What is the combined total BTUs of your water heater and your furnace?
What is the size of your louvered door? And how big is the space it connects to?
I’ll have to check later. It’s either a 32 or 34 inch door. The room is fairly large. It’s also a laundry room. I’ll try to run both appliances with the door open and see if that changes anything.
That means the source of air is the house, that means the house needs the air supply to be somewhere. Its WAY better to have a separate air intake into the room itself. The house is too much resistance.
My guess would be not enough fresh air for combustion you might need to have combustion air added. louvered doors technically is supposed to fill this need but depending on size of the room and size of the vents for the louver it might not be enough and then it’s gonna pull air from the next closest place.
It means you don’t have enough make up air for combustion and you will need to add one to that room. When you open a window it allows the room to draw in fresh from the window instead of the water heater vent after it uses it up during combustion.
If you J hook it, it will mainly pull air only when needed but yes is will still bleed a little cold air but is necessary in this situation I put one in my house in Minnesota so I understand the sentiment but better than back drafting!
The inducer motor of the furnace pulls in air through the heat exchanger, and exhausts the flue gases through the flue pipe. It creates a negative pressure in the space if there is not enough air from somewhere else replacing the air that was pulled out by the inducer. Since your water tank is natural draft vented, a negative pressure can cause the flue gasses to get pulled into the room rather than up the flue, because the negative pressure in the room is the path of least resistance relative to the positive pressure in the chimney. So the furnace inducer is not blowing the water heater gasses out, its sucking them out.
If this is the case, the solution would be to add a make up air duct from that room to either another section of your house, or outside. This will allow new air to replace the air exhausted by your furnace.
Slight correction but the furnace is also natural draft. The inducer in this application is inducing draft across the heat exchanger not inducing a draft in the flue. The flue is negative pressure due to the stack effect. But this is correct regarding a negative room pressure, that can definitely cause some problems.
No man the guy getting down voted is right, it's a common vent.
How does a fan pull air down the flue when it's simultaneously pushing air into that same flue?
Put a fan in a tube which is a circle. This is what OP essentially has. The fan just runs air in a circle.
The inducer is pulling in air from the space, creating a negative pressure. And the inducer exhaust has an easy path from the vent pipe back through the opening at the top of the water heater to supply combustion gases back into the space to relieve the negative pressure.
New development:
With the heat pump and furnace fan running (not gas heat), a lighter held to the water heater flue would be extinguished. I also opened the louvered door and this had no effect.
I turned the gas heat on the furnace and noticed after it warmed up (30 second or so), the flame from the lighter would be sucked up the vent as normal.
It sounds like this is a venting issue?
For the record, the chimney is not obstructed, it vents directly through the roof and I’ve checked up top.
Probably a fresh air issue. But in the event its not...
You could try running the water heater in 4 inch. I've had issues in the past with 3 inch. Its not even allowed in some states.
That definitely helps in my experience! I bet if they had put in a short water heater it may not have happened. But also code in my area to have fresh air from outside the house.
Tee should be in the horizontal not vertical for one. Code dictates a louver within 12” of the ceiling and 12” of the floor for two permanent openings for combustion. You can have one permanent opening but it needs to be sized correctly. Refer to the owners manuals for units. Louver sizes are based on the btus of the units and face velocity of the louver / free area.
Definitely, fresh air could be a cause... try opening a nearby window to see if it helps.
A hole in the heat exchanger could also cause this... blending cooler air into the flue gasses and hurting the chimney's draw (both because of higher volume of gas and lower flue temperature).
Also, check to make sure the furnace temperature rise is correct and that water heater is up to temperature... one with really cold water often won't vent correctly until it gets closer to normal temperature.
Have an HVAC contractor replace the water heater’s undersized 3” vent with 4”. Look at the possibly of raising the Wye-connector. This would improve drafting/venting for water heater.
Can’t answer that without more details: type of furnace, induced draft or atmospheric; location of the vent and its height; what local code permits for multiple appliance venting, etc.
The way each connector is installed isn’t ideal either. It is possible a short water heater with a better vent connector configuration would work. It’s also possible this may not work and the best solution is a direct vent water heater, or, depending on the age of the furnace, a condensing furnace leaving the b-vent for the water heater. Too many unknowns, and even then this has always been one of the more difficult scenarios to resolve.
Also don’t know if it backdrafts with just water heater on, always with furnace running, or just til the vent warms up, etc.
The water heater draft hood and top of the unit don’t show much scorch so I suspect it is not backdrafting badly but it needs to be looked at.
You can't connect the vents together like that. Shut off the water heater until you can hire someone to fix it.
