Furnace Burner

Good question that I would also like to hear about, I noticed mine does that too when the heat has to run for a good amount of time, I would say 30+ minutes, like when first heating the trailer up. The fan will run continuously but the burner cycles.

What is a lot of heat? The heat should be venting out of the vent as part of normal operation.

It is always nice to have brand/model so better/direct answers can be provided.


Generic answers.

Furnace is 'limiting' (caused by high limit switch operating due to lack of air flow, high gas pressure or faulty limit switch)

Furnace sail switch is not staying made up due to low 12V DC power.

Furnace has a cracked heat exchanger......

Provide more info....brand/model and what is happening (fan continues to run, burner shuts off/fan stops, burner lights then drops out etc.)
More info provided the better ideas as to problem. Especially when one can not see/hear furnace operate.

The burner section of the furnace exhausts to the exterior of the rig. That is normal.
You said the furnace cycles on and off, how often? This should happen about 6 times per hour. There is an adjustment on some thermostats to change cycling but it also changes the temperature on/off times. It is the Attenuator. This is on mechanical thermostats.
It is possible that the furnace is shutting down via the over heat sensor. Furnace over heats sensor opens then cools off and restarts. Could be caused by restricted air movement with in the furnace or ducting in the rig.

What RV do you have?
What's the square footage of your RV?
What size furnace (BTUs) do you have?
How often does it cycle?
How cold is it outside?
What is the thermostat set at?

Answer these questions and maybe we won't have to guess with an answer to your question.

RV furnace tech is at about the level of home furnaces from 1970s.
No really, you waste something like 30 to 40% of you heat with them. Unlike modern home furnaces that are 90+%

Understand how these systems work. The flame in the furnace heats the metal surface of the heat exchanger and then exhaust outside, carrying the Carbon Monoxide, soot, and unburned fuel with it.

The blower takes air from inside the trailer, passes it over the surface of the heat exchanger (On the side opposite the flame area), warming it and then pushing it into the trailer.

The air around the flame and the air that goes into the living space never mix!!

Should the flame be on with no interior air moving over the surface of the heat exchanger, the exchanger will eventually melt. Two safety systems prevent this. The "sail switch" is a paddle in the air flow that checks on how much air the blower is putting out. If there is not enough airflow to protect the heat exchanger, the flame is cut off. The other system is "after run" on the blower. After the thermostat hits the set point and shuts the flame down, the blower continues to run until the surface temperature on the heat exchanger is within the safe range. (Oil or gas fired household furnaces do the same thing.)

As others have said, not knowing what trailer, how cold is it outside, and/or anything else makes it difficult to troubleshoot (if there is actually trouble).

Could it be that it is so cold out and the trailer is so poorly insulated (as many are) that the thermostat is calling for the flame to come back on before the blower finishes the "after run" from the previous cycle?

Too many variables for any more than generalizations.

According to Suburban
RV Furnaces are 70-80% and water heaters are 70%

Size and air flow capabilities are the problems.......along with less than adequate insulation/air leakage in RVs.

Thanks for the replys...it sounds to me like this may be my problem...

"Furnace is 'limiting' (caused by high limit switch operating due to lack of air flow, high gas pressure or faulty limit switch)"

Probably isn't something I can fix, so I'll get a furnace guy to check it out.

I think like so much equipment installed in these rv,s is borderline in capacity.I switched out our SF35 suburban furnace for a new 42Q,this is night and day difference in heating our 35.5 newmar ,very quick to heat and So Quiet.

alboy wrote:
I think like so much equipment installed in these rv,s is borderline in capacity.I switched out our SF35 suburban furnace for a new 42Q,this is night and day difference in heating our 35.5 newmar ,very quick to heat and So Quiet.

7,000 btu heat increase........should heat rig up quicker