WH PVC vent pipes can be used only with power-vent /exhaust motor equipped hot water heaters also known as an induced draft gas water heaters (also CPVC and ABS but some manufacturer and local restriction may apply to all 3 types).
Furnace and / or water heater chimney requires proper placement and termination to safely discharge exhaust gases from the appliances combustion (fuel burning) process. Improper chimney placement and termination is often responsible for back-drafts, excessive corrosion of the chimney pipe walls, condensation, and Carbon Monoxide contamination of your house.
The rule explained on the pictures is just one of the several that have to be applied when installing the chimney. Make sure that you have a licensed professional performing gas water heater & furnace flue sizing calculations while replacing or adding any of those appliances in your home.
Natural (gravity) and motor [...]
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. The conditions explained in this post also apply to the wood burning appliances.
Consequences of improperly assembled vent pipes, problematic chimney flue, deteriorating chimney walls can be sometimes fatal – fire and Carbon Monoxide poisoning are topping the list.
Therefore, combustion products from your furnace, water heater, wood burning fireplace, and any other appliance that requires some kind of an exterior vent / chimney, have to be properly properly discharged to the exterior.
Certain requirements [...]