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heyobie's avatar
heyobie
Explorer
Nov 05, 2016

Suburban Furnace SF24 fixed - FYI

So I bought a used TravelMaster Class C 1988 (which became Coachman) with 30,000 miles. Trying to get everything working. Cold weather is here and the furnace wasn't working right.

I got it working after spending most of the day on it. If I knew what I learned today, I didn't have a problem and would not have wasted my day. So I am passing this on.

Anyway, the system would try to start up, but would not light. So I downloaded the manual and read all 22 pages twice. I removed the gas line to make sure gas was working and that it was bled. I then tested power to the circuit board. I then removed the igniter and tested it. Read all the posts on this site. Re-read the manual and followed the procedure related to this problem. I even stuck a butane lighter in the furnace when the igniter was trying to fire.

Then I saw something in the manual about battery being properly charged. I had a new RV battery which read about 12.5. I thought it might be a little low. So I started up the Ford Econoline 350. I read 14.5 going to the furnace. I then recycled the furnace, placed the lighter in the hole and **** if it didn't light. Of coarse it went out since the flame detector was out with the igniter. Put it all back together and it worked like a champ.

So moral of the story is turn on the vehicle when you test it. Mine would have fired right up.

I also saw some old posts that a guys furnace would work fine for 3 hours and then the flame would go out. I imagine he was running on battery and the battery got tired in the night. 12.5 volts is not enough to satisfy the SF24, so it cuts out the flame and lets the fan run.

If this helps anyone, my day is not wasted.

Obie
Class C
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Nov 05, 2016
    Not sure which Suburban Furnace model you have...didn't make an SF24
    NT24 or SF25 but not an SF24

    Regardless.......

    12.5V DC from battery is more than adequate. (that is almost a fully charged battery 90%)


    Factory specs:

    DC Voltage
    12V DC standard
    10.5V DC Minimum and 13.5VDC Maximum (higher then 13.5V DC and damage can occur to circuit board)

    Gas Pressure
    11" WC Minimum and 14" WC Maximum

    IF it only functions when engine is running and alternator is charging then you still have a problem
    Your furnace is designed to properly function off battery voltage.

    If it now works off battery voltage then it just needed cleaning of contacts, connections etc.
    Doesn't take much 'resistance' to stop DC ---a little corrosion here/there, a loose connection, poor contact and the need for higher DC voltage

    But 12.5V DC is MORE then adequate and capable of making that Suburban Furnace run
  • ronfisherman's avatar
    ronfisherman
    Moderator
    Nov 05, 2016
    As already said 12.5 volts should be enough. When mine did not work. Removing circuit board and cleaning contacts solved problem when not firing up.
  • heyobie's avatar
    heyobie
    Explorer
    Nov 05, 2016
    Sorry. NT24 SP. MY bad. Wouldn't mind removing board but not sure how to remove board. It looked like I had to remove the whole unit from the cage.
  • charlessfolly's avatar
    charlessfolly
    Explorer
    Nov 05, 2016
    Had a no ignition problem on a class b motor home furnace.Turned out blower motor was seizing up and not pushing enough air to actuate the "sail" switch.batteries were fine.replaced motor and unit worked much better than it had ever worked before.
  • ronfisherman's avatar
    ronfisherman
    Moderator
    Nov 05, 2016
    heyobie wrote:
    Sorry. NT24 SP. MY bad. Wouldn't mind removing board but not sure how to remove board. It looked like I had to remove the whole unit from the cage.

    Looking at parts list. Your model would be difficult to get at board.
    But the SF models can be gotten at. Though not always easy.
  • DrewE's avatar
    DrewE
    Explorer II
    Nov 05, 2016
    Is that 12.5V at the battery, or 12.5V at the furnace? If you have a poor connection somewhere, the voltage at the furnace could easily be a good bit less than at the battery. Making sure all the connections are clean and tight might be all it takes.

    As others have said, if it really needs more than 12.5V to work properly, then it's not very useful for operation off of a battery as even a tiny bit less than full charge will drop below 12.5V. (That is, of course, assuming your voltmeter is accurate.)
  • Bordercollie's avatar
    Bordercollie
    Explorer
    Nov 07, 2016
    Corroded or loose 12vdc connections, including grounds at the furnace , or anywhere else in the furnace power circuit can reduce actual voltage to the furnace. Connections to the thermostat can be poor too. On an old rig, spade automotive type connectors and ground screw connections can be slightly corroded lowering voltage to furnace, and solenoid valves, etc. may be balky. Cleaning circuit board connections is always good. Removing, checking and replacing fuses and tightening fuse block screws can "fix" low voltage problems. Amazing that furnaces work properly with all the lockout/safety features.
  • heyobie's avatar
    heyobie
    Explorer
    Nov 07, 2016
    When I was underneath the RV checking things out a month ago, I saw some scary grounds on the frame with spade connectors. The connectors and the screws looked bad and I said to self, 'Gotta clean up all the grounds'. But I didn't get to it. Had bigger fish to fry. Engine ran real bad. Changed plugs, wires, cap and rotor. Runs like new. The rotor was rusted on the tip. Gas gauge didn't work. Dropped tank and installed new sender. Mint.

    So I needed to get the heat running so I could work on it in fall/winter. I will get to the contacts as suggested. It is working fine on 12.5 volts now. It must have just needed a little more current to get through the bad contact, where ever it is. I will clean it up. Thanks again.

    Next project is the Onan generator. I was told it worked until mice chewed some wires. Wish me luck
  • Bordercollie's avatar
    Bordercollie
    Explorer
    Nov 10, 2016
    Make sure that the ground wire from generator to chassis is clean and tight. Generator starter must get good 12 volt dc power to start. May start easier using the start switch on the generator or with RV engine running. Starting via the remote on/off switch and emergency start switch on the dash requires that a number of connections and switches are good and that generator is in good tune.

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