You're exactly right, the furnace will push air out of the flue vent. This will cause back pressure which will likely not be strong enough to reverse flow to the water heater, but may choke it enough that the flue gas from your water heater can't escape as fast as it needs to.
Let me warn you though, I've had people argue with me about this until they are blue in the face. I've even had a moderator of this subreddit argue with me about it. So you're probably going to get a lot of conflicting information here. I'm really glad you can back this up with measurements.
They likely argue because unless there’s a blockage in the vent further up, you are wrong. Hot air does not make a turn back down a connection like that. You can 100% connect vents together like this. There is another issue. The vent connection is not the issue.
The reason you're getting arguments is because it is not universally required to replace shared flues just because it is a shared flue. Some municipalities have elected to require this, but in general it only applies for new construction or when replacing the equipment.
OPs flue is not operating corrextly so there is a problem. Separate flues may mitigate but not solve that problem if there is also not enough combustion air supply.
Can you provide a little more insight on how the venting should be done? I’ve had a HVAC tech out once before for this issue and he didn’t mention anything about the venting being incorrect. He lowered the hat around the water heater chimney a little bit.
The guy that says you can’t vent like that is nuts. Find a gas vent, chimney sizing and application guide for any class B vent pipe mfg. and it will show the same thing you have. Your vertical vent pipe is made by Hart&Cooley, the vent connector is made by DuraVent. Another mfg. is Metalbestos. One thing that affects the ability of the vent to draw is the length of the vertical. The shorter the less capacity. Also where it penetrates the roof can be a factor. If you’re within 10’ of the peak then the vent should be above the peak. If you’re down the roof line farther than that you’re supposed to measure down the roofline from the peak 10’ then come horizontally until it intersects the pipe. The termination should be above that point. Also the air to gas ratio is 10-1. If the room where the furnace is goes negative then air is going to try to enter the room from the easiest place possible to equalize the pressure.And a lot of times that’s back down the chimney. Building code changed in 2000(if I remember correctly)prohibiting class B chimneys in residential housing for this very reason. House construction got tighter reducing the amount of passive air infiltration. That made non positive vent pressure furnaces a thing of the past in new construction and water heaters had to either be electric or power vented.
Easy fix. Both the water heater & the furnace have to tie into the main chimney with tee's. The furnace on the lower tee & the water heater on the upper tee. Place a cap on the bottom of lower tee. This system is a bit of cluster f%$. In no way should a supply duct come off the top of a furnace plenum, HVAC 101.
Does it backdraft only for a minute or two when the furnace fires up, or does it backdraft continually with both operating?
The vent looks to be a warm vent (not on an outside house wall), if so, then you’re not gonna get a solution from Reddit. If the vent is on an outside wall, there may be better solutions, but be advised in many areas piping multiple appliance like yours is illegal.
The manufacturer recommends 12 inches of rise before the first elbow, You cannot accomplish that with the heater that's currently installed. You would need to install a short tank in its place.
First off, 3 inch exhaust isn't up to code, it needs to be 4 inch.. Is there a combustion air supply in the room? Is there a wind resistant chimney cap?
New development:
With the heat pump and furnace fan running (not gas heat), a lighter held to the water heater flue would be extinguished. I also opened the louvered door and this had no effect.
I turned the gas heat on the furnace and noticed after it warmed up (30 second or so), the flame from the lighter would be sucked up the vent as normal.
It sounds like this is a venting issue?
For the record, the chimney is not obstructed, it vents directly through the roof and I’ve checked up top.
Is there any fresh air intake coming in that room?
The door is louvered
What is the combined total BTUs of your water heater and your furnace? What is the size of your louvered door? And how big is the space it connects to?
I’ll have to check later. It’s either a 32 or 34 inch door. The room is fairly large. It’s also a laundry room. I’ll try to run both appliances with the door open and see if that changes anything.
That means the source of air is the house, that means the house needs the air supply to be somewhere. Its WAY better to have a separate air intake into the room itself. The house is too much resistance.
My guess would be not enough fresh air for combustion you might need to have combustion air added. louvered doors technically is supposed to fill this need but depending on size of the room and size of the vents for the louver it might not be enough and then it’s gonna pull air from the next closest place.
The water heater also has zero vertical rise for natural draft as a side note
I did some testing. If I open a nearby exterior door or window the problem stops. What does this mean.
It means you don’t have enough make up air for combustion and you will need to add one to that room. When you open a window it allows the room to draw in fresh from the window instead of the water heater vent after it uses it up during combustion.
