The photo below shows typical combined furnace water heater venting.
Furnace and water heater venting diagram.
The conditions explained in this post also apply to the wood burning appliances.
Smith power vent gas water heaters are equipped with an electric blower fan that is installed on top of the water heater which requires an electrical outlet to be close to the water heater.
The result is a back draft of combustion products out of the water heater s draft hood and into the building.
This structure generally uses the furnace vent pipe through roofbecause the b venting pipe is designed in vertical manner.
With a replacement high efficiency furnace the existing vent pipe does not work for water heater only because the vent is too large.
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Also known as the type b this vent is operating on the natural air convection.
The blower pushes the exhaust and excess heat through the exhaust venting.
The old vent pipe is too large because it served both the furnace and water heater.
Gas water heater vent codes standards.
High e furnaces have separate venting.
Consequences of improperly assembled vent pipes problematic chimney flue deteriorating chimney walls can be sometimes fatal fire and carbon.
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When the vent is withdrawing air from the house it expels the gas outside through the vent.
In comparing the venting of exhaust gases from a high efficiency furnace boiler or water heater with the venting of exhaust gases from a conventional heating system it will be immediately obvious that the high efficiency equipment exhaust is produced at a low enough temperature that it is vented through comparably small diameter plastic piping.
The lower btuh of a gas water heater compared with a heating furnace or boiler will very often be unable to heat the chimney interior sufficiently to develop a draft.