is not the one people expect.

Forest River's Part 573 report for recall 24V-047 covers 2023 and 2024 Coachmen Prism Class C motorhomes, 213 units. The defect, in their own words: "The furnace may short cycle due to not having the appropriate amount of square inches of cold air return, and an inconsistent number and size of heat ducts installed to the furnace."

The risk statement is what makes it valuable: "If the furnace continues to short cycle, it may overheat and cause a puncture in the burn chamber, allowing carbon monoxide poisoning. This condition may lead to personal injury or death." The stated cause is a "manufacturing error", and the owner-facing warning is that "a user may notice the furnace is short cycling."

Read the chain in order, because it is the whole subject: not enough return air, so the chamber overheats, so it punctures, so carbon monoxide enters the coach. The remedy was free replacement of the cold air return and the ducting. Not a new furnace. Ducting.

One correction worth having, because it circulates: recall 23V-049 is frequently described as a furnace or carbon monoxide recall and it is not. It covers 2020 to 2023 Winnebago Solis, Travato and Ekko units fitted with a Suburban drop-in cooktop, where "the internal aluminum burner tubes connecting the gas valves to the cook top burners can fracture from excessive vibration or shock experienced through RV travel", causing a propane leak. It is a gas leak recall about a cooktop. The furnace and carbon monoxide recall is 24V-047.

Carbon monoxide has no smell, and the symptoms look like flu

The Consumer Product Safety Commission open their fact sheet with the sentence that matters: "Carbon monoxide (CO) is a deadly, colorless, odorless, poisonous gas."

Because it is undetectable by any sense, the only human warning is how you feel, and the symptoms are deliberately unalarming. The CDC: "The most common symptoms of CO poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion. CO symptoms are often described as 'flu-like.'" They add something every owner should know: "People who are sleeping, drunk, or under the influence of other substances can die from CO poisoning before they have symptoms."

The CPSC publish the levels, and they explain why a mild exposure is easy to dismiss:

For a workplace comparison, OSHA set the permissible exposure limit at 50 ppm averaged over an eight hour period.

And a distinction worth holding onto: eye irritation, a headache with stinging eyes, or a smell of exhaust is not carbon monoxide. CO has no smell and does not irritate. Those symptoms point at something else burning where it should not, such as dust or debris on a hot surface, which is its own problem but a different one. It also means the absence of irritation tells you nothing.

Your alarm has a service life, and the end of it is silent

A carbon monoxide alarm is not a permanent fixture, and the way it fails is quiet. Kidde state that their alarms have a proven life of "seven to ten years", and that as of 2013 all of them carry a "10-year life". The line that matters is what happens at the end: "Since CO alarms cannot detect carbon monoxide in end-of-life mode, it's important to take precautionary steps." The manufacturer and the National Fire Protection Association both recommend replacing a carbon monoxide or combination alarm every seven to ten years, and Kidde put their own alarms at a ten year life.

So an alarm past its date is not a slightly degraded alarm. It is not an alarm. It will still sit on the wall, it will still have a test button that makes a noise, and it detects nothing. The one thing you can do about that is look at the date code on the back and replace the unit when it has passed.

When an alarm does sound, the steps are the ones the Consumer Product Safety Commission publish: "Call your emergency services, fire department, or 911." Get everyone outside and do a head count, and "DO NOT reenter the premises until the emergency services responders have given you permission."

The insect screen question, settled by the makers

Owners commonly fit mesh screens over furnace vents to keep insects out, and forums recommend it. Both major furnace makers prohibit it, in explicit language.

Suburban: "Do NOT install screens over the vent for any reason. Screens will become restricted and cause unsafe furnace operation."

Dometic, under critical operation warnings: "Do NOT install screens over the intake air or exhaust vents for any reason." Their limited warranty also excludes "any item that restricts airflow including but not limited to bug screens."

The reason is the same one the recall demonstrates. A restricted vent reduces airflow, which is exactly the condition that makes a furnace overheat. If insects are a problem where you camp, the answer is inspection and cleaning, not a screen.

