cb_db
New Member
Long story but I'll try to keep it short. 11 year old WH. Atmospheric vent. 10 year old mid-efficiency furnace. Both share a B vent. 4" vent on both furnace and WH and B vent is 5". Both are installed in a closet in my garage. Combustion air is provided by a 8x8 hole in the foundation wall to the outside. WH vents ABOVE furnace which I've read is the correct configuration. BUT, the WH immediately vents to an elbow into the B vent (they are right net to each other so the run is what it is.
I've been preparing to swap the WH out with a new unit but noticed signs of backdrafting. Some light deformation of my plastic escutcheons is visible. After extensive testing I ended up providing more combustion air to the space by removing a restrictive grille and making my own with 1/4" mesh fabric.
I'm following what is suggested to test for proper drafting. All exhaust vents on, dryer, kitchen vent hood and two burners on high for good measure. I get zero backdraft when the WH is lit on its own. All is well. What does happen is no matter how long the WH has been running once the inducer fan on the furnace kicks on I get some degree of backdrafting. Anywhere from 10-45 seconds depending on how cold the flue is. Once the fan turns off and the furnace fires up the gases get sucked out immediately. I DO NOT get backdraft during the furnace's mid-cycle inducer start up (not sure what the term is). I believe that's because the flue is at its hottest and draft is not a problem.
So I know this isn't an air supply issue. But from what I've read those inducer fans always create some amount of backdraft. In fact I read that the Building Performance Institute allows up to 60 seconds of backdrafting from an atmospheric-venting appliance.
What I'm thinking is installing a short WH so I can get another 9-10" of vertical vent before the elbow...maybe that might help. But logic tells me the fan is always going to push back some of that exhaust...like connecting two straws to a single one with a wye, blowing in both but whoever is blowing harder will win (the inducer fan).
Also, I'm sure the air temp isn't helping but there's not a whole lot I can do about that since it's in an unconditioned space. I've noticed my dad's WH backdrafts when his inducer fan kicks on. It's not as extreme though.
Sorry for the ramble. I'm just at a loss over if this is as serious of an issue as I've made it in my head. Never had any issues in 10 years other than the slightly deformed plastic escutcheons. Pic attached which shows the setup.
I've been preparing to swap the WH out with a new unit but noticed signs of backdrafting. Some light deformation of my plastic escutcheons is visible. After extensive testing I ended up providing more combustion air to the space by removing a restrictive grille and making my own with 1/4" mesh fabric.
I'm following what is suggested to test for proper drafting. All exhaust vents on, dryer, kitchen vent hood and two burners on high for good measure. I get zero backdraft when the WH is lit on its own. All is well. What does happen is no matter how long the WH has been running once the inducer fan on the furnace kicks on I get some degree of backdrafting. Anywhere from 10-45 seconds depending on how cold the flue is. Once the fan turns off and the furnace fires up the gases get sucked out immediately. I DO NOT get backdraft during the furnace's mid-cycle inducer start up (not sure what the term is). I believe that's because the flue is at its hottest and draft is not a problem.
So I know this isn't an air supply issue. But from what I've read those inducer fans always create some amount of backdraft. In fact I read that the Building Performance Institute allows up to 60 seconds of backdrafting from an atmospheric-venting appliance.
What I'm thinking is installing a short WH so I can get another 9-10" of vertical vent before the elbow...maybe that might help. But logic tells me the fan is always going to push back some of that exhaust...like connecting two straws to a single one with a wye, blowing in both but whoever is blowing harder will win (the inducer fan).
Also, I'm sure the air temp isn't helping but there's not a whole lot I can do about that since it's in an unconditioned space. I've noticed my dad's WH backdrafts when his inducer fan kicks on. It's not as extreme though.
Sorry for the ramble. I'm just at a loss over if this is as serious of an issue as I've made it in my head. Never had any issues in 10 years other than the slightly deformed plastic escutcheons. Pic attached which shows the setup.
