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No heat or hot water: Codes, Causes & Fixes

When a gas boiler gives no heat or hot water, the display usually shows why: low system pressure, an ignition lockout, a frozen condensate pipe on condensing models in cold weather, or a thermostat or timer that isn't calling for heat. Safe checks are limited to reading the pressure gauge, confirming the gas supply and other gas appliances work, checking thermostat settings and pressing reset once. If the fault returns, call a licensed heating technician; if you smell gas or a CO alarm sounds, leave the building and call your gas utility or 911.

11Codes
3.0Average severity
18%High or critical
0%DIY-friendly
Severity distributionSeverity distribution

All codes (11)

All Severity 3+ Severity 4+
10 Rinnai · Water Heater / Boiler The Rinnai can't breathe: a frozen condensate line, a blocked vent terminal or a dirty air… High 11 Rheem · Water Heater / Boiler Rheem tankless didn't light at startup. Rheem's own first fix: check the gas shut-off valve… Moderate 11 Rinnai · Water Heater / Boiler The Rinnai tried to light but never proved a flame – gas off, air in the line or low gas… Moderate 12 Rinnai · Water Heater / Boiler The Rinnai lit but then lost its flame or stopped sensing it – usually gas supply, venting… Moderate 29 Rheem · Water Heater / Boiler Condensing Rheem tankless: acidic condensate can't drain, so the heater stops before it floods… Moderate E003 Navien · Water Heater / Boiler The heater sparked and opened its gas valve, but never saw a flame – often a closed valve or no… Moderate E012 Navien · Water Heater / Boiler The burner lit, then lost its flame signal mid-cycle – think gas pressure, grounding or a dirty… Moderate E02 A. O. Smith · Water Heater / Boiler The tank water got hotter than the safety limit, so the ECO cut power to both elements – reset… Moderate E04 A. O. Smith · Water Heater / Boiler The electronic thermostat can't power the upper element; the lower one still works, so hot… Low E110 Navien · Water Heater / Boiler The fan can't move enough air through the vent system – most often a clogged intake filter… High LC Rinnai · Water Heater / Boiler Mineral scale is coating the heat exchanger – the Rinnai needs a descaling flush; the digit… Low

Symptoms

Not heating 71Won't start a program 55Not cooling 41Leaking water 17Not draining 16Not filling with water 14Door won't lock or open 13Not spinning 9Low system pressure 1