Noritz Error Code 20 — Low Water Flow

Error Code 20 on Noritz tankless water heaters (NR, NC, NRC series) means the unit has detected insufficient water flow to safely operate. Noritz units require a minimum flow rate to activate the burner — without adequate flow, the heat exchanger can overheat. Error 20 prevents this.

Jump to Fix

• Most Likely Cause
• Diagnosis Steps
• Parts

Minimum Flow Requirements

• Noritz NR98-OD / NR83-OD: Minimum flow = 0.26 GPM
• Noritz NRCP199-OD (condensing): Minimum flow = 0.26 GPM
• Below this flow rate, Error 20 is triggered and the burner will not ignite

Common Causes {#most-likely-cause}

Step-by-Step Diagnosis {#diagnosis}

Step 1 — Clean the inlet filter screen This is the most common fix:

1. Shut off cold water supply to the unit
2. Disconnect the cold water inlet connection
3. Remove the stainless mesh filter screen
4. Rinse under running water and clean with a soft brush
5. Reinstall and test

Step 2 — Check cold water supply pressure

• Supply pressure should be 15–150 psi at the unit
• Use a pressure gauge on the cold inlet service port if available
• Check for partially closed shutoff valves at the unit, water meter, or pressure regulator

Step 3 — Inspect the flow sensor

• The flow sensor (turbine-type) is inside the unit on the cold water inlet piping
• Remove and inspect for scale deposits, sediment, or debris in the turbine chamber
• Spin the turbine wheel — it should rotate freely
• Flush with water — do not use tools in the turbine housing

Step 4 — Test the flow sensor

• Reassemble and measure flow with a bucket and stopwatch at the hot water outlet
• If flow is adequate (>0.26 GPM) but Error 20 persists: flow sensor is failed — replace

Step 5 — Check entire hot water distribution

• In heavily scaled systems, scale inside pipes reduces fixture flow below the minimum
• Disconnect the hot outlet and test flow directly from the unit
• If unit flow is fine but fixtures are slow: problem is downstream scale

Reset Procedure

Noritz units reset automatically when flow is restored:

1. Fix the root cause (clean filter, open valve)
2. Open a hot water fixture
3. Error 20 should clear as soon as flow exceeds minimum
4. If Error 20 doesn’t clear with adequate flow: flow sensor is failed

Replacement Parts {#parts}

Pro tip: If Error 20 occurs intermittently rather than constantly, check for partially open fixtures or a check valve that is sticking. Intermittent flow loss below the minimum triggers Error 20 briefly before flow returns.

Related Articles

• American Water Heater Error Codes — Complete Guide
• AO Smith Water Heater 3 Flashes — What It Means and How to Fix It
• AO Smith Water Heater 4 Flashes — What It Means and How to Fix It
• A.O. Smith Water Heater Error Codes Guide
• Bradford White Water Heater Error Code 1 — Pilot Outage Fix