Follow the rules for venting or risk premature failure.
Last month, I addressed the changes in the classification of venting systems that impact the venting of a water heater. A standard tank-type, gas-fired water heater is a Category I appliance. Also in Category I are many residential furnaces and boilers. However, furnaces and some boilers use fan-assist combustion. As a result, this category of appliances is classified as Category I, Fan, whereas a water heater is Category I, Nat.
Combining a fan-assist combustion appliance with a natural burning combustion appliance requires attention to detail in sizing and installing the vent system. The fan-assist venting must match with the natural venting to allow the vent system to operate properly. Failure to follow the rules for venting can result in the premature failure of a vent system. This, of course, can lead to the escape of carbon monoxide into a building. A lethal dosage of carbon monoxide is 0.04 percent (not 4 percent, 0.04 percent) in the breathing atmosphere. That is a rather small quantity of CO. Hence it is imperative that you get the venting system right.