The conditions explained in this post also apply to the wood burning appliances.
Furnace and water heater venting diagram.
Gas water heater vent codes standards.
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 most common problem associated with water heater venting is a condition called backdrafting in which exhaust gases from the water heater fail to exit the home via the vent and instead end up in the house backdrafting can have many causes but it is most commonly due to poor vent design or installation and or an imbalance of air volume in the home.
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.
With a replacement high efficiency furnace the existing vent pipe does not work for water heater only because the vent is too large.
Proper furnace water heater venting is an extremely important issue which depends on the performance of the house chimney chimney flue or other exhaust assemblies.
When the vent is withdrawing air from the house it expels the gas outside through the vent.
Proper venting prevents backdrafting.
Once a high efficiency condensing furnace removes this excess heat that conventional furnaces vent to the outside of a home the leftover mixture is acidic water.
Also known as the type b this vent is operating on the natural air convection.
This combination of carbon dioxide and water would damage the type of vents used in conventional systems which run vertically out of a home.
The photo below shows typical combined furnace water heater venting.
Traditional gas fired forced air furnaces produce hot combustion exhaust gasses and therefore need metal vent pipes or chimneys.
Consequences of improperly assembled vent pipes problematic chimney flue deteriorating chimney walls can be sometimes fatal fire and carbon.
In contrast modern high efficiency condensing furnaces exhaust much cooler gasses and need only plastic pipe materials such as pvc cpvc or abs for their exhaust vents some high efficiency furnaces also include a plastic pipe for intake area and all types.
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The old vent pipe is too large because it served both the furnace and water heater.
High e furnaces have separate venting.