Rheem Tankless Water Heater Error Code P1: Meaning & How to Fix It

By the ErrorCodePro Editorial Team · Last reviewed October 20, 2026

Rheem tankless water heater error code P1 means: There isn't enough water flow to run the unit. [1] Keep using it? Yes. Use a higher flow rate. Start with the owner checks below, beginning with: increase the flow.

Smell gas? Leave the building now. Do not touch light switches, phones or appliances inside. Call your gas utility or 911 from outside. CO alarm sounding or headache, dizziness or nausea? Get everyone outside and call 911. Gas valves, burners, venting and wiring are for a licensed technician only. See our safety policy.

Quick facts

Code	P1
Also shown as	None listed by the manufacturer
Brand / appliance	Rheem tankless water heater (rtgh high-efficiency non-condensing tankless gas water heaters (use and care manual))
System	Water fill
Official meaning	There isn't enough water flow to run the unit. [1]
Keep using it?	Yes. Use a higher flow rate
DIY-safe?	Owner checks only (listed below)
Support	Rheem tankless support, 1-866-720-2076

On this page: Meaning · Steps · Parts · When to call a technician · Related codes · FAQ

What error code P1 means on a Rheem tankless water heater

There isn't enough water flow to run the unit. [1]

This is a water fill code. Codes and wording can vary by model, so check your owner's manual for your model number.

How to fix Rheem tankless water heater error code P1 (safe owner checks)

Owner checks only: on heating, cooling and water-heating equipment, limit yourself to reading the code and checking the thermostat, breaker and service switch, air filter, condensate drain, that the gas supply valve is open (no disassembly) and that vent terminals are clear of snow or debris. Reset power only if your manual allows it.

Safety first: disconnect power before you remove any panel or reach into the sump or drum. Do not touch wiring, the control board or the motor. If you see water near electrical parts, stop and call a technician. These steps are based on Rheem's published guidance [1], rewritten for clarity.

Increase the flow. Open the hot water fixture further to increase the water flow.

Parts that may be involved

Exact parts depend on your model number. This page lists part types only, never part numbers.

No specific part is implied by the manufacturer's guidance for this code.

When to call a technician

The code continues at normal flow: dealer or qualified service technician
Gas valves, burners, igniters, flame sensors, venting/flue or CO concerns: licensed HVAC technician or plumber
Refrigerant work: EPA 608-certified technician only
Smell of gas: leave and call the gas utility or 911
Anything involving wiring, the control board, the motor or the water valve electrics
The appliance is under warranty: contact Rheem tankless support, 1-866-720-2076 first

Frequently asked questions

What does error code P1 mean on a Rheem tankless water heater?

Rheem lists P1 as follows: There isn't enough water flow to run the unit.

Is it safe to keep using the tankless water heater with P1?

Yes. Use a higher flow rate.

Can I fix Rheem tankless water heater error P1 myself?

Only with the owner checks on this page, starting with: increase the flow. If it persists, call a technician or Rheem tankless support, 1-866-720-2076.

Sources

https://files.rheem.com/blobazrheem/wp-content/uploads/sites/2/RTG-Use-and-Care.pdf (official)

Last reviewed: October 20, 2026. ErrorCodePro is not affiliated with Rheem.