Every wood burning fireplace should have a dedicated flue. Many old houses have two fireplaces on two different stories sharing one flue. Back-to-back fireplaces sometimes also share one flue. These are unsafe because the exhaust products from one fireplace can find their way back into the house through another fireplace.
Where this condition is found, experts often recommend sealing off one of the fireplaces.
Many authorities will permit the connection of two gas appliances to one flue, for example, even if the appliances are on separate floor levels.
Wood stoves can sometimes be connected with other appliances to the same flue.
Oil fired appliances usually have to be at the same story or floor level. Two oil burning appliances can be on the same flue if they are on the same story. Where a wood stove and an oil-fired furnace, for example, are connected to the same flue, they must be on the same floor and the wood stove connection must be below the oil connection. Generally speaking, the heaviest fuel is connected lowest.
These rules vary, depending on location and authority. Sometimes the building code authority and gas utility, for example, are not in agreement.
In attached, row or townhouses, it's common to find one chimney serving two dwellings. The chimney may have four flues. Two flues would be for a furnace (or boiler) and a fireplace in one house, the other two flues would be for a furnace (or boiler) and fireplace in the other dwelling. Sometimes these flues are traded back and forth between the houses, sometimes unintentionally. One flue should not serve appliances in different homes.
You won't always be able to tell, but be careful with this situation. It's possible that you have more appliances connected to the flue than it seems. Where you are unsure, we recommend further investigation. Look up (or down) the chimney as far as possible, for openings.
This condition may be the result of -
appliances added or rearranged
Fire and life safety hazards are the risks.
This requires checking the number and locations of flues (not just chimneys) on the outside, and comparing this with the number, type and location of appliances throughout the house. Watch for two wood-burning fireplaces on one flue (two gas fireplaces may be all right). Watch for wood and oil burning appliances sharing a flue (this may be all right if on the same floor level, and the wood stove enters the chimney below the oil vent). Watch for a wood-burning fireplace and any other appliance on the same flue (not acceptable).