Oxygen Depletion Sensor (ODS)

Disclaimer: Diagnosis and repair should only be undertaken by qualified personnel.


As noted in previous posts, complete combustion of fuel requires a sufficient quantity of air. Without the right amount of oxygen, incomplete combustion will occur and produce a dangerous amount of carbon monoxide. The American National Standards Institute mandates that unvented gas heaters to be automatically turned off when the specified level of oxygen is not achieved. The automatic shut-off is controlled by the oxygen depletion sensor or ODS pilot system.


The ODS system is part of the pilot assembly designed to regulate the flame performance. The pilot flame is consistent only within a very narrow range, so if oxygen drops below the acceptable level, the flame will lift off the thermocouple, which reduces the millivolt signal going to the gas valve.


An improperly adjusted flame can be caused by the following conditions:


  1. If the flame lifts or blows off the pilot hood, this is an indication that you have too much gas pressure.
  2. A small and weak flame is an indication that the gas pressure is too low.
  3. Debris in the primary air opening produces a large yellow tipped flame through the natural gas orifice.


To adjust the pilot, one must complete adjustments according to the manufacturers instructions. If the pilot is adjusted properly, there will be a steady blue flame that engulfs the upper one third  (some appliances require 1/2 to 3/4 Рrefer to manufacturers instructions) of the thermocouple /thermopile.


Gas flows to the ODS pilot via a precision-bored orifice that pulls in primary air via the air shutter, and is then ignited by the spark discharge from the spark electrode of the piezo igniter. When ignited, the pilot burner ignites the main burner, heats the thermocouple that powers the gas control valve, and shuts off gas flow when the oxygen level around the appliance drops below a predetermined level. The system should be set to shut down if the oxygen level reaches 18 percent.