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Water Heater Backdraft

My waterheater is located in the basement of a two story house. I also have a wood burning fireplace in the basement that has an outside air intake. The waterheater is vented with a 3" duct that is to a "y" connection on a 6" vent duct that runs to the roof. The 3" waterheater duct is orphaned as the 6" duct was once also attached to our old furnace.

Issue is that when I burn a fire in the fireplace, I get a backdraft down through the water heater 3" duct. I have to open a basement window to re-establish venting but as soon as I close the window, the backdraft comes back.

Should I put a booster fan on the 6" duct or should I run a 3" or 4" duct through the middle of the 6" duct? Or???

Re: Water Heater Backdraft

When the fireplace is operation it is creating a substantial draft which is creating negative pressure inside your house. Opening the window relieves the draft's "search/demand" for make-up air. When you close the window, it draws air from whenever it can get it. It will seek the path of least resistance....which in this case is or includes the flue for your WH.

What you need to install is a combustion and draft air inlet... in and dedicated to the firebox of the fireplace. Trying to overcome the WH backdraft problems with a power-vent or similar will leave the two in a battle for make-up air and most likely your house is tight enough that both of them cannot be satisfied at the same time. If you should try the power-vent route...both devices will most likely starve for air... and flame roll-out will still likely occur on the WH... resulting in CO gases being injected into the house....and the fireplace may do the same...spewing both CO and smoke inside the house. In addition...oxygen depletion inside the house to some degree is a likely probability.

Your fireplace will also run more beneficially to you with its own combustion and draft air provision as it won't be sending the heated air from inside the home up the chimney.

It's either that or keep cracking a window to provide make-up air for the fireplace and to prevent the WH backdraft...and to keep yourself alive.

PS- Another potential option is to install a "draft intake vent/opening" on the outside of the chimney itself. This would be above the fireplace damper and would provide the bulk of the air consumed/sent up the chimney via draft. Most of the combustion air will still come from the house interior, but not so much of the heated air inside the house will get drawn up into the draft and out the chimney. (This draft vent has to be constructed such that rainwater cannot enter/opening sloped downward....but wind can sometimes still be a problem...depending upon how your house is situated and what surrounds it... or not. A little door on/over this exterior vent can be a big plus in that regard, but means you'd have to make a trip outside to open and close it.)

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