There are some restrictions for gas and oil fuel burning appliances venting into the same chimney but based on IRC (International Residential Code), NFPA (National Fire Protection Association), and IFGC (International Fuel Gas Code) such configuration is permitted.
The NFPA 31 document applies specifically to oil burning appliances and states that oil burning equipment is permitted to be vented in combination with other oil burning equipment, and with some restrictions fuel gas appliance.
IFGC article 503.5.7.4 dedicated to combination of gas and oil fuel burning appliances:
A listed combination gas- and oil fuel-burning appliance shall be permitted to be connected to a single chimney flue. The chimney flue shall be sized to properly vent the appliance.
2006 IRC article 24126.96.36.199 also permits oil and gas burning appliances into the same chimney.
However, there are limitations associated with combining oil and gas fuel common venting:
Gas-fired appliances are not permitted to be vented in an exterior clay tile lined chimney in New England / this NFPA requirement may also apply in other areas. If this is the type of chimney you have, you should contact your local building department first and verify requirements in your area.
The temperatures of propane and/or natural gas-fired appliances exhaust gases are not sufficient to maintain adequate draft during the cold season. Cooler exhaust gasses will condensate on the chimney’s clay tiled flue surface and most likely spill dangerous carbon monoxide
into the property.
Therefore, an approved and insulated metal liner is required for the gas burning appliances vented into an exterior clay lined chimney flues to ensure proper draft and venting.
When the combination of oil and gas burning appliances are vented into the same chimney flue, the small diameter vent connector which in most cases serves the gas burning appliance must be vented into the chimney flue above the larger diameter vent connector.
The chimney flue utilized for common venting of a gas and oil burning appliance must be large enough to properly vent the exhaust gasses from both appliances operating at the same time, but at the same time it must be small enough to provide sufficient draft for the smallest appliance only.
Various jurisdictions adopt and utilized different codes and different code editions because of some specific geographical location and weather conditions that may not occur anywhere else. For your own safety you should verify local requirements for the gas and oil burning appliance venting into a common chimney.
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