Suburban SF-25F Furnace tries to start three times but fails to stay on

ThomasLM · January 11

I have a Suburban SF-25F RV Furnace that is not working properly. I followed the instructions to start and here is what it does:

- Blower comes on

- You can hear the igniter click

-then you can feel and hear the burner blowing hot exhaust out

- however it only lasts about 20 seconds then turns off and

- then cycles two more times doing the same thing

- then after the third attempt the blower runs for about 5 minutes and turns off.

What have I done:

- Removed the furnace and checked the "sail switch" (after isolating it I held multimeter to it and move the switch up and down, continuity when on and off as I moved the switch.

- Then I isolated the High Temp switch and had continuity thru it.

- then I disconnected and reconnected all wires to ensure goo connection.

BUT IT STILL DOES THE SAME THING. Any ideas

lappir · January 12

Sounds like it's not getting enough fuel.

If you only have a 20lb tank and it's much below freezing you won't have enough fuel to keep it running unless you heat the tank. A friend of mine found that out the hard way last winter. Let his big tank get low and then just asked me to put the little one on if the big one went empty. I did, and asked for a fill of the big tank, but apparently not soon enough.

Rod

Wrknrvr · January 12

Where did you get your last propane fill. If it was in the south, then can mix butane in with Propane.

Are you in cold weather. Below 33 degrees. If it has burned all the LP off, it will act like it has been. LP will supply gas at below -40 degrees.

Butane will not provide enough vapor to operate below 33 degrees, I think that’s the temperature point.

Now if the fuel supply is good?

Then I think there is a ground problem where the flame sense is. It senses a very small amount of dc voltage through to flame to ground, as it is burning.

Any time I pull a Suburban furnace out to check things. I remove the burner assembly in the burn chamber. They are stainless steel screws, but what you are describing, I would bet that is the problem. It is best to use a crows foot wrench to take the supply tube loose. Remove the burner assembly, and the mounting bracket. Sand or file all contact surfaces. Then reassemble unit.

Maybe youtube it.

Kirk W · January 12

1 hour ago, ThomasLM said:

BUT IT STILL DOES THE SAME THING. Any ideas

That is a very common symptom and nearly always indicates that the ignition probe has failed. The same probe that supplies the spark to light the furnace also detects the heat of the burning propane. If it does not send a signal back to the circuit board that it detects the heat from the burning propane, it will try 2 more times and then go into safety lockout. If you turn the furnace off and after a brief pause turn it back on it will repeat that same sequence. Over the many years that I have had an RV and also helped others repair their RV furnaces, I have probably seen that issue about 20 times and nearly all of them were the ignition probe.

If you do not have one, you can download a copy of the Suburban Service Manual from this link. On page 7 of that manual the troubleshooting guide addresses your symptoms.

Edited January 12 by Kirk W
Add information.

Wrknrvr · January 12

While your at it. I would suggest to put a new igniter in the furnace.

Ray,IN · January 12

3 hours ago, Kirk W said:

That is a very common symptom and nearly always indicates that the ignition probe has failed. The same probe that supplies the spark to light the furnace also detects the heat of the burning propane. If it does not send a signal back to the circuit board that it detects the heat from the burning propane, it will try 2 more times and then go into safety lockout. If you turn the furnace off and after a brief pause turn it back on it will repeat that same sequence. Over the many years that I have had an RV and also helped others repair their RV furnaces, I have probably seen that issue about 20 times and nearly all of them were the ignition probe.

If you do not have one, you can download a copy of the Suburban Service Manual from this link. On page 7 of that manual the troubleshooting guide addresses your symptoms.

Where IS that like button? As usual Kirk is exactly right.

Edited January 12 by Ray,IN

ThomasLM · January 12

17 hours ago, Kirk W said:

That is a very common symptom and nearly always indicates that the ignition probe has failed. The same probe that supplies the spark to light the furnace also detects the heat of the burning propane. If it does not send a signal back to the circuit board that it detects the heat from the burning propane, it will try 2 more times and then go into safety lockout. If you turn the furnace off and after a brief pause turn it back on it will repeat that same sequence. Over the many years that I have had an RV and also helped others repair their RV furnaces, I have probably seen that issue about 20 times and nearly all of them were the ignition probe.

If you do not have one, you can download a copy of the Suburban Service Manual from this link. On page 7 of that manual the troubleshooting guide addresses your symptoms.

