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Old 02-24-2011, 01:27 AM   #1
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Still have occasional air flow problem from w/heater pipe


I had 2 90+ furnaces installed a couple weeks ago(first new 1 had blower issues so contractor replaced). I have been having occasional air flow being pulled back down the w/heater flue pipe. It doesn't do it every time but what concerns me is that maybe the old flue pipe(he re-lined with smaller flex pipe)and the new concentric pipe is about 14 inches apart. The contractor doesn't think the pipe distance is the problem. He told me to get in my crawlspace while the furnace is on and look for a major leak in my ductwork. He said he has seen this before creating some type of back flow. I haven't done this yet but was wondering If this sounds possible? I do know my house is pretty tight with plaster walls, and noticed tonight when I would open the doors in the back bedrooms that the vents would blow harder.I guess the return vent in the hallway would make it blow out more...and would blow slightly less doors closed. He doesn't seem like he wants to do anything about it now and cant blame him since he's already went above and beyond replacing 2 furnaces. Until I get this lined out I'm keeping 2 CO detectors hooked up( has not sounded with newest install).Also a 4" plastic cap came off so thinking this was pullng air I sealed with a piece of sheet metal to make a better sealed system(might of helped some). Thanks in advance!

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Old 02-24-2011, 07:49 AM   #2
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Still have occasional air flow problem from w/heater pipe


I read this twice but still don't understand what the actual problem is. You give alot of info but very little of any use. Is the furnace not lighting because of the flue problem or is it running then shutting down? I don't know a whole hell of alot about heating since the average winter temp down here in the winter is only 55 but I know some. If he told you there was air leakage on your supply ducts causing a negative pressure in the house and that negative pressure was effecting the flue of the furnace then it would have to be an extreemly large leak. You would notice air being sucked through outside doors and windows. Are the furnaces attic mount, basement mount, or closet? Are the close together? Pic might help some of these northern boys well versed in these units help you more.

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Old 02-25-2011, 12:38 AM   #3
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Still have occasional air flow problem from w/heater pipe


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Originally Posted by college A/C View Post
I read this twice but still don't understand what the actual problem is. You give alot of info but very little of any use. Is the furnace not lighting because of the flue problem or is it running then shutting down? I don't know a whole hell of alot about heating since the average winter temp down here in the winter is only 55 but I know some. If he told you there was air leakage on your supply ducts causing a negative pressure in the house and that negative pressure was effecting the flue of the furnace then it would have to be an extreemly large leak. You would notice air being sucked through outside doors and windows. Are the furnaces attic mount, basement mount, or closet? Are the close together? Pic might help some of these northern boys well versed in these units help you more.
Pretty simple. The new furnace and hot water heater is in the same room right next to each other. He ran new 2" pvc pipe for the furnace exhaust and fresh air return for the burners but kept the water heater in the old 6" flue pipe. He did re-line the w/heater flue with a smaller flex pipe because he thought probably wouldn't draw as good without the furnace useing it too. Every now in then when the furnace comes on and the blower starts it causes air to flow back down from the roof (outside) and through the opening at the top of w/heater. It is so strong it will blow a lighter out. It doesn't do it everytime and seems to have got better since I screwed a piece of sheet metal over a hole on the bottom of the furnace where a cap came out during installation.

I agree that it would have to be a large leak to cause this type of problem. I'm wondering if he didn't put the furnace concentric pipe and water heater pipe to close together on the roof at 14 inches...Could this cause such a back flow?? One time it pulled exhaust fumes from the water heater in while it was heating. I need to call the county for the correct code and crawl under the house this weekend and check the duct-work. Thanks for any input!
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Old 02-25-2011, 02:44 AM   #4
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Still have occasional air flow problem from w/heater pipe


You have a four inch chimney liner inside a 9in x 6in and a whole bunch of unconditioned space that stays cold and keeps the liner cold too. This will cause the water heater to back flow to the point of blowing out the pilot.

