How House Works

Rheem code P1 (not enough water flow): what to check

By How House Works EditorialBased on Rheem manualsChecked against sources Updated

Rheem display

P1

“Not enough water flow to operate the unit.”

RTGH-3, RTG-3, RTG-R-3, Ruud Mid-Efficiency · RTGH-3 Use and Care Manual, p. 75; RTG-3 Use and Care Manual, p. 77; RTG-R-3 Use and Care Manual, p. 74; RTGH-3 and RTG-3 Troubleshooting Manual, p. 34; RTG-R-3 Troubleshooting Manual, p. 27; Tankless Water Heater Use and Care Manual AP18000-2, p. 26
Owner can check

The RTG-R-3 chart adds "Water flow was low during recirculation cycle." and asks you to confirm there is no debris in the inlet water filter (RTG-R-3 manual, p. 74). The troubleshooting manual gives 0.4 GPM as the minimum flow to activate the heater (AP21822 tech manual, p. 34).

What it means

P1 means the water flow is too low for the heater to run. It needs at least 0.4 gpm (1.5 L/min) to start RTGH-3 and RTG-3 Troubleshooting Manual, AP21822, p. 34; RTG-R-3 Troubleshooting Manual, AP24838, p. 27; RTGH-3 Use and Care Manual, p. 38. The RTG-3 and RTG-R-3 manuals add that it needs at least 0.25 gpm (1.01 L/min) to keep running RTG-3 Use and Care Manual, p. 36; RTG-R-3 Use and Care Manual, p. 36. Operation also needs at least 14 psi (97 kPa) of water pressure, and 40 psi (276 kPa) for the maximum flow rate RTGH-3 manual, p. 38.

The chart’s remedy is to increase the water flow from the fixtures RTGH-3 manual, p. 75.

Check these first

Most of these steps come from Rheem’s troubleshooting chart for “Not enough or no hot water” RTGH-3 manual, p. 72; RTG-3 manual, p. 74; RTG-R-3 manual, p. 71.

  1. Open the hot tap all the way. The main burner goes off when the incoming water volume is inadequate RTGH-3 manual, p. 72.
  2. Make sure the water shut-off valve to the heater is completely open RTGH-3 manual, p. 72.
  3. Clean the aerator on the tap if it is clogged or dirty RTGH-3 manual, p. 72.
  4. If the water piping may be frozen, let it thaw RTGH-3 manual, p. 72.
  5. Before you touch the inlet water filter, drain the heater as the manual describes; its warning is about scalding. Turn the heater off, close the gas shut-off valve, unplug the heater at least 10 seconds later, run the hot water until it is cold, close the water shut-off valve and open the drain valve RTGH-3 manual, p. 69; RTG-3 manual, p. 71; RTG-R-3 manual, p. 68.
  6. Clean the inlet water filter at the base of the cold water inlet line. On RTG-R-3 heaters the P1 row asks for this check directly RTG-R-3 manual, p. 74. Unscrew the filter without tapping or forcing it, rinse it under running water, using a soft brush for heavy sediment, and screw it back until snug without overtightening RTGH-3 manual, pp. 66–67; RTG-3 manual, pp. 68–69; RTG-R-3 manual, pp. 65–66. To put the heater back in service, close the drain valve, open the water shut-off valve, run the hot taps until the air is out, reconnect power, open the gas shut-off valve and press POWER ON/OFF RTGH-3 manual, p. 70; RTG-3 manual, p. 72; RTG-R-3 manual, p. 69.
  7. If P1 is still shown, run the reset sequence from the chart section and call for service if the code stays RTGH-3 manual, p. 74.

