Wood fireplace smoke entering your house through an adjoining chimney flue pipe is quite “popular” and annoying issue. There are a few things you can try in order to correct this condition. Extending the water heater flue is one of those options. Some wood burning fireplace manufacturers recommend having a furnace and / or water heater chimney flue at least 16”-18” higher than the wood fireplace flue.
However, draft problems might arise from having such flue extension installed, especially when dealing with low ambient temperatures (that would depend on your geographical location) and having only a single water heater with 3” draft hood discharging into a 6” flue. An exterior chimney with 3 exposed walls (one wall shared with the house) might escalate the problem.
Even that based on a 7 times rule
you can have up to 7” diameter chimney flue to vent a 3” diameter draft hood water heater, during some cold days the water heater might have problems with proper discharge of exhaust gases. The extension will become much colder than the flue section hidden behind the chimney walls causing condensation and compromising exhaust discharge.
For interior chimneys (only small portion exposed above the roof) and with longer water heater cycles (heating water after taking a long shower) the flue extension should not be a problem. Just to make sure that there’s no Carbon Monoxide spillage into the basement, make sure that you have a Carbon Monoxide detector installed in that area (you can check Carbon Monoxide detector locations
for more info.).
Second option is to use wind directional cap on the fireplace flue. Those chimney caps improve draft and direct smoke further away from the chimney.
Another way to prevent smoke from entering adjoining flue is to eliminate the negative pressure inside the house – that’s the most important way! To make our homes more energy efficient we seal the gaps and holes forgetting that all fossil fuel burning appliances require combustion air / oxygen to support clean combustion process. Additionally, turning any of the house vents ON (kitchen, bathroom, clothes dryer, etc.) creates a vacuum inside your home and so called make-up air must be provided.
For example; if your fireplace uses only combustion air from the house’s interior (no extra combustion air port inside the fireplace), once the flue gets warm it creates suction that pulls the air out from your house eventually creating negative pressure. Unless you have a direct vent fireplace that uses 100% of exterior air for combustion you will need to re-supply that air that goes through the chimney from somewhere else - the water heater flue.
As the air is drawn down the water heater flue (whenever it is cold), it picks up smoke exiting from the fireplace’s flue. The solution would be to provide makeup air to the house and make sure that you have sufficient amount of combustion air for the water heater (and any other appliance(s)) in your basement to eliminate negative pressure issue.
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