Furnace Help

[Kach]

Our 5’er has a Dometic furnace with Airexcell thermostat. It has been short-cycling recently.

The problem is worse when it is cold (first turned on) and seems to improve when it runs for a while. When first turned on, it will only fire for a couple seconds at a time. As it warms up, it will improve to 4-5 minutes before cutting the gas and then re-fires 30 seconds or so later.

I’ve replaced the sail switch and control board (Dinosaur) with no change. Is there an easy way to isolate the furnace from the thermostat? I know I need to look into the high limit switch and propane pressure next, but trying to be smart before I keep throwing parts/money at it.

I’d expect the high limit switch to not be the problem since it’s worse starting from cold then gets better, but who knows?

[opnspaces]

Do this by the labels on the back of the thermostat, not by the wire colors which might be different
Take the thermostat off the wall and turn it around and take a picture of the wire connections.
On the back of the thermostat identify the wires that are connected to the R and W terminals. R is 12v + and W is out to furnace. When everything is cold connect a jumper between the R and W terminals Or alternatively take the wires off the R and W terminals and twist the wires together. Does the furnace still short cycle?

[Kach]

The furnace is a DMFD35121 Dometic. 

Thermostat is Airxcel 9430A3382 or very similar - controls AC, fan high and low as well as furnace. I’m thinking it’s calling for heat appropriately since when the burner turns off, the fan seems to continue running for longer than the 30 seconds or so at the normal end of cycle. 

the furnace is mounted horizontally below fridge and has no access hatch to the outside - everything I try involves me disconnecting the gas and pulling the whole unit!

[StirCrazy]

I wish people on the internet would stop telling people to change the sail switch anytine there is an issue, they rarely are the issue, if it does any kind of cycle the sail switch is fine. 

it could be the high temp limit, but I would put that one down the list, as when it is cold the furnace is working harder to heat it up, so I coud see that.  it could be the thermostat, but like opnspaces mentions we would need to know more about the furnace and type, , it could be a few other things including old wires and lose conections.  if it was me I would start with the flame sensor/pilot tube, then the thermostat if that doesnt fix it

[opnspaces]

Typically the furnace is controlled by two wires coming from the thermostat. The problem is your thermostat almost assuredly has more than two wires on it.  To offer meaningful advice we probably need
The year of the RV
The model of the Dometic furnace
The Model of the Airexcel thermostat.

Once we have that information we can look at the appropriate install or troubleshooting guide to see which wires are for the thermostat.