I’ve always believed it was improper to mix gas/fuel oil in same flue. Yesterdays inspection had both (fuel oil water heater/LP boiler) into same class A. What was odd to me, the boiler was installed in 1996 and had a county approved sticker on it.
Here’s some pics. Also, check out the great beam cutting job!
I didn’t think you were supposed to mix fuels either, but my codecheck book says “OK to vent gas & Oil to the same chimney” IRC 2426.5.6.2 UMC 809.0
dear lord. did we fix this mess? haha
i suppose codes are different everywhere, but here, you can have 2 different gas’ in the same chimney, but not in the same flue. it must be 2 different entries into the chimney
Years ago we tried to run a wood stove and a oil furnace in the same chimney. Insurance shut us down and made us put in another chimney. Again every area is different as is insurance companies.
Lawrence
Same chimney OK, but into different flues,… most jurisdictions don’t even allow two different wood burning fire places to expel into the “same” chimney flue. Check local codes.
Wood stoves (and other solid fuel burning appliances) or fireplaces must have a dedicated flue per the IRC. However gas and oil equipment can share the same flue in many areas (e.g. per the IRC), although some consider it poor practice. Your local mileage may vary … 
what about the furnaces that are Oil, Wood, and Coal with three different heatin chambers and one flue. With a heat exchanger for the water heater.
Thats a special case where the equipment is designed/listed for common venting. But since it uses solid fuel it should not be vented in common with any other equipment (e.g. IRC M1801).