If you’re seeing this code, you’re likely standing in a cold shower or hearing the unit’s fan run incessantly without the burner firing. You might have even heard a loud “pop” or “banging” noise (kettling) right before the error appeared. Don’t sweat it just yet—while the unit is protecting itself, the underlying cause is usually a maintenance issue or a simple sensor trip that we can troubleshoot together.
| Category | Specification |
|---|---|
| Difficulty Level | Intermediate (Requires Multimeter Knowledge) |
| Estimated Time | 45 – 90 Minutes |
| Tools Needed | Phillips Head Screwdriver, Digital Multimeter, Needle-Nose Pliers |
| Estimated Cost | $0 (Reset/Cleaning) to $60 (New Switch) |
Symptoms of a High Limit Trip
In my thirty years of turning wrenches, I’ve found that the unit usually talks to you before it dies. If you’re dealing with Error 20, look for these specific physical signs:
🛠️ Safety First: Read Before Repairing
Don’t guess the wiring. Get the official PDF to see the exact schematics.
- The “Cold Water Sandwich”: You might get a few seconds of hot water followed by a total lockout and an ice-cold stream. This happens because the unit starts, detects an overheat condition almost immediately, and kills the gas valve.
- Flashing “20” on the Remote: The digital controller will flash the code “20” and likely beep to alert you that the safety circuit is open.
- Audible Banging or Vibrating: If you hear a sound like marbles rattling inside the unit (kettling), that’s a sign of localized boiling. The water is getting too hot because it can’t flow properly, triggering the switch.
- The Fan Runs Constantly: The unit will often keep the combustion fan running at high speed to try and cool down the heat exchanger after the error is triggered.
Technical Explanation of the Fault
When Error 20 hits, it means the electrical circuit through the High Limit Switch (or the Overheat Wrap) has been broken. Here is why that happens from a mechanic’s perspective:
- Scale and Mineral Calcification: This is the #1 killer. If you have hard water, calcium deposits build up inside the copper piping of the heat exchanger. This scale acts as an insulator. The burner is blasting heat, but because of the scale, that heat can’t transfer into the water. The copper pipe itself gets dangerously hot, tripping the switch.
- Incorrect Gas Pressure: If your gas manifold pressure is set too high, the flame is essentially “over-firing.” It’s like red-lining an engine; the unit produces more BTUs than the heat exchanger can dissipate, leading to a rapid temperature spike that the safety sensors catch.
- Faulty High Limit Switch or Overheat Wrap: Like any electrical component, these sensors can fail due to age and heat cycling. The switch is a bimetal disc that opens the circuit at a specific temperature. Over time, “fatigue” sets in, and the switch might start tripping at lower, normal operating temperatures or simply fail in the “open” position.
- Internal Obstructions: Occasionally, a piece of debris or a failed internal part can lodge in the flow path. Low flow through the heat exchanger while the burner is at high fire will cause an instant overheat.
Troubleshooting & Replacement Instructions
Step 1: Safety First & Power Down
Before you even touch a screwdriver, pull the plug on the unit or flip the dedicated breaker. Close the gas shut-off valve located underneath the unit. We’re going to be working near electrical terminals and water lines; you don’t want a surprise spark or a gas leak while your head is inside the cabinet.
Step 2: Remove the Front Cover
Use your Phillips head screwdriver to remove the four screws holding the front panel in place. In most Noritz models, these are at the corners. Lift the panel up and out. Set it somewhere safe where it won’t get scratched. Take a moment to look for any scorched wires or obvious signs of leaking around the heat exchanger.
Step 3: Locate the High Limit Switch
Look for the heat exchanger (the large copper component). Near the top, you will see two thin wires leading to a small circular component clipped to the pipe. This is the High Limit Switch. On some newer models, you may also see a “thermal fuse” or “overheat wrap” which is a thin white wire wrapped around the entire exchanger. If that wrap is burnt or broken, the circuit is dead.
Step 4: Perform a Continuity Test
This is where the wisdom comes in: don’t just guess. Set your digital multimeter to the “Ohms” (Ω) or “Continuity” (Beep) setting. Pull the two wire leads off the High Limit Switch using your needle-nose pliers (pull by the connector, not the wire). Place your multimeter probes on the two terminals of the switch.
The Verdict: If the meter beeps or shows close to 0 Ohms, the switch is fine—the problem is elsewhere. If it shows “OL” (Open Loop), the switch has tripped or failed and must be replaced.
Step 5: Inspect for Scale Buildup
If the switch is tripped, you need to know why. Check your maintenance logs. If it’s been more than a year since your last flush, the unit is likely scaled up. You can’t just reset the code; you must descale the unit using a flush kit and food-grade white vinegar for 60 minutes to remove the minerals that caused the overheat in the first place.
Step 6: Replacing the Switch (If Necessary)
If the switch is confirmed dead via your multimeter, unsnap it from the copper pipe. Snap the new OEM Noritz switch into the exact same position. Reconnect the wire leads. Ensure they are snug; a loose connection here can cause resistance and lead to a false Error 20 code in the future.
Step 7: Reassemble and Test
Replace the front cover, restore gas, and plug the unit back in. Open a hot water tap slowly. Watch the unit fire up. If the code is gone and the water stays hot, you’ve nailed it. If it trips again immediately, you likely have a gas pressure issue that requires a professional with a manometer.
How to Prevent Error 20
I always tell my customers: “Take care of the machine, and the machine will take care of you.” Here is how you keep Error 20 from coming back:
- Annual Descaling: This is non-negotiable for tankless heaters. If you have hard water, minerals will settle in the heat exchanger. Flushing the unit once a year with a pump and vinegar keeps the heat transfer efficient and the temperatures within the safe range.
- Install a Water Softener or Scale Shield: If your water is particularly “heavy,” a pre-filter system can catch those minerals before they ever reach the heater. This can extend the life of your heat exchanger by a decade.
- Monitor Gas Pressure: If you recently had a new gas appliance installed (like a pool heater or a large range), it could be affecting the pressure at your water heater. Have a tech check the “static” and “load” gas pressures to ensure the Noritz isn’t being starved or over-fed.
FAQ
Frequently Asked Questions
Q: Can I just bypass the High Limit Switch to get hot water tonight?
A: Absolutely not. I’ve seen what happens when these units melt down. Bypassing a safety limit is a fire hazard. If the switch is open, it’s for a reason. Replacing a $50 switch is much cheaper than replacing a $2,000 unit or dealing with a house fire.
Q: My multimeter shows continuity, but I still get Error 20. Why?
A: You likely have a “ghost” trip or a loose connection at the control board. Trace the wires from the switch all the way back to the PC board. If the wires are frayed or the plug on the board is loose, the board thinks the switch is open. Also, check the “overheat wrap” (the white wire around the exchanger) for any tiny breaks.
Q: Is Error 20 the same as Error 11 or 12?
A: No. Errors 11 and 12 are ignition and flame loss issues. Error 20 is strictly about temperature safety. While they all result in no hot water, Error 20 specifically tells you the unit is physically too hot, whereas 11/12 usually mean it can’t get hot enough (no fire).