Furnace burner cycling on & off

[MStat]

Brought home our new TT last week to prep for a combination maiden voyage and anniversary trip.

Turned furnace on, blower started, ran for a period of time then the burner lit. Placed hand over floor register and felt just a slight draft of warm air coming out. All three registers felt the same. I did the same thing when we picked it up, commented on low air flow, but guy at the dealer said that was normal they all felt that way. So OK, I thought, we bought our last trailer nine years ago so maybe something changed in furnace technology.

Well, guess I was wrong on that count.

What warm air was coming out, maybe two-three minutes worth, starts to turn cool, the blower is still running, then after a period of maybe a minute or so, I hear the burner light off a second time. The air starts getting slightly warm, stays warm for maybe 2-3 minutes, then starts to get cool again. Blower is still running, then again after abut a minute or two, I hear the burner light off again. Never had a furnace that the burner would light, go out then relight.

Something new maybe? Some new technology in furnaces to keep temperatures lower.

Two hours later, same cycle still repeating itself trailer has warmed up about 10 degrees. It was about 35 outside. No way would it keep up in colder temperatures.

Yes it's a larger trailer than what we had, but our other would warm up in 15-20 minutes, furnace running constantly and the burner would never go out.

Put thermometer in floor register and air temperature right at point of duct connection would peak out at about 120, but then would drop off.

Pulled off cold air return to check for blockage and did not see any.

Put same thermometer into duct right at furnace discharge and air temperature there reached 140 before dropping back off.

It would appear that there be a gremlin with either the burner or air flow thru furnace.

Thoughts or ideas on what else should I (we, it's going back to dealer Saturday) be looking at?

Thank you

[MStat]

Thanks for all the replies.

Yup on shore power

No discharge near t-stat

Talked to wife's co-workers and they echo the comments made. She works at a commercial HVAC company.

Furnace firing too hot and/or restricted air flow.

Dealer gets to worry about it. Have only had it for a month

[prichardson]

Sounds like restricted air flow thru the furnace causing it to cycle the burner on high limit. Since you mention low air flow at all the registers the return may be undersized. Try running with it disconnected. If that does not increase air flow the ductwork to the registers is probably restricted.

[Old-Biscuit]

Cycling of main flame is indicative of high limit switch operation.
The high limit switches can be 130*F, 170*F or 190*F depending on make/model.

Temperature Rise (TR) affects cycling.
The typical RV furnace has an average 90*F TR.......return air temp subtracted from discharge air temp (measure after furnace has been operating 5 mins with a fast acting thermometer).
They are rated for a maximum of 130*F TR.

140*F at discharge could be tripping high temp limit. (what is return air temp?)
Problem could be restricted air flow, could be high propane pressure, could be faulty high limit, could be slow motor speed

Which brand & model?

[Boband4]

Is it possible that an air vent is blowing right at the thermostat? This could cause quick temp changes causing the thermostat to be cycling on and off.

[time2roll]

If it is like mine the ducting is collapsed and restricted causing over heating of the firebox and cycling the burner against the high limit switch. The low airflow sure is suspect.

I found the issue well after warranty was expired. Became aware when camping below freezing the first time. Used a mirror in the duct to confirm the issue. Removed the furnace and propped open the ducting with some square aluminum stock. Helped a lot but still cycling. This summer added an additional vent. Have not really tested it yet.

I would call the manufacturer of the RV and also the tech line of the furnace to build your confidence to insist on a warranty repair from the dealer that is already resistant and in denial.

Good luck.

[Ductape]

Sounds to me like you have an air restriction. Could be in the discharge or the return. Not enough air flow > higher temperature in the heat exchanger > burner cycling on the high temperature limit control. Kinked ductwork maybe?

[enblethen]

Couple of items come to mind.
First is the thermostat may not be functioning correctly. Remove the cover and see if it has two wires. If it does connect them together to by pass the thermostat. furnace should come on run for a short period of time 15-20 seconds then burner should light and run. Let it run in that mode for 10-15 minutes. Separate the two wires. the burner should shut off, furnace goes into cool down and then shut off in 15-30 seconds.
Next the furnace flame sensor is not in the flame and cycles the furnace burner on and off.
Next the furnace is dirty and the fan does not keep the sail switch closed and shuts off power to circuit board shutting off burner then gets a little more power back and starts again.
Make and model of furnace?
Are you connected to shore power or running off battery?