Furnace won’t start

[Flip]

Thermostat calls for furnace to turn on. Furnace attempts 4 times then quits. The furnace has been working but will quit after a few cycles. I was having to turn the thermostat off and then back to heat and it worked for last couple of days. This evening it quit. As mentioned, calls for heat and attempts to start 4 times and nothing. It usually attempted once and then started on second attempt.

[Country Campers]

Thermostat calls for furnace to turn on. Furnace attempts 4 times then quits. The furnace has been working but will quit after a few cycles. I was having to turn the thermostat off and then back to heat and it worked for last couple of days. This evening it quit. As mentioned, calls for heat and attempts to start 4 times and nothing. It usually attempted once and then started on second attempt.

Here is a few questions.

How cold is it where you are camping?

Are you sure that the propane tanks are full?

What model and brand is the furnace and thermostat?

Does the stove top burners light and run on high?

Brian

[Flip]

Thanks for quick response.

40’s during the day/upper 20’s at night

yes on propane bottles. Actually just filled them up and tried agai before posting.

Furnace - Suburban - SF35Q Thermostat - Airxcel

Yes, all three burners burning on high.

Philip

[Flip]

Additional details - when furnace was working, we tried all three burners on high and kicked furnace on and it had no effect on the flames. One dealer’s service tech said the thermostat needed replaced. Actually chuckled when I told him what the thermostat looked like. Ordered one today but now quit completely.

[TedS]

Judging by no change in the cooktop flame when the furnace heat is called for it may be that the furnace solenoid gas valve is not opening. Check the connections on the valve.

[Scott'n'Wendy]

I'm not sure stove top burner flame is affected when the furnace starts. Maybe, never tested that. But I tend to agree with TedS. If you are hearing the blower running, then hear the ignition 'clicking' sounds, the furnace airflow requirements are met - the sail switch has closed and you moved on to the ignition sequence. So the only thing missing to create flame is fuel. A 12v gas solenoid should have a resistance somewhere around 50ohms. Could check it with a meter. Or place your finger on it and see if you feel it operate when the ignition sequence starts - if its in a place where you could safely do that.

[howson]

Scott'n'Wendy -- the only time (I know of) where the stove top will be affected by the furnace is if the regulator can't sustain 11" of water column under a 50% load. Only way to accurately measure this is with a manometer.

Flip -- does the gas ignite and is there a flame? If there's a momentary flame but then it suddenly quits, that could mean the electrode is not sensing the flame. When that happens the circuit board will shut down the gas valve. This could be a bad electrode or for some reason it's not centered in the flame.

Also, if the furnace is responding to a call for heat (by turning on the furnace fan) I doubt there's anything wrong with the thermostat.

Assuming you have an auto-changeover regulator connected to the propane cylinders, it's a long shot but you can give this a try:

1) Close both valves on the two propane cylinders
2) Move the primary arrow that's pointing to one cylinder to the other cylinder.
3) Turn on the cylinder (slowly) that has the arrow pointing to it.

Try the furnace.

If it works, repeat the experiment by trying the other cylinder as the primary. What's the result?

Howard

[howson]

Scott'n'Wendy -- you're spot-on with the resistance measurement (if the OP is familiar with using an ohmeter). The only "gotcha" is if the two valves (there are always two valves for safe operation) are measured in parallel the resistance will be 1/2 of the expected value. The only way to truly measure each valve independently is to remove each of them from the circuit. (I'm sure you know that...post is for those reading who may not know. If you're that reader and don't understand--ask the question! Scott or myself or any number of other forum members can explain.)

Howard

P.S. Pic below is a generic one from my school handouts.

[DarnDave]

Flip I've had two of those t-stats fail. If you remove it from the wall, and knew what wire color controlled what, you could jumper the wires temporarily to see if the furnace will light. I suggest this since it was working. The colors and functions should be listed in the furnace manual. Also, if you have other thermostats in other rooms of your unit, you can scroll through the display and see if heat is a choice. If so, and the previous test worked, you could swap tstats until your new one shows up. I have three of these; two have "heat" as an option although only one actually controls the furnace. (covid supply chain issues strike again i'm sure)

Dave

[howson]

Flip -- reading back through this over my first cup of coffee and realized we missed the first thing that should be checked (!). Are you plugged into shore power or are you on battery only? Do the lights dim when the furnace is trying to light? Anything below 10.5vDC and the furnace's circuit board will shut down.

And DarnDave has a point--if the thermostat was intermittent it could cause the issue. (All the thermostat does is send 12vDC or the signal to "provide heat" and then drop 12vDC when the temperature set at the thermostat is satisfied).