Either that or you have a birds nest partially blocking the flue gases from venting.
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Old 02-25-2011, 05:34 AM   #5
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Still have occasional air flow problem from w/heater pipe


Sounds more like return duct/system leakage in the room with the furnace. And a supply leak in the duct system.

Have your wife open and close the bedroom doors(15 to 30 seconds between opening and closing) while the furnace is on. And check the water heater for back draft while she does this.

Doesn't sound like its a termination problem.
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Old 02-25-2011, 10:37 PM   #6
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Still have occasional air flow problem from w/heater pipe


You may need another vent directly from the outside into the basement to provide the furnace (and water heater) with enough combustion air.

The International Residential Code (sp?) gives the calculations needed for the number of square inches of intake vent for the number of BTU's of the furnace.

If you open a window, does this problem go away?

(Many modern furnaces have an intake pipe so semi-heated basement air is not wasted up the chimney.)

When a room has a heat vent and no return vent of its own, then when you close the door, the air blown out the heat vent causes a positive pressure resulting in less air blown out compared with having the door open.
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Old 02-25-2011, 10:40 PM   #7
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Still have occasional air flow problem from w/heater pipe


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Originally Posted by AllanJ View Post
You may need another vent directly from the outside into the basement to provide the furnace (and water heater) with enough combustion air.

The International Residential Code (sp?) gives the calculations needed for the number of square inches of intake vent for the number of BTU's of the furnace.

(Many modern furnaces have an intake pipe so semi-heated basement air is not wasted up the chimney.)
That does not apply to direct vent gas appliances.
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Old 02-26-2011, 01:26 AM   #8
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Still have occasional air flow problem from w/heater pipe


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Originally Posted by beenthere View Post
Sounds more like return duct/system leakage in the room with the furnace. And a supply leak in the duct system.

Have your wife open and close the bedroom doors(15 to 30 seconds between opening and closing) while the furnace is on. And check the water heater for back draft while she does this.

Doesn't sound like its a termination problem.
I will have to try this and let you know.
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Old 02-26-2011, 01:58 AM   #9
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Still have occasional air flow problem from w/heater pipe


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Originally Posted by AllanJ View Post
You may need another vent directly from the outside into the basement to provide the furnace (and water heater) with enough combustion air.

The International Residential Code (sp?) gives the calculations needed for the number of square inches of intake vent for the number of BTU's of the furnace.

If you open a window, does this problem go away?

(Many modern furnaces have an intake pipe so semi-heated basement air is not wasted up the chimney.)

When a room has a heat vent and no return vent of its own, then when you close the door, the air blown out the heat vent causes a positive pressure resulting in less air blown out compared with having the door open.
Its a downflow furnace with the main air supply pulled in through 1 large vent(aprox. 18"x18") from above and out of the wall behind it which is the hallway.

There is a hole in the floor right in front of the water heater covered with about a 4"x9" grill. I'm not fond of it because it lets cool air into the room but the contractor said to leave it there because it helps bring fresh air in from the crawl space. Also it doesn't seem to make a difference If the furnace room door is open or closed when the w/heater flue is pulling air. By the way the furnace room is only about 10'x9' and sits off the kitchen without its own heating vent.
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Old 02-26-2011, 02:04 AM   #10
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Still have occasional air flow problem from w/heater pipe


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Originally Posted by hvaclover View Post
You have a four inch chimney liner inside a 9in x 6in and a whole bunch of unconditioned space that stays cold and keeps the liner cold too. This will cause the water heater to back flow to the point of blowing out the pilot.

Either that or you have a birds nest partially blocking the flue gases from venting.
This makes sense, but so far hasn't blown out pilot. No birds nest they just replaced a new cap too and its caged really good.
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Old 02-26-2011, 02:58 AM   #11
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Still have occasional air flow problem from w/heater pipe


The higher the chimney the longer it will take to warm up to draft properly.

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