How House Works

Rheem code 90 (blocked vent or air intake): what to check

By How House Works EditorialBased on Rheem manualsChecked against sources Updated

Rheem display

90

“Air intake or vent exhaust opening may be blocked.”

RTGH-3, RTG-3, RTG-R-3 · 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. 61; RTG-R-3 Troubleshooting Manual, p. 56
Owner checks, then technician

The charts print one row for "90 or 99", with a second cause: "The vent pipes on the vent termination may not be connected properly." The troubleshooting manual says 90 occurs before the unit goes into ignition and 99 after it shuts down (AP21822 tech manual, p. 61).

What it means

Code 90 means the heater found a blockage in its venting before it went into ignition. The troubleshooting manual says the heater checks for a blockage during its pre-purge and post-purge cycles, that the same code appears if the heater was run before the vent was installed, and that 90 occurs before ignition while 99 occurs after the unit shuts down RTGH-3 and RTG-3 Troubleshooting Manual, AP21822, p. 61; RTG-R-3 Troubleshooting Manual, AP24838, p. 56.

The owner charts print 90 and 99 in one row with two causes. A blocked air intake or vent exhaust opening is for the owner to clear. Vent pipes not connected properly at the termination go to a dealer or qualified service technician 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.

Check these first

  1. Close all hot water taps. Turn the heater off with the POWER ON/OFF button on the remote control, wait about 5 minutes, turn it back on, then open a hot tap and check the display RTGH-3 manual, p. 74; RTG-3 manual, p. 76; RTG-R-3 manual, p. 73.
  2. If 90 is still shown, close the tap, turn the heater off, unplug it for about 30 seconds and plug it back in. Keep the heater turned off while you check the venting RTGH-3 manual, pp. 66, 74.
  3. Go outside to where the air intake and vent exhaust end, and remove anything blocking them. Leave at least 12 in. (30 cm) between the air intake opening and any obstruction RTGH-3 manual, pp. 66, 75; RTG-3 manual, p. 68; RTG-R-3 manual, p. 65.
  4. On RTGH-3 heaters, the owner maintenance list also includes a visual check of the vent screens, removing any debris or blockage RTGH-3 manual, p. 68.
  5. Restart the heater, open a hot tap and check the display. If 90 is still shown, turn the heater off, write down the code and call for service RTGH-3 manual, p. 74.

Do not run the heater if the vent system shows signs of leaking exhaust RTGH-3 manual, p. 64; RTG-3 manual, p. 66; RTG-R-3 manual, p. 63.

When to call a licensed technician

  • Vent pipes that are loose or not connected properly at the termination. The chart sends this to a dealer or qualified service technician RTGH-3 manual, p. 75.
  • Code 90 that returns with the openings clear.
  • Any change to the vent run. All three manuals require venting to follow national and local codes and the vent manufacturer’s instructions, and forbid venting through a chimney RTGH-3 manual, p. 20; RTG-3 manual, p. 21; RTG-R-3 manual, p. 21.
  • RTGH-3 vent limits: only 2 or 3 in. pipe of approved materials RTGH-3 manual, p. 20. With no elbows, the longest equivalent run is 60 ft (18.3 m) of 2 in. pipe or 150 ft (45.7 m) of 3 in. pipe; a standard 90° elbow counts as 6 ft (1.83 m), and DIP switch settings depend on vent length RTGH-3 manual, p. 21.
  • RTG-3 and RTG-R-3 direct-vent limits: 3 in./5 in. (7.6 cm/12.7 cm) UL-approved Category III coaxial stainless steel vent pipe, or concentric vent pipe approved by the water heater manufacturer RTG-3 manual, p. 21; RTG-R-3 manual, p. 21. With no elbows, the longest equivalent run is 43 ft (13.1 m) RTG-3 manual, p. 22; RTG-R-3 manual, p. 22.
  • The yearly vent inspection. The RTGH-3 manual says the venting system should be inspected annually to make sure every section is secure and airtight, and that qualified service personnel can advise RTGH-3 manual, p. 64. The RTG-3 and RTG-R-3 manuals assign that inspection to a qualified service technician RTG-3 manual, p. 66; RTG-R-3 manual, p. 63.

The manuals warn never to run the heater unless it is properly vented RTGH-3 manual, p. 3; RTG-3 manual, p. 21; RTG-R-3 manual, p. 21.

Notes for different models

Code 99 shares this chart row. Rheem’s troubleshooting manual also lists 99 as a “hard lockout” whose reset is done behind the front cover AP21822 tech manual, p. 61.

IKONIC heaters show A090, and their chart adds a third cause, “Blocked vent termination or inlet air filter.”, with a call to a dealer or qualified service technician IKONIC Use and Care Manual, p. 107. IKONIC models have an inlet air filter that Rheem recommends inspecting and cleaning once a year IKONIC Use and Care Manual, p. 97. The RTGH-3, RTG-3 and RTG-R-3 manuals give owners no air filter procedure.

On Ruud Tankless Mid-Efficiency heaters the chart uses 05 for a blocked air intake or vent, and 99 for a faulty blower motor Tankless Water Heater Use and Care Manual AP18000-2, p. 26.

Related Rheem codes

  • 99 “Air intake or vent exhaust opening may be blocked.”
  • 10 “Incorrect inlet air volume”
  • 14 “Water heater is overheating.”

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 (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.