How to Replace a Thermostat on a Commercial Fryer

Reading Time: 3 Minutes

Replacing a thermostat on a range is easy. Replacing one on a fryer? That’s where things can get tricky.

The stakes are higher because the sensing bulb lives inside a vat of 350°F oil. If you don't seat the packing nut correctly, that oil will weep out, run down the capillary tube, and drip directly onto the gas valve or burner assembly. That is a fire hazard waiting to happen.

Here is the "no-leak" procedure for swapping a standard RX-style gas fryer thermostat.

1. Drain It

Allow the frypot to cool for 10 minutes before draining. After draining, allow the frypot to cool completely before proceeding. 

Some techs try to work around a half-full tank to save time. Don't.

• Drain the vat completely. You need access to the probe guard and clamps inside the tank, and you can't get a proper seal on the stuffing box if oil is seeping through during installation.

2. The Removal

• Unscrew the stuffing box nut (also called the packing nut) located on the outside of the fry tank.
• Inside the tank, remove the bulb clamps.
• Pro Tip: Before pulling the bulb out through the tank wall, straighten the capillary tube gently. If you yank a coiled bulb through the hole, you might damage the threads on the tank fitting.

3. The Installation

• Pre-bend: Uncoil the new thermostat capillary carefully. Never put a hard kink in the tube; it will stop the fluid expansion and kill the stat immediately.
• Slide it in: Feed the bulb from the outside in.
• Seat the Bulb: Clamp the bulb firmly in the exact same position as the old one. If it touches the burners or heating elements, it will read false high. If it touches the cool zone, it will read false low.

4. Sealing the "Stuffing Box"

This is the money step.

• Slide the new packing washer and nut down the capillary.
• Thread it into the tank fitting.
• Hand-tighten, then give it a 1/4 to 1/2 turn with a wrench. You need it tight enough to compress the washer around the capillary tube to form a seal, but not so tight that you crush the tube.

5. The Leak Check & Calibration

• Fill the vat with water (or oil) above the probe line.
• Check the exterior nut for weeping.
• Light the pilot and set the dial to 350°F.
• Check the temp with a trusted digital thermometer (placed in the oil, not touching the metal sides).
• Adjust: If the dial reads 350°F but your thermometer reads 325°F, pull the knob and adjust the set screw inside the stem.
• Insert a thermometer or pyrometer into the frypot, placing the probe bulb within 3 inches of the bottom. Ensure the thermometer/pyrometer tip does not touch the bottom or sides of the frypot. 
• Allow burners to cycle on and off several times, then recheck the oil temperature.
• If the temperature matches the thermometer temperature, the controller is calibrated. If not, repeat.

Need an RX Thermostat?

Most commercial gas fryers use the Robertshaw RX series. Find it here, and if you want to save, AllPoints also stocks the OCM equivalent with the exact same thermostat specs. In stock and ready to ship today. Search "RX Thermostat" at AllPointsFPS.com.