Any time there is combustion of any type fuel, from a Rinnai heater to a car engine. There is always a small amount of Carbon Monoxide given off. But, it's usually in small insignificant amounts.
Since your model is a Vent-free or flue-less model, the chances are a little higher. This usually occurs if the heater has not been properly and regularly maintained. Keep in mind, flue-less heaters take in household air for combustion, heat it and send it back out. Whatever is floating in the household air (duct, lint, pet hair, etc) is pulled into the heater. These things can clog the burner(s), the pilot and the Oxygen Depletion Sensor (ODS) and cause incomplete combustion. Which translates into higher than normal Co levels. However, your unit is equipped with an ODS, that will shut the gas off to the heater, if the household oxygen level reaches 25%. Which is not the danger level, which is 19%. Once the Oxygen level returns to 30%, the ODS will release the gas valve and gas will return to the pilot, so that you can relight it.
When properly maintained and operated according to the Owners Manual (see link below) they are totally safe. In the last 20 years, millions of vent-free/flue-less heaters have been sold and operated throughout the world. There has never been a death related to their use.
Certainly, if you or your family think you are experiencing signs of carbon monoxide poisoning, stop using the heater and have a qualified Rinnai technician check and service the heater. If you have other gas appliances in the house, it may be one of them causing the symptoms and not the heater. You may want to have those check and serviced, too.
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