When to call a licensed technician

  • P1 stays with the tap wide open, a clean aerator and a clean filter.
  • Hot and cold water lines reversed at the heater. The chart’s fix is to reverse them RTGH-3 manual, p. 72, and Rheem says plumbing should be done by a qualified plumbing contractor RTGH-3 manual, p. 38.
  • Low house water pressure. Rheem’s sizing formula adds pipe and faucet pressure losses and a safety margin of more than 5 psi (34 kPa) to the 14 psi minimum RTGH-3 manual, p. 38.
  • The troubleshooting manual’s checks, written for technicians and service professionals only AP21822 tech manual, p. 7: debris in the heat exchanger, which calls for flushing; a blocked water flow sensor; a plumbing cross-over in the house; and a flow reading in maintenance mode, where more than 0.4 gpm with no firing points to the control board AP21822 tech manual, p. 34.

Notes for different models

The RTGH-3 troubleshooting chart sends you to page 75 for the water filter, but the cleaning steps are on pp. 66–67 RTGH-3 manual, p. 72. The RTG-3 and RTG-R-3 charts point to the right pages RTG-3 manual, p. 74; RTG-R-3 manual, p. 71.

IKONIC heaters use T113; their service manual describes it as a flow rate below 0.4 gpm IKONIC Use and Care Manual, p. 107; IKONIC Technical Training and Service Manual, p. 65. The chart for Ruud Tankless Mid-Efficiency heaters prints the same P1 row as the -3 charts Tankless Water Heater Use and Care Manual AP18000-2, p. 26.

Related Rheem codes

  • 1L “Water heater has buildup of lime deposits.”
  • 63 “There is an abnormality in the pump's operation.”
  • T113 “Not enough hot water to operate the unit.”

All Rheem codes

Rheem is a trademark of its owner. How House Works is not affiliated with or endorsed by Rheem.

Sources

  1. Rheem Manufacturing Company. RTGH-3 Use and Care Manual. AP21820 Rev 06, no printed date; cover title: Super High Efficiency Condensing Tankless Water Heater Use and Care Manual; the same file is posted for Ruud RUTGH models; p. 1 is the unnumbered cover. Accessed Oct 12, 2026.
  2. Rheem Manufacturing Company. RTG-3 Use and Care Manual. AP22009 Rev 06, no printed date; cover title: High-Efficiency Non-Condensing Tankless Water Heater Use and Care Manual; the same file is posted for Ruud RUTG models. Accessed Oct 12, 2026.
  3. Rheem Manufacturing Company. RTG-R-3 Use and Care Manual. AP23962 Rev 02, no printed date; cover title: High-Efficiency Non-Condensing Tankless Water Heater with Recirculation Use and Care Manual; the same file is posted for Ruud RUTG-R models. Accessed Oct 12, 2026.
  4. Rheem Manufacturing Company. RTGH-3 and RTG-3 Troubleshooting Manual. AP21822 Rev 02, no issue date printed; cover title: High and Super High Efficiency Tankless Water Heater Troubleshooting Manual; written for technicians and service professionals only (p. 7). Accessed Oct 12, 2026.
  5. Rheem Manufacturing Company. RTG-R-3 Troubleshooting Manual. AP24838 Rev 01, no issue date printed; cover title: High Efficiency Tankless Water Heater with Recirculation Troubleshooting Manual; written for technicians and service professionals only (p. 6). Accessed Oct 12, 2026.
  6. Rheem Manufacturing Company. IKONIC Use and Care Manual. AP22745 Rev 07, no printed date; cover title: Condensing Tankless Water Heater Use and Care Manual; Ruud's S and SR Series pages post Rev 06. Accessed Oct 12, 2026.
  7. Rheem Manufacturing Company. IKONIC Technical Training and Service Manual. AP23209 Rev 03, no printed date; written for technicians and service professionals only (p. 4). Accessed Oct 12, 2026.
  8. Rheem Manufacturing Company (posted on ruud.com). Tankless Water Heater Use and Care Manual AP18000-2. AP18000-2 Rev 03, no printed date; cover title: Tankless Water Heater Use and Care Manual, Noncondensing 199,900 Btu/h, 180,000 Btu/h, and 150,000 Btu/h Models; the manual prints no model numbers and no -3 suffix; ruud.com posts it on its Tankless Mid-Efficiency 70, 84 and 95 product pages (2026-10-12). Accessed Oct 12, 2026.