Thank you that was a very detailed reply and appreciate you taking the time to answer. I will see about getting the part.

oldjohnt · January 14

I agree with the fine gents above, here's my take: In over 50 years using RV furnaces Ive had more Ignitor Tip/Flame Sensor Circuit problems than circuit control boards going bad. When the t stat calls for heat the blower runs and IFFFFFFFFFF theres adequate air flow a good working sail switch closes,,, the gas valve opens,,, HV is sent to the Ignitor Tip,,,,,,,,and an adequate spark ignites the flame.......After that so long as Flame Sensor (Ignitor tip and circuit board) circuit senses flame the gas valve remains open HOWEVER IF NO FLAME IS SENSED (tip or circuit) the gas valve closes. I would remove clean and inspect the ignitor tip and its air gap, look for carbon traces deposits or hairline cracks, check all connections and grounds in the ignitor wire and circuit plus the control board etc.

NOTE The fact that it at least first ignites means the HV spark is adequate making it less likely its a SERIOUS problem allowing an INITIAL SPARK (could still be a control circuit board issue) however as the flame sense voltage signal from tip to control board is so small it could still be an Ignitor Tip issue instead of the board. The ignitor tip is much cheaper than a new board so look at it and connections and grounds FIRST

PS I just remembered a couple minor easy to fix issues some years back that gave me trouble. One was the air gap between the ignitor tip and burner was incorrect and another was corrosion right on the tip EASY PEASEY perhaps ???

John T Live in the RV in Wauchula Florida

Edited January 14 by oldjohnt

Ray,IN · January 14

It it's been mentioned already I'll repeat it. The surface area of LP is the limiting factor when converting to gas/vapor. A 20# cylinder has a small surface area, and at these low temperatures often cannot boil enough to supply the required gas/vapor. A solution to this is, warm the cylinder/tank. Heating "pads" are made for this purpose.

That is another reason why 250G and larger tanks are horizontal instead of upright.

You could have a 1,000G vertical tank that will not have the surface area to supply gas/vapor in enough quantity to supply, say a grain dryer for a silo.

reference: https://propanewarehouse.com/helpful-information/lp-gas-properties/

Edited January 14 by Ray,IN

D&J · January 14

One other thing to check is the ground, remove the igniter and clean the area where it's screwed to the bracket. In the past I would just run a ground wire to the control panel.

Denny

oldjohnt · January 15

15 hours ago, D&J said:

One other thing to check is the ground,

DITTO and X2 Any loose, resistive, rusted or corroded connection, ground or otherwise, can cause a voltage drop and the flame sense circuitry operates on very low voltages to start with CHECK ALL WIRES CONNECTIONS AND GROUNDS !!!!

John T

Kirk W · January 15

Let us know when you get the part. It's not difficult to replace. For the future, if the sail switch doesn't close there will be no sparking attempt for the furnace to light and the propane valve never opens.

Hope you aren't suffering with the cold! 😊

oldjohnt · January 17

On 1/15/2024 at 9:54 AM, Kirk W said:

For the future, if the sail switch doesn't close there will be no sparking attempt for the furnace to light and the propane valve never opens.

EXACTLY I certainly agree with Kirk on that.......If yours is as you report working with the gas valve opening,,,,,,,Ignitor Tip and circuit control board functioning firing an HV spark igniting the flame,,,,,,,,,,But the gas valve closes and flame goes out THE SAIL SWITCH ISNT THE PROBLEM (if its bad it never lights in the first place as Kirk noted) but as we agree above more likely an Ignitor Tip (air gap, corrosion, crack, carbon trace, wiring, failure) or the control circuit board or a bad ground or connection. The Ignitor Tip is an easy replacement while the board is a bit more difficult but so often Ive had success merely cleaning the tip or adjusting the air gap and checking grounds and connections versus the more expensive board.

PS as far as sail switches are concerned, Ive had more problems with them being dirty or stuck instead of an electrical failure. Other causes when they don't close are the air flow isn't sufficient or even low voltage.

Good luck let us know what you find

John T Live in the RV at the Florida Flywheelers in Fort Meade Florida

Edited January 17 by oldjohnt

Wrknrvr · January 17

If you are still having trouble with the way it is acting? Did you clean the base the the igniter sits on? Those screws and the contact surface is how the flame sense completes the ground loop for the pc board to sense the flame is burning?