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Atwood 37022 High Temp Limit Switch for HydroFlame 8500 Series Furnaces -- NBK 20297 Direct Replacement
Atwood 37022 high temperature limit switch NBK 20297 direct replacement for HydroFlame 8500 series RV furnace models
Atwood High Temp Limit Switch For HydroFlame 8500 Series Furnaces - Direct Replacement
  
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Part Number: 37022
Manufacturer: Atwood

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Atwood190F High-Temp Limit SwitchDirect Replacement -- NBK 20297Atwood 37022 High Temp Limit Switch for HydroFlame 8500 Series Furnaces

When your HydroFlame 8500 series furnace shuts off unexpectedly or won't ignite, a failed high-temperature limit switch is one of the most common culprits. The Atwood 37022 is a direct-fit 190F limit switch engineered to restore safe, reliable furnace operation -- no modification required. This is the same switch the furnace shipped with, sourced as NBK part number 20297.

The Right Part for Your HydroFlame Furnace

The high-temperature limit switch is a critical safety component inside your RV furnace. It monitors heat exchanger temperature and cuts power to the burner if temperatures climb above the rated threshold -- protecting the furnace and your rig from overheating damage. When the switch fails (open circuit) the furnace will lock out even at normal operating temperatures, mimicking an ignition or airflow problem.

Atwood part 37022 (also listed as NBK 20297) is a 190-degree Fahrenheit bi-metal limit switch designed as an exact replacement for the furnace models listed below. If your HydroFlame 8500 series furnace is shutting down on limit, failing to ignite, or showing a limit lockout fault, inspect this switch before replacing costlier components.

190F Trip Point

Rated to open the circuit at 190 deg F, matching the original factory specification for 8500 series furnace heat exchanger protection.

Direct Plug-In Fit

Designed to replace the original Atwood 37022 without wiring changes or bracket modification. Fits the existing mounting location and connector.

Broad Model Coverage

Compatible with 8516, 8520, 8525, 8531, and 8535 furnaces in both III and IV configurations, plus 1522 and 2334 two-stage models.

Lightweight Service Part

Weighs approximately 0.5 lbs. Easy to carry as a spare for full-timers and extended-season campers who can't afford furnace downtime.

How the Limit Switch Works

The limit switch is a normally closed bi-metal safety device wired in series with the furnace control circuit. Under normal operation the switch remains closed and current flows freely. If the heat exchanger temperature reaches 190 deg F -- caused by blocked airflow, a dirty filter, a failed blower motor, or another fault -- the bi-metal disc snaps open and interrupts the circuit, shutting down the burner. Some switches auto-reset on cooldown; others latch open and require manual reset or replacement. If your furnace is consistently tripping the limit, address the root cause (airflow restriction, blower performance) before fitting the new switch.

Direct Replacement Fit: Atwood 37022 (NBK 20297) drops straight into the original mounting location on all listed 8500 series models. No adapter harness, no drilling, no splicing required.

Compatible Furnace Models

Verify your furnace model number on the data plate inside the furnace access door before ordering. This switch fits the following HydroFlame models:

  • 8516-III and 8516-IV
  • 8520-III and 8520-IV
  • 8525-III and 8525-IV
  • 8531-III and 8531-IV
  • 8535-III and 8535-IV
  • 1522 2-Stage
  • 1522 LD 2-Stage
  • 2334 2-Stage
Identifying Your Furnace: The model and serial number plate is typically located on the furnace cabinet inside the access compartment. Match the full model number including the Roman numeral suffix (III or IV) to confirm fitment.

Troubleshooting Tips Before You Replace

A high-temp limit switch that trips repeatedly is usually a symptom, not the root cause. Before installing the new switch, check the following:

  • Return air filter -- clogged filters are the leading cause of limit trips on HydroFlame furnaces
  • Combustion air inlet and exhaust outlet -- inspect for blockage, insect nests, or ice in cold climates
  • Blower motor -- confirm the wheel spins freely and the motor runs at full speed on startup
  • Duct registers -- ensure all supply registers in the rig are open and unobstructed
Maintenance Tip: Replace the furnace filter at the start of each heating season. Restricted airflow is the primary cause of limit switch failures on 8500 series HydroFlame furnaces. A clean filter extends the life of the limit switch and the blower motor.
Heads Up: If the furnace continues to trip the high-temp limit after replacing the switch, the underlying airflow or blower problem must be diagnosed and corrected. Installing a new switch without resolving the root cause will result in repeated lockouts or premature switch failure.

Part Identification

Atwood Part Number37022
NBK / Vendor Part Number20297
ManufacturerAtwood (HydroFlame)
Component TypeHigh-Temperature Limit Switch
Weight0.5 lbs

Switch Specifications

Trip Temperature190 deg F (87.8 deg C)
Switch ActionNormally closed, opens on over-temperature
Device TypeBi-metal thermal limit switch
Circuit RoleSafety lockout -- wired in series with furnace control board

Compatible Furnace Models

8516-III / 8516-IVCompatible
8520-III / 8520-IVCompatible
8525-III / 8525-IVCompatible
8531-III / 8531-IVCompatible
8535-III / 8535-IVCompatible
1522 2-StageCompatible
1522 LD 2-StageCompatible
2334 2-StageCompatible

What's in the Box

Included1x Atwood 37022 high-temp limit switch
Not IncludedMounting hardware, wiring connectors, installation tools
Important Information
  • Confirm your furnace model number -- including the III or IV suffix -- before ordering. This switch is not confirmed for earlier 8500 series revisions without the III or IV designation.
  • Turn off the furnace and allow it to fully cool before accessing internal components. The heat exchanger and surrounding parts can retain dangerous temperatures long after shutdown.
  • Disconnect 12V power to the furnace at the breaker or fuse panel before removing or installing the limit switch.
  • Repeated limit trips after replacing the switch indicate an unresolved airflow or blower issue -- do not continue operating the furnace without diagnosing the root cause.
  • This is a furnace service part. Installation should be performed by someone comfortable working with RV gas appliances. If in doubt, consult a qualified RV technician.


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