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What are the codes regarding venting gas and oil appliances into same flue

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I have a customer who has a Weil McLain 366 oil fired boiler and a 40 gal gas water heater .  A chimney contractor installed a 6 in stainless " slinky type " chimney liner and I have an interrmittant poor drafting situation.  The boiler manufacturer requires 7 in just for boiler.  My concern is that the liner comes right into basement and the contractor did not create a breeching for particulate accumulation and that the connection for the gas water heater is on the horizontal not on the verticle.

Do you know of a code whch will clarify this situation?
asked in Heating System by oildr71 (120 points)
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1 Answer

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Hi Oildr71,

Sorry for a delay, I wasn’t around yesterday. I did answer similar question here:


The chimney flue diameter dimensions (there are exceptions but you’d have to contact appliance’s manufacturer for guidelines of this particular boiler):


M1805.3 Size of chimney flues.
The effective area of a natural draft chimney flue for one appliance shall be not less than the area of the connector to the appliance. The area of chimney flues connected to more than one appliance shall be not less than the area of the largest connector plus 50 percent of the areas of additional chimney connectors.

Exception: Chimney flues serving oil-fired appliances sized in accordance with NFPA 31.

The following should answer connector concern. In general: the material / connector type used outside of the chimney (between the appliance and the chimney) must be listed / approved to be used with oil burning appliances. Flue liners are often designed and listed to be used only as a liner and should not continue beyond the chimney wall / into the basement.


M1805.2 Masonry chimney connection.
A chimney connector shall enter a masonry chimney not less than 6 inches (152 mm) above the bottom of the chimney. Where it is not possible to locate the connector entry at least 6 inches (152 mm) above the bottom of the chimney flue, a cleanout shall be provided by installing a capped tee in the connector next to the chimney. A connector entering a masonry chimney shall extend through, but not beyond, the wall and shall be flush with the inner face of the liner. Connectors, or thimbles where used, shall be firmly cemented into the masonry.


M1803.2 Connectors for oil and solid fuel appliances.
Connectors for oil and solid-fuel-burning appliances shall be constructed of factory-built chimney material, Type L vent material or single-wall metal pipe having resistance to corrosion and heat and thickness not less than that of galvanized steel as specified in Table M1803.2.





Less than 6



6 to 10



Over 10 through 16



Just click “comment” below if you have additional questions.

answered by darekrudy (21,730 points)