I'm at my wit's end with my 2003 Alpine coach's Suburban SF-42 furnace! This thing fires up fine initially, but then it starts making these awful rumbling sounds and actually backfires through the intake - scared the hell out of me the first time it happened. The worst part is it's not heating worth a damn even though it runs constantly.
I've already cleaned what I could reach and checked for obvious blockages, but I'm wondering if this could be an air-fuel mixture issue? The unit is over 20 years old now, so I'm prepared for the possibility that major components might need replacing. Could this be related to gas pressure problems, or maybe the igniter is going bad and causing delayed ignition? I really need to understand what's actually causing these rumbling and backfire symptoms so I know what to focus on next.
Furnace rumble and backfire indicates air-fuel mixture problems. Check gas pressure at manifold, deep clean burner by soaking, and inspect heat exchanger. Delayed ignition from weak spark or dirty igniter also causes this. May need replacement on 20+ year unit.
Your Suburban SF-42 is experiencing combustion air starvation, which is causing the flame to rumble and backfire through the intake. The symptoms you're describing - immediate ignition followed by flame disturbance, continuous running without adequate heating, and backfiring through the intake - all point to insufficient primary air mixing with the gas at the burner. Even though you've cleaned the manifolds and burner orifice, and the burner holes appear open, there's likely a restriction in the primary air intake system or a problem with the gas valve pressure regulation. The fact that your water heater and stove work normally indicates your propane supply and pressure are fine, so this is isolated to the furnace's air-to-gas mixture.
In your 2003 Alpine with the SF-42, flame rumbling and intake backfiring occurs when the primary air (the air that mixes with gas before combustion) can't enter the burner assembly properly. This creates a rich burn condition where there's insufficient air for complete combustion. The flame becomes unstable, creates pressure waves that cause the rumbling sound, and can actually reverse flow back through the primary air intake, causing the popping you're hearing.
Common causes include: a partially blocked primary air shutter or intake screen, debris in the burner tube itself that compressed air couldn't dislodge, improper gas valve pressure (verify correct pressure specifications with dealer), or a warped/damaged burner that's affecting air flow patterns. Sometimes spider webs, dust, or even small insects can create restrictions that aren't immediately visible. The continuous running without adequate heat output suggests the flame is so disrupted that it's not transferring heat efficiently to the heat exchanger.
SAFETY WARNING: Working with gas appliances can create fire, explosion, and carbon monoxide hazards. Ensure propane is completely shut off at both the appliance and tank level before beginning work. If you smell gas at any point, stop immediately and ventilate the area.
You'll need a few specific tools beyond basic hand tools: a manometer or gas pressure gauge capable of reading 0-15" water column (SF-42 operates at manufacturer specified pressure for LP (typically around 3.5" WC - verify with manufacturer specs)), a small mirror or inspection camera to look inside the burner tube, pipe cleaners or flexible wire brushes, and a multimeter. Have compressed air ready, and non-flammable degreasers specifically designed for gas appliances. You'll want good lighting and possibly a small flashlight to inspect the primary air shutter area.
Have your multimeter ready to check the gas valve operation, and make sure you have the furnace manual or wiring diagram available. The SF-42 has specific gas pressure requirements and limit switch locations you'll need to reference.
Start by completely removing the burner assembly from your SF-42 - don't just blow air across it. Follow proper gas line disconnection procedures (check local codes - many jurisdictions require certified gas technicians for gas line work) and perform leak testing upon reassembly using soapy water solution on all connections. Look for the primary air shutter at the base of the burner tube where it connects to the gas valve. This adjustable opening controls primary air flow and often gets restricted by debris or spider webs. Clean it thoroughly and ensure it moves freely. Check that the air shutter opening matches the specifications in your manual (verify correct opening specifications with dealer for LP gas).
With the burner removed, inspect the entire length of the burner tube with your mirror or camera. Look for any obstructions, corrosion, or damage inside the tube that compressed air might not have cleared. Use pipe cleaners or flexible brushes to physically scrub the interior. Pay special attention to the mixing area where the gas orifice feeds into the burner tube - this is where proper air-to-gas mixing occurs.
Check the gas valve pressure with your manometer. Connect it to the outlet pressure tap on the gas valve and verify you're getting the correct pressure per manufacturer specifications with the furnace running. If pressure is off, you may need professional gas valve adjustment or replacement (gas valve adjustment typically requires certified technician). Also verify that all limit switches are functioning properly with your multimeter - a faulty high-limit switch can cause erratic operation.
Before reassembling, inspect the heat exchanger for any signs of sooting or damage that might indicate long-term poor combustion. Clean the flame sensor rod with fine steel wool or emery cloth to ensure proper flame detection. Reassemble everything, ensuring all connections are tight and the burner is properly positioned relative to the gas orifice.
Final Testing: After reassembly, turn on the propane supply and test the furnace operation. Listen for any remaining rumbling or backfiring. Check that the flame burns blue and steady without yellow tips or flickering. Verify proper heating operation by monitoring air temperature output. If problems persist, shut off the gas supply immediately.
If you've thoroughly cleaned the burner assembly, verified gas pressure, and checked all components but the rumbling and backfiring continue, you're likely dealing with a warped burner tube, damaged gas valve internals, or heat exchanger problems that affect air flow dynamics. These issues require professional diagnosis and potentially expensive parts replacement.
Gas valve replacement on the SF-42 runs $200-300 plus labor, and involves precise pressure adjustments that require professional tools. If the heat exchanger is cracked or severely damaged, you're looking at $400-600 in parts alone, and at that point replacement of the entire furnace might be more economical. Additionally, if you're not comfortable working with gas pressure testing or if your manometer readings don't make sense, call a certified RV technician. Improper gas appliance work can create carbon monoxide hazards or fire risks that far outweigh the repair costs.
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