Start with the failed step
This procedure applies to Suburban NT-series furnaces. If the furnace is not an NT model, stop and use its own manual. Diagnose in the order the furnace operates; do not replace parts before finding the failed step.
| Step | What should happen | If it fails |
|---|---|---|
| 1 | Thermostat calls for heat. | Check thermostat and wiring. |
| 2 | Blower starts and reaches about 75% rpm. | Check 12V, polarity, relay, and motor. |
| 3 | Sail switch closes through the limit switch. | Check airflow, sail switch, and limit switch. |
| 4 | Module board waits 12-18 seconds, opens gas valve, and sparks. | Check gas supply, electrode, and module board. |
| 5 | Flame is sensed and the burner stays lit. | Stop on lockout; do not keep sparking. |
Lockout is not automatic re-ignition
If flame is not sensed or the burner fails to light, the furnace goes into 100% lockout. The gas valve closes and stays closed until the thermostat circuit is broken. Re-ignition will not occur automatically.
No-blower branch
If the thermostat calls but the blower does not run, check 12 volts DC at the furnace, wiring polarity and connections, thermostat continuity, the thermostat relay, and the motor.
Stop conditions
- Do not run the furnace with the electrode wire disconnected or the electrode assembly removed.
- Do not use a battery charger to check the ignition; use a 12-volt battery.
- Keep the electrode-to-ground gap at 1/8 inch.
- Stop and use qualified service for gas leaks, repeated lockout, or a defective module board.
Source: Suburban NT Series Service Manual
Yearly furnace maintenance
| Task | Action | Stop |
|---|---|---|
| Gaskets | Inspect all furnace gaskets yearly; replace any that leak or deteriorate. | Stop if the combustion chamber will not seal. |
| Motor | Leave the permanently lubricated motor sealed; do not oil it. | Stop if the motor is noisy or seized. |
| Gas connections | After any service, leak-check all gas connections inside and outside the furnace. | Stop if any leak is found. |
| Ignition | Keep the electrode gap at 1/8 inch, keep the electrode wire connected, and use a 12-volt battery for checks. | Stop if the electrode or module board is damaged. |
If a gas leak, repeated lockout, or damaged module board is found, stop and use qualified service.
Furnace install checks
| Check | Requirement | Stop |
|---|---|---|
| Venting | Vent to the outside, draw combustion air from outdoors, keep the vent unobstructed and sealed, and separate exhaust from intake. | Stop if the vent is shared, blocked, or unsealed. |
| Gas pressure | Confirm LP gas pressure of 10.5 to 13.0 inches of water column. | Stop if pressure is outside that range. |
| 12V supply | Confirm 12-volt DC from a battery or converter. | Stop if the supply is not 12V DC. |
| Return air and ducting | Confirm return air and ducting against the model installation manual. | Stop if airflow or ducting is unknown. |
| Model scope | This procedure applies to Suburban NT-series furnaces. | Stop if the model is not an NT-series furnace. |
After installation, leak-check all gas connections and verify the furnace follows the normal operating sequence before returning it to service.
Replacing the furnace
| Check | Requirement | Stop |
|---|---|---|
| Model match | Match input BTU and 12V amperage to the old NT model. | Stop if the replacement is undersized or draws more current. |
| Venting | Use the same outside vent path with separate exhaust and intake. | Stop if the vent path cannot be reused. |
| Return air and ducting | Match the replacement model installation manual. | Stop if airflow or ducting is unknown. |
| Gas and power | Confirm 10.5-13.0 inches water column and 12V DC. | Stop if either is out of range. |
| Gaskets and leaks | Replace deteriorated gaskets and leak-check all gas connections. | Stop if the chamber will not seal or a leak is found. |