Dear Dave,
Hi. My furnace burner cycles constantly. It starts up, burner kicks in, and after 15 or so seconds the burner shuts off. The fan keeps running and the burner kicks back in after maybe 30 seconds, then shuts off again after 15 seconds. This repeats until the rig gets up to temperature, then the furnace and fan shut off as if everything was normal. So while eventually I do get enough heat, it takes forever and causes the fan to run way longer than it should. This happens whether on battery, shore power, or when running the generator. Maybe it’s the thermocouple, but it seems like that would shut the burner off immediately and not after 10 or 15 seconds. Thank you! —Greg, 2017 Thor Windsport 34J
Dear Greg,
Since you did not give a make and model number of the furnace, I’ll assume it is a Suburban. Most RV furnaces operate in a similar manner so it should apply to other brands as well. Since the amount of time between intervals is somewhat strange, in my opinion, I sent your question off to my Suburban contact, and he stated the same concern. He stated that the 15-second length is shorter than a limit switch issue and longer than a faulty flame sensor.
Faulty flame in RV furnace
When the burner is running, the flame creates a closed circuit. That means it touches the electrode and sends voltage to the module board indicating there is flame and to keep the gas valve open. If the flame is low, or does not touch the electrode due to misalignment, or if it is low gas pressure, it could be the regulator getting weak and something else is pulling LP at the same time such as the water heater. Make sure everything is off that uses LP or have the pressure tested at the furnace.
Another issue I have found is the LP lines can get oil in them from the refining process of the LP and it affects the operation of appliances. However, I would go through the other troubleshooting steps first. If it is a misaligned electrode, there is a procedure to realign it which requires removing the furnace chamber assembly. This typically only takes about 7 seconds.
Limit switch
Limiting or cycling of the limit switch happens due to excess temperature. In that case, the limit switch creates an open circuit and no power is sent to the module board. When this happens, the module board shuts down the gas valve but the blower motor keeps running to purge the combustion chamber and cool down the limit switch. It takes about 5 minutes for the limit switch to cool, at which time it closes and sends power to the module board and it attempts to reignite.
Issues that could cause excess temperature are restricted ductwork, blocked or dirty return air, high motor amp draw, and a weak limit switch. Check all the vents and ductwork to make sure nothing is closed or blocked and there is good airflow. You could check the airflow with an anemometer that measures CFM. You can find them on Amazon here.
According to the service manual for Suburban, the CFM depends on the model and BTU you have. Here is the list. You can also determine the amp draw of the motor with this chart as well.
I would suggest starting with the easy items such as vents, cold air return, and airflow. Then you can determine your level of comfort digging into the flame sensor and limit switch.
One last item could be the wiring. I remember years ago trying to start a furnace in a prototype motorhome in 20-degree weather and it would not start. When I called my tech support they told be it was too cold! The wiring to the thermostat was not the proper gauge and, sure enough, when it got over 30+ degrees it would start.
I have also found this an issue with solder points in module boards that have a hairline crack. When it is cold, the crack shrinks enough to create an open circuit and not allow power through. When it heats up, it expands and creates a closed circuit allowing power to go to the module board. This is a real gremlin to find! I found it in a module board by applying heat with a hair dryer for several minutes and it worked.
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RV furnace has low airflow and isn’t hot. Why?
Dear Dave,
The RV furnace has always worked well, but this last week it seems it isn’t heating the interior as quickly as it did a few months ago. I checked the vents and nothing is blocking them. The air blowing is warm but used to be much warmer. And while the heat is on, I can’t light the stove burners; I have to turn the furnace off, and then the stove will light. That has never happened before, so I think the two may be related. —Cheryl, 2016 Coachmen Leprechaun 220QB
Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
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The first thing I would check is blocked register(s). DW loves throw rugs, she bought several, I didn’t pay any attention until first cool spell, setting the thermostat the furnace would try to light then shutoff. Halfway into my trouble shooting I noticed a throw rug covering half of a register, taking it off the furnace started and ran perfect. DW said she wanted that rug there, I told her she had a choice, move the throw rug or put on long underwear, the rug was moved. As an aside, it might be the name of the RV.
We had a new trailer a few years ago and the furnace would go on/off constantly. We found out while fixing something else that there was 8 feet of ducting shoved and squished into a 12″ deep x 18″ wide by 9″ tall chase under a cabinet. Apparently, the installer at Forest River didn’t have his/her scissors so he/she just shoved all of it into the space. We took it out, cut it down to about 16″ and reinstalled and the furnace worked perfectly.
Dave, Reread the initial question. The writer stated 15 seconds, not 15 minutes as you listed in your response. I would suspect the flame sensor.
Thank you, Dave! I am not handy, so I tend to rely on service technicians. However, your column is extremely helpful in educating me so I can better talk to technicians, as well as better understand if they are any good. Thank you for the education! 🙂