Suburban Furnace problem

[jasonsherman]

I've used this furnace all winter without a problem. On a recent trip, it stopped working.

The blower starts, but never clicks on. The blown-air is cold. It runs for about 5 seconds and shuts off.

It's getting plenty of 12v power. LP tanks are full and the connections look fine.

What's my next step?

How do I diagnose?

Thanks,
Jason


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[909kenu]

I'm no expert. But while searching for my own furnace issues, I gained some knowledge.

Check the sail switch. Maybe check with a meter or temporarily bypass the switch to see is burner will ignite.

[Wingeezer]

I believe if you do a google search, you should be able to find specific trouble shooting charts that could help.

Many times appliance problems I have had have been resolved just by pulling and replacing all connectors - ie just corroded connectors.


Brian.

[jasonsherman]

Here's an update:

The furnace is a Suburban NT-40 from 2001.

The sail-switch looks like it is working and all the wires appear to be OK.

My next step is to check the gas supply. I'm replacing the 20+ year old regulator and some gas lines. I'll keep posting updates.

Thanks,
Jason

[jasonsherman]

FYI: The following is from the Furnace Manual:

C. Blower Runs but Burner Does Not Light:
1. Make sure the manual shut-off valve is in the "ON" position. 2. Possible air in the gas lines. Reset the thermostat and cycle the furnace several times to bleed the system. 3. Check the gas supply. Be certain 11" WC pressure is present while the furnace is operating. If not, adjust the pressure regulator. 4. Check the wiring for proper polarity. 5. Check for proper clearances down the sides and across the top for return air to feed the room air blower. Each model furnace has a minimum requirement of return air outlined in the installation manual. With improper return air, the air volume may not actuate the sail switch that sends voltage to the module board. All models must have I" clearance along the sides and across the top except the NT-34, -42 and -45 models, which require 2" on the sides and top.
6. Check the ducting of the furnace to determine proper airflow away from the furnace. See the installation manual for correct duct requirements. If the ducts have severe bends or kinks in the hose, the airflow may be restricted enough to cause the microswitch to bounce back from the volume of air hitting the cabinet front. This again will not allow voltage to the module board. Many times, a furnace will operate satisfactorily on the bench or without the cabi.net door on the unit. Check for microswitch energizing when this happens. Correct ducting for proper airflow and/or replace the microswitch (could be stiff). 7. If the furnace will ignite with the cabinet front off, but will not with the cabinet front secured, and the ducting is free, check the gaskets around the burner access door. If the burner access is not properly sealed, air may be deflected from the cabinet front into the burner area, thus disturbing air and gas mixture for proper ignition. 8. Check for 12 volts DC on both sides of the limit switch. If there is voltage on one side of the switch, but not on the other, replace the switch. If voltage is present on both sides of the switch, proceed to Step 9. 9. Check for 12 volts DC at the power terminals on the module board (refer to the wiring diagram - red wire positive and yellow wire negative). If no voltage, check wiring from the limit switch. Correct wiring. 10. If sparking is audible approximately 15 seconds after the blower starts, but no ignition, check the high-tension wire for continuity, grounding and secure connections. Repair if necessary. 11. Make sure electrode is not grounding to burner, the gap between the spark probe and the ground probe is 1/8" (see Figure 7), and the ceramic insulator is not cracked, broken or sooted. Adjust, clean or replace as necessary. 12. If sparking is not audible, check the module board and observe if the neon bulb on the circuit board is flashing during the trial for ignition. Remove the white plastic wire connector from the circuit board. Clean the board terminals on back with a soft rubber eraser. Check the little pin terminals in the plastic connector for good contact. Reinstall connector. If the tube does not flash, replace the module board. 13. Check for voltage at the gas valve during the trial for ignition. If no voltage and connections are okay, replace the board. 14. If the valve does not open with 12 volts DC present, replace the valve. 15. If the electrode sparks and the valve opens but the burner will not light, check the gas supply for 11" WC pressure to the main burner office. If 11" is present to the valve but not to the orifice, replace the valve. Check vent and air intake installation. Check the main burner for correct relationship to the electrode assembly (burner should be 3/16" from the spark probe and sawports or charge ports directly under the spark gap). Check main burner for soot build-up in ports (see Figure 8). Clean cast-iron burners by passing a hacksaw blade through the sawports, being careful not to enlarge the openings. Wire brush stainless tube burners to remove build-up. 16. Check the combustion air wheel for proper rotation. Insufficient air will not sustain the flame if the wheel is installed backward.

[jasonsherman]

It's a bad Sail-Switch. Here's how I figured it out.

The switch was constantly on (it always allowed current through it). I disconnected one of the wires on the switch. I started the furnace and then reconnected the switch. The furnace ignited and hot air's working. If you have questions, let me know.

Thanks for all the help and someone else can use this info.

[909kenu]

Jasonsherman,

Congrats!!!! So, was it the sail switch or the connection?

[jasonsherman]

I'm pretty sure the switch is bad. It doesn't turn off, so it's always allowing power through it. It should be off until the blower comes up to speed. When I unplug it, start the furnace and the reconnect the switch during start-up, the furnace works.


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