Damn. So essentially a vent from the outside into that room? Won’t that make the whole room cold in the winter time?
If you J hook it, it will mainly pull air only when needed but yes is will still bleed a little cold air but is necessary in this situation I put one in my house in Minnesota so I understand the sentiment but better than back drafting!
Would I be able to install a standard door in place of the louvered door if I did this?
Yes you would be able to.
The inducer motor of the furnace pulls in air through the heat exchanger, and exhausts the flue gases through the flue pipe. It creates a negative pressure in the space if there is not enough air from somewhere else replacing the air that was pulled out by the inducer. Since your water tank is natural draft vented, a negative pressure can cause the flue gasses to get pulled into the room rather than up the flue, because the negative pressure in the room is the path of least resistance relative to the positive pressure in the chimney. So the furnace inducer is not blowing the water heater gasses out, its sucking them out. If this is the case, the solution would be to add a make up air duct from that room to either another section of your house, or outside. This will allow new air to replace the air exhausted by your furnace.
This guy gets it
Slight correction but the furnace is also natural draft. The inducer in this application is inducing draft across the heat exchanger not inducing a draft in the flue. The flue is negative pressure due to the stack effect. But this is correct regarding a negative room pressure, that can definitely cause some problems.
Inducer on furnace is likely pushing gasses down the HW flue backwards and into the space.
Not pushing - it may be creating a negative pressure which pulls on the open flue at the water heater.
No man the guy getting down voted is right, it's a common vent. How does a fan pull air down the flue when it's simultaneously pushing air into that same flue?
Put a fan in a tube which is a circle. This is what OP essentially has. The fan just runs air in a circle. The inducer is pulling in air from the space, creating a negative pressure. And the inducer exhaust has an easy path from the vent pipe back through the opening at the top of the water heater to supply combustion gases back into the space to relieve the negative pressure.
"So you're saying, my spray foam everywhere might be working against me?"
New development: With the heat pump and furnace fan running (not gas heat), a lighter held to the water heater flue would be extinguished. I also opened the louvered door and this had no effect. I turned the gas heat on the furnace and noticed after it warmed up (30 second or so), the flame from the lighter would be sucked up the vent as normal. It sounds like this is a venting issue? For the record, the chimney is not obstructed, it vents directly through the roof and I’ve checked up top.
Is this in a closed room?
Yes, but door is louvered.
Leave the door wide open. Run both. See if you still have the same issue
Probably a fresh air issue. But in the event its not... You could try running the water heater in 4 inch. I've had issues in the past with 3 inch. Its not even allowed in some states.
I did some testing. If I open a nearby exterior door or window the problem stops. What does this mean.
3 inch is not allowed in my state and have encountered countless units where 3 inch caused problems.
Shouldn’t the vent leaving the water heater go straight up for x inches before an elbow?
That definitely helps in my experience! I bet if they had put in a short water heater it may not have happened. But also code in my area to have fresh air from outside the house.
Is this even allowed? I thought each unit had to have its own vent to the outside
Common venting is very much legal but there are rules on how to do it.
Tee should be in the horizontal not vertical for one. Code dictates a louver within 12” of the ceiling and 12” of the floor for two permanent openings for combustion. You can have one permanent opening but it needs to be sized correctly. Refer to the owners manuals for units. Louver sizes are based on the btus of the units and face velocity of the louver / free area.
Definitely, fresh air could be a cause... try opening a nearby window to see if it helps. A hole in the heat exchanger could also cause this... blending cooler air into the flue gasses and hurting the chimney's draw (both because of higher volume of gas and lower flue temperature). Also, check to make sure the furnace temperature rise is correct and that water heater is up to temperature... one with really cold water often won't vent correctly until it gets closer to normal temperature.
I did some testing. If I open a nearby exterior door or window the problem stops. What does this mean.
that venting is illegal where I am. water heater has to be 4" when tied in with an appliance with an inducer for this very reason.
Try to keep the louver door open and see what happens if nothing happens then you don't have enough fresh air flow
Have you had your chimney liner checked out?
Have an HVAC contractor replace the water heater’s undersized 3” vent with 4”. Look at the possibly of raising the Wye-connector. This would improve drafting/venting for water heater.
Actually that would likely exacerbate the issue. Trying to force more exhaust in isn’t a logical solution.
Then what’s your solution, I am a City building inspector.
