Furnace burner cycling AKA "Limiting"

[Daryles]

We are currently in Fishing Bridge RV Park in Yellowstone. Cool (mid 50⁰F) daytime temps. Cold at night (mid 30⁰F) and we are at 7,800ft altitude. Altitude can be a factor with propane appliances.
Our furnace starts, blows and fires (you are all familiar with the sounds a Furnace burner makes (miniature jet engine).
The burner cycles on and off every few seconds becoming a longer and longer "on" cycles until it runs normally.
I downloaded Dometic service manual and looked up the Troubleshooting chart.
I was surprised when I read the description regarding "Limiting" (see screen shot).
I decided to try simple things first. Take the cover off and clean it. It was already clean (including the sail switch). I reseated all the connectors and put the cover back on. Let's see what it does tonight. Same thing.
I was reluctant to disassemble the burner to clean the burner, flame sensor, inside the chamber etc since I did not have any pipe dope to seal the gas line when reassembling. I could get to the limit switch easily and test it or by pass it for testing purposeses. I noticed on my Mopeka propane tank sensors one of my tanks was almost empty. When I checked the auto switching valve, it was pointed to the low tank. I switched the valve to the other FULL tank and refilled the empty one. I started thinking about the TS chart and what I read in the operation description in the service manual. One of the things was to check for correct propane pressure (11" of water, I dont have one of those pressure meters).
It started me thinking, low propane tank, cold night time temperature, high altitude. This combination could cause low propane line pressure and cause "Limiting".
Last night the furnace ran perfectly on the FULL propane tank.
Hope this helps someone down the road.

[Mikendebbie]

“We are currently in Fishing Bridge RV Park in Yellowstone.”

…great write up and Thankyou…but I can’t feel too bad for you after that opening statement! I’m in hotter than hades Texas! Now don’t you feel bad for me? Guess I’m just jealous.

[Jay & Bobbie]

That is good information.
We are heading that way late July. Hopefully we may get by with fireplace or heat pump if necessary.
I think it would be safe to say the 11" WC is calibrated at or below 3,000 feet. Rising to above 6,000 feet will change that calibration quite a bit (I think decreasing the water column). I'm glad simply filling the tanks to capacity did the trick.
Happy travels,

[Lee-CI]

Daryle glad you got your furnace issue sorted out. Will definitely have to keep your findings remembered. Hope you folks thoroughly enjoy your stay in Yellowstone.

Mike & Debbie - you well know that we have but two seasons in our beloved Lone Star state, one of which can last for 9 or 10 months.....

[Bourbon County]

The problem with any gas fuel burner at high altitudes is shortage of oxygen to support combustion. The air is thinner and oxygen content is slightly lower. Residential gas appliances marketed in high altitude regions have a smaller size orifice jet and the the efficiency does go down some too. I believe the industry standard altitude for calibration is 2000 feet, you're nearly 4X that.

You can buy some time during short trips to high altitude areas by reducing the gas pressure slightly. I never though about it, but a manometer might be a good thing to carry. You're dealing with some extremely low pressure, <0.5 PSI, I don't believe there is a gauge on the market that would be accurate at that level. There are digital models that are fairly inexpensive, and you could always buy a mangnehelic gauge in the correct range. In either case, make sure the meter and hose is rated for LP gas.

Check the note on the troubleshooting guide, you're supposed to adjust gas pressure with all gas appliances running. You will need to fire up the cooktop and water heater as well as the furnace while checking & adjusting.

[Daryles]

Update:
We are now in KoA in St. Mary (East Glacier) Montana. Elevation 4,551'. It was 32⁰F last night when I turned on the furnace at 3am (the fireplace and bedroom 350W plug in wall heater could not keep up).
The furnace fired up and ran perfectly.
So I'm guessing altitude played a big roll in the furnace "cycling" issue.
View looking out the back window this morning.

[Mikelff]

I have a propane heater that I use for high altitude and freezing temperatures. So far it works great. Keeps my tank warm and maintains good pressure, especially when the tank is low. I also use it for my propane grill in the winter. I grill out all year long so the heater helps. Freezing temps also lowers propane performance and a warm tank makes a difference.

[rames14]

Daryles, we had a similar issue and I chalked it up to altitude. Well, had the same issue at a lower altitude- wouldn’t fire after 40 attempts. Took it to a mobile tech who tore the whole thing apart without finding anything - until he was reassembling. The screw that holds the igniter was stripped. That is also the ground screw. New larger screw and it runs perfect, regardless of altitude. I had talked to Dometic, had Keystone check the propane pressure all to no avail. We camp near 10,000 feet and had never had an issue until this camper. Now we’re ready again.