Furnace Burner Cycling at startup

OP...you have enough discharge ducts.
How is airflow from each register...lacking could be sign of collapsed duct

How is return airflow. Nothing stacked up next to furnace OR obstructing return air??

Check exhaust ......obstructed will cause heat build up (mud dabbers. nests)

Propane pressure.
Turn ALL stove top burners on HIGH one at time.'
Do flames recovery quickly when another is lit?
Are all flames same....height/color?

With flames on ....turn furnace on.
Get same short cycling?
Do stove top flames diminish or stay steady?

Could be the flame sensor - if it's not in the right location (bent mounting bracket?), it could take a couple of cycles to heat up so the controller knows the flame is actually coming on.

Thanks for all the feedback. Here's a followup.

- the led on the circuit board is not flashing.
- the specs on this furnace call for a min 36 sq inch of discharge area. My furnace has 4 - 4" flexible ducts. The furnace is installed in a vertical configuration with two 4" discharge ducts on the top and two 4" discharge ducts on the rear, plus a 2" discharge duct on the rear. Each 4" duct provides 12 sq inches. So, it appears that I have 54 sq inches of discharge area. All the outlet registers are full open. Air flow should not be an issue.
-Someone mentioned that it could be thermostat related. More information on this please.
-Today I made a request to Dometic for suggestions on this issue. No feedback from Dometic as of this posting.

Thanks again for all the responses.

sgip2000 wrote:
Ever since Dometic bought Atwood, Atwood has gone to ****. Cheap, Chinese junk now.

Explain your expertise to state such ignorant nonsense. Doug

sgip2000 wrote:
Ever since Dometic bought Atwood, Atwood has gone to ****. Cheap, Chinese junk now.

Check out what is manufactured in Greenbrier TN

Ever since Dometic bought Atwood, Atwood has gone to ****. Cheap, Chinese junk now.

I agree it sounds like it is cycling against the high limit switch.
Probably need to add one or two additional ducts to get more airflow... unless you can repair the existing ducts to flow much better.

Mine did this for 8 years. I had tried to open the collapsed and poorly installed ducts for some time. Finally gave in and added two more 4" round ducts through the cabinet. Works perfect and tons more heat.

Sounds like it is operating off of the 'high temp' switch---called 'limiting'

Temp comes up to fast a rate of increase..furnace overfiring...high temp switch opens shutting down DC to circuit board. Fan continues to run...high temp switch cools/closes/DC back to circuit board/t-stat still calling for heat so burner fires.....cycle repeats

Causes:
Blocked ducts, too many registers closed/obstructed ----resists air flow
Restricted return air -----minimizes amount of air back to furnace
LP System Pressure.......too much fuel/overfiring
Defective high temp limit switch.....190*F switch #31091

If you remove outside access panel you should be able to see the LED on the circuit board
1-Flash with 3 second pause.......Limit switch/airflow issue



AFM Series Furnace

That could even be a thermostat related.