Furnace Problem. I think or ???

[mikectx]

2020 Keystone Montana Model 3121RL Purchased new October 2019
(Have owned two other Montana’s in past 20 years with zero furnace problems)
2017 Silverado 3500 6.6L Diesel

Commentary: We are full-timers. It is a real pain to take the RV to service centers. Wait all day and then be told “we will order the part, Have a good day”. Then call in two weeks and be told “it is working like it is supposed to”. They don’t seem to like warranty work and want to do everything they can to discourage customers. Okay I’m officially off soapbox, onto the furnace problem.

Furnace: Dometic 35000 BTU/Hr Model: DMFD35131
Propane Tanks: Full
• Checked gas stove burners no change in flame output.
• Removed furnace fuse and replaced with a similar fuse.
• No flashing lights on furnace control circuit board

Problem: Furnace will continue to short cycle every 10 seconds or so (does not go in lockout) for about 15 minutes and then starts and runs until temperature set point is reached. Blower goes into cool down and stops. When temperature drops below set point it will short cycle then restart in a shorter time and sequence normally. Subsequent times same thing.

Action with Dealer (60 miles one way): Technician observed problem and said it was flame sensor. Told me they ordered flame sensor. Called two weeks later they said Keystone would not cover cost as “it was working as it was supposed to”. Had to leave town so didn’t go back to dealer.

Action with Mobile repair tech: Observed problem. Said it was gas valve. They changed gas valve, checked and tightened all propane connections, then changed electrode. It started while they were here then checked two days later when it was cold and same problem.

Action with Dometic: Sent this information to Dometic. Response: We do not provide technical support over the phone as we need a tech to diagnose the unit and test components. Sent me service manual.

This is a sequence I found on an internet site. I added my observations, actions and results so far.

1) The thermostat triggers a request for heat, sending a current to the RV furnace.
Action: Relocated Bedroom Thermostat to Living Room Thermostat
Results: No Change
2) The current powers the time delay relay which in turn passed power to the RV furnace blower. The blower spins up and pulls air from the Air Return creating a flow of air through the heat exchanger and out of the ducts. Another fan pulls air into the combustion chamber to feed the furnace.
Observation: Blower continues to run the entire time. Air is coming out of
all ducts.
3) As the blower speed increased the fan air causes the sail switch to close.
Action: Changed sail switch
Results: No Change
4) Power then flows through the High Limit Switch and onwards to the Control Circuit Board.
Action: Changed limit switch and Changed control board
Results: No Change
5) As power is delivered to the control board it opens the gas supply and causes the DSI igniter to spark. The gas lights and creates heat within the combustion chamber.
Action: Replaced gas valve and electrode
Results: No change
Observation: Burner lights then goes off. This action repeats for 10-15 minutes then the burner remains lit until thermostat reaches set temperature and then burner goes off and then after a short period fan turns off
6) Heat will pass into the heat exchanger, heating the air passing through the Ducts.
Action: Removed ducts from heater to determine if duct was problem
Results: No change

Observation: Ambient temperature seems to affect the short cycle startup time. Example It was in the 80’s here the other day. Short cycle time was about 2 minutes and then subsequent time even shorter. Starting to think propane is warmer therefore more pressure. Could it be propane regulator?
Action: Adjusted regulator screw clockwise ¼ turn then ½ turn. Seemed to effect a change but then tried again in the morning when outside temp was in the 40’s
Result: Same old problem.

Any help would be greatly appreciated.