Lance furnace

Barring something being really wrong or a control board problem, it's the thermostat that controls the flame burning time. Check for any furnace warmed air being blown at or being redirected towards the thermostat area. I've seen it happen in RV's several times that warmed air being blown towards the thermostat will tell the bimetal it's now warm enough to shut the gas flow off, so it does.

When first warming a cold RV inside, turn the thermostat up at least 5 degrees higher than your final temp desired because all the interior walls and objects are still cold and the cold coming off the still cold inside surfaces and objects cause the furnace to run for short period cycles until all the interior surfaces and objects warms also. Once equal warmth is acheived, turn the thermostat back down to your desired living level.

This same cycling occurs in a cold home temp set to 55 degrees that's being brought back to 72 degrees and we have it happen every year when we return from our couple winter months in Florida. Once the home's total interior gets up to temp, the quick short cycling quits.

There is a thing called a heat anticipator in your thermostat that can be adjusted to control the length of the heat periods and length of time between cycles

If you can read this it tells you how to adjust it , this is probably the same stat you have ,Robertshaw , if not most stats are similar

thanks for the diagram...I increased the number by one and it did rum longer and then when the burner stopped the blower air was much warmer, we'll see tonight how it works

thanks again

okan-star

x2 I had to adjust mine and that cured the problem.

I didn't mention the thermostat heat anticipator as the OP didn't say what year his Lance is and the model years and thermostat moldels used had varied over the years. Glad to see his has the adjustable anticipator and after the TC normalizes evenly inside and there's still short burn time, the anticipator adjustment (if there is one) is the next normal step to adjust for flame burn time.

Well, I just had to coment on this subject. Travelnutz pretty much nailed it. Heating systems for the most part are closed loop. If the abient air recirculating inside is so cold that it cools the firebox down, then the blower control will close down so that the burner can raise the firebox temp back up. When the firebox reaches a certain temp then the blower switch closes and the blower engages, as the abient temp increases so does the blower on time. When the set abient temp is reached then the burner will close and then the blower.

I find it better on high heat than auto,it gets it up to temp pretty fast.

Rollin wrote:
Well, I just had to coment on this subject. Travelnutz pretty much nailed it. Heating systems for the most part are closed loop. If the abient air recirculating inside is so cold that it cools the firebox down, then the blower control will close down so that the burner can raise the firebox temp back up. When the firebox reaches a certain temp then the blower switch closes and the blower engages, as the abient temp increases so does the blower on time. When the set abient temp is reached then the burner will close and then the blower.

I don`t belive that the burner could ever be on and the blower not.
The sail switch controls the blower and keeps it running until it reaches a set temp after the burner is off, to exhaust gas on the combustion side of the heater to outside

Agreed, not knowing what heater you have but the thermostat turns on heater-
the fan/blower is what signals controls that its ok to fire burner. When tstat turns off- it turns off burner, fan continues to run to cool the box down.
Unless this operation can be verified as 'normal' If your seeing burner on and no fan running- on a typical horizontal rv furnace, I believe you have problem.

Edit: response to Rollins comment-not OP

The blower is running, according to the OP.

The problem is the furnace burner is only running for 2 minutes, then shutting off.

This could be one of two problems:

1. A vent is blowing hot air right on the thermostat.
2. The burner is sensing that it is getting too hot, and shutting down to prevent a heat exchanger meltdown.

If your heat exchanger is caked with dust it can prevent the efficient transfer of heat to the air being blown through by the fan.