Based on the symptoms, though, I still doubt the thermostat is the cause. The furnace is trying to light so the symptom points to a propane-related issue.

[TNFSolitude]

ours exhibited the same symptons several years ago. In our case it was the circuit board. Had it replaced at the following National Rally, gratis. Good luck going forward.

[Flip]

Thanks to everyone for your suggestions and help. TNSFSolitude, you hit the nail in the head. Just replaced the control board and we are up and running. Thanks again for everyone’s time. Have a good one

[J & J Bumblebee]

I'm not sure stove top burner flame is affected when the furnace starts. Maybe, never tested that. But I tend to agree with TedS. If you are hearing the blower running, then hear the ignition 'clicking' sounds, the furnace airflow requirements are met - the sail switch has closed and you moved on to the ignition sequence. So the only thing missing to create flame is fuel. A 12v gas solenoid should have a resistance somewhere around 50ohms. Could check it with a meter. Or place your finger on it and see if you feel it operate when the ignition sequence starts - if its in a place where you could safely do that.

There may be an additional problem according to the service manager at my dealer.

We got a wave of snow and low temps last week So, I fired up our heater in the trailer. Propane tanks are full, the stove works perfectly, the heater comes on and blows cold air until it decides to shut down

The furnace is easily accessible from the outside so, I popped the cover. I could smell propane and I heard the igniter clicking. I checked the sail switch unplugging it then checking with a multimeter. That was fine, so I left the cover off and closed the sail switch manually, and no flame. I figured it was the control board and since it's under warranty I called the dealer. He thought the igniter was misaligned, so I'm taking the trailer to them on the 17th of this month.

In the meantime, I plugged in a heater to keep it warm inside since I don't winterize.

I had no idea that the igniter could be misaligned. We'll see what the solution is.

John

[howson]

There may be an additional problem according to the service manager at my dealer.

We got a wave of snow and low temps last week So, I fired up our heater in the trailer. Propane tanks are full, the stove works perfectly, the heater comes on and blows cold air until it decides to shut down

The furnace is easily accessible from the outside so, I popped the cover. I could smell propane and I heard the igniter clicking. I checked the sail switch unplugging it then checking with a multimeter. That was fine, so I left the cover off and closed the sail switch manually, and no flame. I figured it was the control board and since it's under warranty I called the dealer. He thought the igniter was misaligned, so I'm taking the trailer to them on the 17th of this month.

In the meantime, I plugged in a heater to keep it warm inside since I don't winterize.

I had no idea that the igniter could be misaligned. We'll see what the solution is.

John

For what it's worth...it could be what the dealer suggests. If you hear the "click click click" of the igniter and smell propane...it should light, even if momentarily. The lack of ignition could be a weak spark, but generally if you can hear the "click" that means there's a good enough spark. Pic below of various electrodes is from an RVSA presentation. As you can see, the electrode could be a local or a remote version, and they are differences between the Atwood (Dometic) and Suburban heater electrodes.

The first step, though, is to double-check you have good 12vDC. Power problems are very common. It is not surprising that the stove top works since most use a piezo ignitor (they aren't dependent on 12vDC to ignite the propane). Some ovens use a powered ignitor. The water heater has an electrode similar to the furnace--does the water heater work on propane?

If the issue isn't power or the electrode, another possibility is the propane may not be directed near the electrode (propane is being dispersed). There's been posts on the forum lately of clogged burner screens. The air intake/exhaust tubes could be plugged up, too--that's not unusual due to mud dobbers or other critters--and if true the reason the furnace won't light is there's no way to support ignition due to the lack of secondary ignition oxygen.

Bottom line: the by-the-book process to find out what's going with the furnace is to remove it from the trailer and bench check it along with a thorough inspection. If that's what the dealer tells you must be done don't be surprised.

[Scott'n'Wendy]

Seeing as you said this started with a wave of snow and continuing cool temps, I would be tempted to blow hot air from a hair dryer on it to ensure there is no ice in or around the ignitor and gas nozzle.
Of course if the ice/snow got in around your control board, that could also be the problem. But it seems most like a gas issue to me. If you hear clicking you are getting a spark then air flow is good (or good enough for the sail sw.) You said you smell gas, but it's possibly redirected away from the ignitor - maybe ice - or not enough gas. It's pretty tough to determine volume of gas by smell alone. Or perhaps it's ignitor alignment, but there would have to be a reason for it becoming un-aligned...and snow wouldn't do it.
I'll be interested to see what the final fault determination is.