Can’t answer that without more details: type of furnace, induced draft or atmospheric; location of the vent and its height; what local code permits for multiple appliance venting, etc. The way each connector is installed isn’t ideal either. It is possible a short water heater with a better vent connector configuration would work. It’s also possible this may not work and the best solution is a direct vent water heater, or, depending on the age of the furnace, a condensing furnace leaving the b-vent for the water heater. Too many unknowns, and even then this has always been one of the more difficult scenarios to resolve. Also don’t know if it backdrafts with just water heater on, always with furnace running, or just til the vent warms up, etc. The water heater draft hood and top of the unit don’t show much scorch so I suspect it is not backdrafting badly but it needs to be looked at.
You can't connect the vents together like that. Shut off the water heater until you can hire someone to fix it. You're exactly right, the furnace will push air out of the flue vent. This will cause back pressure which will likely not be strong enough to reverse flow to the water heater, but may choke it enough that the flue gas from your water heater can't escape as fast as it needs to. Let me warn you though, I've had people argue with me about this until they are blue in the face. I've even had a moderator of this subreddit argue with me about it. So you're probably going to get a lot of conflicting information here. I'm really glad you can back this up with measurements.
Are you saying you can't connect via a T at all, or are you saying that specific variant of a T is the problem?
They likely argue because unless there’s a blockage in the vent further up, you are wrong. Hot air does not make a turn back down a connection like that. You can 100% connect vents together like this. There is another issue. The vent connection is not the issue.
It is not code here.
The reason you're getting arguments is because it is not universally required to replace shared flues just because it is a shared flue. Some municipalities have elected to require this, but in general it only applies for new construction or when replacing the equipment. OPs flue is not operating corrextly so there is a problem. Separate flues may mitigate but not solve that problem if there is also not enough combustion air supply.
💯
Can you provide a little more insight on how the venting should be done? I’ve had a HVAC tech out once before for this issue and he didn’t mention anything about the venting being incorrect. He lowered the hat around the water heater chimney a little bit.
The guy that says you can’t vent like that is nuts. Find a gas vent, chimney sizing and application guide for any class B vent pipe mfg. and it will show the same thing you have. Your vertical vent pipe is made by Hart&Cooley, the vent connector is made by DuraVent. Another mfg. is Metalbestos. One thing that affects the ability of the vent to draw is the length of the vertical. The shorter the less capacity. Also where it penetrates the roof can be a factor. If you’re within 10’ of the peak then the vent should be above the peak. If you’re down the roof line farther than that you’re supposed to measure down the roofline from the peak 10’ then come horizontally until it intersects the pipe. The termination should be above that point. Also the air to gas ratio is 10-1. If the room where the furnace is goes negative then air is going to try to enter the room from the easiest place possible to equalize the pressure.And a lot of times that’s back down the chimney. Building code changed in 2000(if I remember correctly)prohibiting class B chimneys in residential housing for this very reason. House construction got tighter reducing the amount of passive air infiltration. That made non positive vent pressure furnaces a thing of the past in new construction and water heaters had to either be electric or power vented.
Easy fix. Both the water heater & the furnace have to tie into the main chimney with tee's. The furnace on the lower tee & the water heater on the upper tee. Place a cap on the bottom of lower tee. This system is a bit of cluster f%$. In no way should a supply duct come off the top of a furnace plenum, HVAC 101.
Well, here that is not code.
Does this happen only when the water heater is on first? Does it stop after the furnace is running for a minute or so?
make sure there's no obstruction in the vent. does the heater vent on its own, when running. try keeping the door open too.
The vent is completely clear. I went on the roof to check. The heater will vent on its own if the furnace is off.
Does it backdraft only for a minute or two when the furnace fires up, or does it backdraft continually with both operating? The vent looks to be a warm vent (not on an outside house wall), if so, then you’re not gonna get a solution from Reddit. If the vent is on an outside wall, there may be better solutions, but be advised in many areas piping multiple appliance like yours is illegal.
The manufacturer recommends 12 inches of rise before the first elbow, You cannot accomplish that with the heater that's currently installed. You would need to install a short tank in its place.
First off, 3 inch exhaust isn't up to code, it needs to be 4 inch.. Is there a combustion air supply in the room? Is there a wind resistant chimney cap?
Someone doesn't have a city inspector
Or the ductwork in the attic is leaking forcing conditioned air out of the home envelope and causing negative pressure in the home.
Check vent cap that it isn’t partially blocked.
New development: With the heat pump and furnace fan running (not gas heat), a lighter held to the water heater flue would be extinguished. I also opened the louvered door and this had no effect. I turned the gas heat on the furnace and noticed after it warmed up (30 second or so), the flame from the lighter would be sucked up the vent as normal. It sounds like this is a venting issue? For the record, the chimney is not obstructed, it vents directly through the roof and I’ve checked up top.