Can a cracked chamber be repaired

No, and the makers are consistent about it. Dometic: "Never attempt to repair the Furnace yourself. Contact a qualified service technician", and "Use only Dometic replacement parts and components, which are specifically approved for use with the Furnace."

Suburban's warranty documentation says the same thing structurally: their flat rate schedule lists "Replace Complete Furnace" and contains no line for repairing a heat exchanger, and their warranty returns require that heat exchangers and "furnace combustion chambers" be "returned … for factory inspection." A part the factory wants back for inspection is not a part intended to be repaired in the field.

Suburban's coverage of the heat exchanger runs through the third to the fifth year, which tells you they expect the part to last a long time and to be replaced rather than mended when it does not.

What to do if you suspect it

Stop using the furnace. That is the first instruction in both makers' materials when the signs are present, and it costs nothing.

Look at the vent for soot before the next use, since soot is the maker's own visual warning and it takes seconds to check. Listen for short cycling, where the furnace lights briefly and shuts down repeatedly, because that is the symptom the recall names as owner-visible.

Do not attempt a repair inside the combustion side. Dometic say never to try, and Suburban route the part back to the factory, and the reason is that carbon monoxide is the consequence of getting it wrong.

Get the annual service the manual specifies. Dometic put the qualified technician inspection on a yearly interval, and their owner checks are monthly.

And if anyone in the coach has a headache, dizziness, nausea or confusion that improves when they go outside, treat it as carbon monoxide until somebody has proven otherwise. Get out, open up, and call for help rather than investigating first.

What this costs

Checking the vent for soot costs nothing and takes seconds. Cleaning a vent costs nothing. Removing a fitted insect screen costs nothing, and the makers say it should not be there.

The annual technician inspection is a modest service charge, and it is the interval the manuals state. A carbon monoxide alarm is inexpensive, and replacing one that has reached its date is the cheapest safety step on this page.

Against that, the alternative is a furnace that has been quietly overheating, a chamber that has punctured, and a failure that the CPSC describe as deadly. The order to spend in: look at the vent, listen for short cycling, remove any screen, and replace the alarm if it is old. None of those is expensive and all four are in the manufacturers' own instructions.

Sources

• Suburban, all gas furnaces operation guide, for the carbon monoxide warning, the soot instruction, the vent and return air inspection, the short cycling and ignition delay warning, and the prohibition on vent screens.
• Dometic DF series furnace operating instructions, for the chamber and vent inspection, the monthly owner check and annual technician service, the carbon monoxide hazard statement, the instruction never to attempt a repair, and the prohibition on intake and exhaust screens.
• Forest River Part 573 safety recall report 24V-047, for the short cycling defect, the burn chamber puncture and carbon monoxide risk statement, the manufacturing cause, the owner-visible symptom, and the ducting remedy.
• Winnebago Part 573 safety recall report 23V-049, for the correction: this is a Suburban cooktop burner tube propane leak, not a furnace or carbon monoxide matter.
• Suburban warranty and claims documentation, for heat exchanger coverage in years three to five, the requirement that combustion chambers be returned for factory inspection, and the absence of a field repair line.
• US Consumer Product Safety Commission, carbon monoxide fact sheet, for the colorless and odorless statement, the parts per million thresholds for symptoms, and the instructions to follow when an alarm sounds.
• CDC, carbon monoxide poisoning basics, for the flu-like symptom list, the warning about sleeping or impaired occupants, and the detector replacement interval.
• OSHA, carbon monoxide poisoning fact sheet, for the permissible exposure limit of 50 parts per million over eight hours.
• Kidde, when should I replace my carbon monoxide alarm, for the seven to ten year proven life, the ten year life statement, and the end-of-life mode in which an alarm cannot detect carbon monoxide.
• Truma, preparing the heater for the travel season, for the instruction to remove dust from the heat exchanger and check the exhaust chimney for damage.