Noritz error code 20: high limit switch tripped

The high limit switch sits on the heat exchanger piping. When water temperature gets close to boiling, it trips, the burner stops, and the unit closes its water control valve so superheated water does not reach your taps.

The meaning below comes from Noritz's own technical knowledge base. Noritz technical support: 866-766-7489.

Common causes

• Scale buildup in the heat exchanger, which Noritz names as the most common cause
• Poor combustion
• Vent obstructions that keep hot exhaust from leaving
• A loose wiring connection or a failed high limit switch

How to reset it

Noritz says code 20 generally requires a hard reset: disconnect the heater's electrical power, wait, then reconnect it.

Why not just keep resetting

Noritz warns that resetting repeated code 20s without finding the cause is likely to end in an error code 14, a blown thermal fuse that cannot be reset and usually means replacing the heat exchanger. A descale now is far cheaper.

What to do

1. Reset once as described above.
2. If the unit has not been descaled in a year or more and your water is hard, schedule a descale.
3. Check the vent terminals outside for obstructions.
4. If code 20 returns after a descale, have the switch and wiring tested.

Before you call

Have the model and serial number from the rating label ready, along with the exact code and when it appears: at startup, during use, or only at high demand. Mention any recent work on the gas, water or electrical supply and when the heater was last serviced. This helps the technician bring the right parts and often saves a second visit.

See every code on our Noritz error codes page.