Noritz Error Code 20: High Limit Switch Tripped (Overheat)

What the code means

The high limit switch is a safety switch mounted on an elbow of the copper heat exchanger, or on the copper pipe leaving the heat exchanger on pre-mix style units. Noritz says it trips when the water approaches roughly 176°F to 205°F, using switches rated 80°C, 94°C or 96°C depending on the unit (smaller units trip lower, larger and commercial units higher). When it trips, combustion stops immediately and the unit closes its water control servo so superheated water can't leave the heater. Scale reduces how well heat passes into the water, so the heat exchanger and other parts hold the heat and overheat. Noritz warns that repeatedly resetting code 20 without fixing the cause can lead to error code 14, which is not resettable and needs a much bigger repair. If the switch or its wiring has failed, the code usually won't clear even when the unit is cold. Noritz's Pro Series models use code 20 for the same high limit switch trip.

Likely causes

• Scale (hard-water mineral) build-up in the heat exchanger, the most common cause
• Dust, dirt or lint build-up causing poor combustion
• An obstruction in the venting that keeps hot exhaust from leaving
• A loose or bad wiring connection between the high limit switch and the main wire harness
• A failed high limit switch

Safe DIY checks

1. Turn the heater off, unplug it or switch off its power, and let it cool for a while. Noritz says that if the switch and wiring are good, the code usually resets when power comes back on and the unit will try to run normally.
2. If the code won't clear even with the unit cold, suspect the switch wiring or the switch itself and call a technician rather than resetting over and over.
3. If you have hard water with no softener or treatment and no record of descale flushes, flush the heat exchanger. Noritz's owner's guide says periodic flushing is necessary in hard-water areas, that scale can damage the heat exchanger, and that scale damage isn't covered by the warranty. Follow the flushing procedure in your model's manual; newer models have a Scale Flushing menu item.
4. Check the outdoor vent and air intake terminations for blockages such as nests, leaves, snow or ice.
5. Look for dust, dirt or lint around the unit. Noritz recommends a combustion cleaning if there are signs of build-up, which is a job for a technician.
6. Keep the temperature setting as low as practical; Noritz notes that lower settings help prevent scale build-up.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 11 Noritz Tankless Water Heater 11
• 12 Noritz Tankless Water Heater 12
• 29 Noritz Tankless Water Heater 29
• 90 Noritz Tankless Water Heater 90
• 10 Bradford White Tankless Water Heater 10

All Noritz codes →

Sources

1. Noritz Knowledge Base: Error Code 20 (EC20, high limit switch triggered, hard reset, switch location, 176-205°F with 80/94/96°C switches, combustion stops and water servo closes, scale most common cause, repeated resets lead to error code 14, poor combustion, vent obstructions, wiring, failed switch, descale flush and combustion cleaning, 866-766-7489)
2. Noritz Knowledge Base: Error Code 20 (Pro Series Models), same high limit switch meaning on pre-mix Pro Series units
3. Noritz NRCR/EZ Owner's Guide: periodic flushing necessary in hard-water areas, scale can damage the heat exchanger and isn't covered by warranty, Scale Flushing menu, lower temperature settings reduce scale