What causes Rheem Tankless Water Heater P1?

Too little hot water demand at one fixture, such as a single low-flow faucet or shower head (opening more hot taps raises the flow). Clogged faucet or shower aerator screens restricting flow. A dirty water inlet filter on the heater. Scale or debris in the heat exchanger restricting flow (Rheem points to its 1L lime build-up flushing procedure). Hot and cold supply lines connected backwards on a new installation. A plumbing crossover in the home, where cold water bleeds into the hot lines through a faulty mixing valve or fixture. A recirculation pump problem on systems with a recirculation line. A blocked or stuck water flow sensor, or a failed control board if the heater reads more than 0.4 GPM and still won't fire.

How do I fix Rheem Tankless Water Heater P1?

Open two or more hot water taps at once. If the heater fires, the problem is low flow at the single fixture you were using. Unscrew and clean the aerator screens on the faucets and shower heads you use most. Do Rheem's reset: turn off the water supply to the heater, turn the remote off, wait 10 seconds, turn it back on, then turn the water back on and check the nearest hot tap. Close the water valves, then remove and clean the heater's water inlet filter, following your manual. If it's a new install, check that the cold supply goes to the heater's cold inlet and the hot outlet feeds the house. Check for a plumbing crossover. Turn off the hot water valve at the heater, then open only the hot side at each fixture, including a washer set to hot only. If water flows freely on the hot side, there's a crossover at that fixture. Pressure-balancing single-handle valves may let no water through during this test if there's a crossover. If the heater hasn't been flushed in a while, flush the heat exchanger with a tankless flush kit as your manual describes. Turn the gas off first and leave it off while you flush.

When should I call a pro for Rheem Tankless Water Heater P1?

A technician enters maintenance mode and reads the flow rate on item 1Y with one or more hot taps open, as Rheem's troubleshooting manual describes. If the heater shows 0.4 GPM or less, they look for restrictions: aerators, the inlet filter, scale in the heat exchanger, and a blocked flow sensor. Rheem says to remove the water control valve and clean it if the flow sensor is blocked. If the heater shows more than 0.4 GPM and still doesn't fire, Rheem's manual says to replace the control board. On recirculation systems, they check the recirculation pump and its piping, since Rheem lists it as a P1 check item. They track down any plumbing crossover and repair the faulty mixing valve or fixture.

Code: P1🚿 Rheem Tankless Water Heater

Rheem Tankless Error Code P1: Not Enough Water Flow (Meaning, Causes, Fixes)

Q: What does Rheem Tankless Water Heater P1 mean?

P1 on a Rheem tankless water heater means there isn't enough water flowing through it to fire. Rheem's troubleshooting manual says the heater needs at least 0.4 GPM to activate. In older manuals, P1 shows when the flow stays under that rate for five seconds. Most of the time the fix is on the water side: open more hot taps, clean faucet aerators, clean the heater's water inlet filter, and on new installs make sure the hot and cold lines aren't swapped. If the heater reads more than 0.4 GPM and still won't fire, Rheem says the control board is next. Rheem's High and Super High Efficiency Tankless Troubleshooting Manual lists P1 as Not enough water flow for operation. It says the heater shows P1 on its built-in controller or remote when the flow is too low to run, and that it needs a minimum flow of 0.4 GPM to activate. Its diagnostic check items are low water flow, the hot and cold supply lines, the water filter, and a recirculation pump if one is installed in the recirculation line. Rheem's older Tankless Gas Troubleshooting Manual (SVC 820) calls P1 a maintenance warning for water flow too low. It shows when flow doesn't reach 0.4 GPM at a 35 F temperature rise for five seconds, and the unit can still run once flow goes up. Raising the set temperature is also listed as a remedy there, since a small temperature rise can keep the burner from engaging at a low flow. P1 is a flow warning, not a burner or gas fault. The heater simply isn't seeing enough water to start heating.

Reviewed 2026-10-25 · Based on 4 manufacturer & service sources

What the code means

Rheem's High and Super High Efficiency Tankless Troubleshooting Manual lists P1 as Not enough water flow for operation. It says the heater shows P1 on its built-in controller or remote when the flow is too low to run, and that it needs a minimum flow of 0.4 GPM to activate. Its diagnostic check items are low water flow, the hot and cold supply lines, the water filter, and a recirculation pump if one is installed in the recirculation line. Rheem's older Tankless Gas Troubleshooting Manual (SVC 820) calls P1 a maintenance warning for water flow too low. It shows when flow doesn't reach 0.4 GPM at a 35 F temperature rise for five seconds, and the unit can still run once flow goes up. Raising the set temperature is also listed as a remedy there, since a small temperature rise can keep the burner from engaging at a low flow. P1 is a flow warning, not a burner or gas fault. The heater simply isn't seeing enough water to start heating.

Likely causes

Too little hot water demand at one fixture, such as a single low-flow faucet or shower head (opening more hot taps raises the flow)
Clogged faucet or shower aerator screens restricting flow
A dirty water inlet filter on the heater
Scale or debris in the heat exchanger restricting flow (Rheem points to its 1L lime build-up flushing procedure)
Hot and cold supply lines connected backwards on a new installation
A plumbing crossover in the home, where cold water bleeds into the hot lines through a faulty mixing valve or fixture
A recirculation pump problem on systems with a recirculation line
A blocked or stuck water flow sensor, or a failed control board if the heater reads more than 0.4 GPM and still won't fire

Safe DIY checks

⚠ Before you start

Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.

Open two or more hot water taps at once. If the heater fires, the problem is low flow at the single fixture you were using.
Unscrew and clean the aerator screens on the faucets and shower heads you use most.
Do Rheem's reset: turn off the water supply to the heater, turn the remote off, wait 10 seconds, turn it back on, then turn the water back on and check the nearest hot tap.
Close the water valves, then remove and clean the heater's water inlet filter, following your manual.
If it's a new install, check that the cold supply goes to the heater's cold inlet and the hot outlet feeds the house.
Check for a plumbing crossover. Turn off the hot water valve at the heater, then open only the hot side at each fixture, including a washer set to hot only. If water flows freely on the hot side, there's a crossover at that fixture. Pressure-balancing single-handle valves may let no water through during this test if there's a crossover.
If the heater hasn't been flushed in a while, flush the heat exchanger with a tankless flush kit as your manual describes. Turn the gas off first and leave it off while you flush.

🛠 When to call a pro

A technician enters maintenance mode and reads the flow rate on item 1Y with one or more hot taps open, as Rheem's troubleshooting manual describes.
If the heater shows 0.4 GPM or less, they look for restrictions: aerators, the inlet filter, scale in the heat exchanger, and a blocked flow sensor. Rheem says to remove the water control valve and clean it if the flow sensor is blocked.
If the heater shows more than 0.4 GPM and still doesn't fire, Rheem's manual says to replace the control board.
On recirculation systems, they check the recirculation pump and its piping, since Rheem lists it as a P1 check item.
They track down any plumbing crossover and repair the faulty mixing valve or fixture.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Tankless water heater flush kit

Pump, hoses and descaler for flushing scale from the heat exchanger.

Search eBay →

⚙

Tankless water heater descaler

Use it with a flush kit if lime build-up is restricting flow.

Search eBay →

⚙

Digital multimeter

For a technician checking the flow sensor and control board circuits.

Search eBay →

Sources

Last reviewed 2026-10-25. Always confirm against the manual for your exact model number.

Rheem Tankless Error Code P1: Not Enough Water Flow (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Tankless water heater flush kit

Tankless water heater descaler

Digital multimeter

Related codes

Sources