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  1. #1
    Join Date
    Apr 2005
    Location
    illinois
    Posts
    257
    One contractor said it would be difficult to get the normal pitch for the intake and exhaust pvc pipes for a 90% furnace. Another said that the pipes could run uphill to about halfway to the outside wall and then could run downhill to the outside wall. The second contractor said the only reason for the pitch is to prevent water from sitting in a low spot and that it didn't matter which way the fluid ran. Is this true?
    Does the exhaust motor run for a few seconds after the burners shut down to clear the exhaust pipe of combustion gas or will this shallow inverted "v" setup trap exhaust at the apex or worse yet let exhaust fumes back into the furnace?
    I also have a limited choice of where the intake and exhaust pipes can leave the house. There is a on that outside wall a nearby electric meter as well as a dryer vent. What problems could occur if the exhaust fumes get pulled into the intake other that reducing the supply of oxygen to the furnace.
    I haven't been able to get a good answer why the exhaust or the intake shouldn't be near the electric meter. Also is it a problem for the exhaust pipe to be near the dryer vent and why?

  2. #2
    Join Date
    Apr 2004
    Location
    Niantic, Illinois
    Posts
    545
    The right way to do it is to run the pitch to drain it back to the furnace. If you run the pitch to the outside the chances of it creating an ice blockage is pretty high. Also it will leave a large frozen patch outside, like when you see a gutter that was blocked and slowly dripping, they freeze over right?

  3. #3
    Join Date
    Jul 2004
    Location
    West TN
    Posts
    983
    The chemicals we use to wash our clothes (bleach, washing powder, fabric softner, etc) are exausted out of your dryer vent. These chemicals, when introduced into the combustion air of the furnace, burn and release an acidic cocktail of chemicals. This will lead to an early grave for your heat exchanger.

    Is your furnace going to be in a basement?
    If so... is there a closet where the pipes can run through to get from basement to attic. THe pipes dont' take up much space so you won't lose much closet space.

  4. #4
    Join Date
    Aug 2002
    Location
    manitowoc wisconsin
    Posts
    4,943
    I have been doing this for twenty years & have not considered pitching pipes in two directions,or for that matter not been able to properly vent a 90% furnace corectly one way or another-choose another contractor.
    Take your time & do it right!

  5. #5
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    Multiple pitches are perfectly acceptable. Most furnaces do some post purging. If the trapping of gases was an issue then I don't imagine they'd allow inverted U shaped exhausts. Now an inverted V shape may be a problem if they can't pitch it so that the water flows to a proper drain. Is it really a V or more a U?


  6. #6
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,943
    Where does that venting method come from?
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  7. #7
    Join Date
    Apr 2005
    Location
    illinois
    Posts
    257
    Thanks for your replies.
    Berad: I see your point but wouldn't the heat from the furnace combustion gas cycling on and off keep the exhaust pipe from freezing. How much condensate is produced?
    The area where the exhaust pipe would come out is not where people would walk but would be near plants and the a/c unit.
    Wormy: I learned about the problem with cleaning substances and your heat exchanger on this site. Is there a problem if the exhaust pipe is near the dryer vent? The best place for the pipes to come out is near a corner and one of the contractors seemed to think that the inlet pipe could be near the corner about 8 feet away from the dryer vent but would locate the exhaust within two feet of the dryer vent, but well away from the intake pipe. Is there any concern about the exhaust pipe and the dryer vent since the dryer fumes couldn't get back to the heat exchanger through the exhaust pipe. The furnace is in the basement but there is no closet in the area. Is it possible to vent up the chimney? The chimney flue would still have an 80% furnace and a 40k btu water heater venting thru the flue.

    Markwolf: Thanks for your reply. I share the same concern. The problem with our house is that much of the exterior wall that could be used for venting has been replaced by a sun room addition which is on a slab or more likely a crawl with no access.

    Irascible: the diagragm gives me hope that the venting will be OK. It seems that in the diagram the arrows labeled "see notes" may point to drains and that the pipes actually drain inside. The notes however clearly state that condensate can drain outside. What do you think about the concern about freezing of the exhaust pipe? Have you ever done a downward pitch to the outside and do you know how well it worked? I don't know if the contractor was talking about a "U" or a "V". He just said he might have to have some of the exhaust pipe slope toward the outside. If the condensate is acidic and can eat out a chimney what will it do to my foundation? Can it eat out a drain?
    Robo: The method was suggested by the contractor. He said the only thing we had to worry about is avoiding a low stop wher the condensate could settle and plug the exhaust up. He also wanted to pull combustion gas from the garage to avoid the inlet pipe from freezing. From others I have
    heard that the inlet pipe should be about 10 inches above the avg snow level and that inlet pipes rarely freeze.


    [Edited by heetseeker on 05-31-2005 at 11:04 AM]

  8. #8
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    Robo,
    I've seen the chart enough times that I knew it could be found in any Trane 90+ install guide. So I picked one and there it was. Does Goodman not allow such a configuration? (Parenthetically, are you not working for them anymore? I'm not current on all the Robo scuttle. )

    heetseeker,
    I don't deal with freezing weather. But it seems to me that if the pipe is pitched towards the exhaust and not back towards a drain like in the picture that you'd get too much water dribbling down the side of your house and freezing. That doesn't sound like a good plan. I’ve personally never installed anything with a negative pitch. But if the manufacturer allowed it then I’d have no problem doing so. My 90+ installs have all used condensate pumps that drained the water to the outdoors onto dirt. That may be what the picture refers to. Draining to the outside is not necessarily the same thing as draining to the outside by means of a negative slope. The condensate isn’t going to eat out a drain. But I wouldn’t let it drain onto a foundation.

    Ultimately the answer is to be found in what the manufacturer of your equipment allows and what local code says. Ask your prospective installer to e-mail you the install guide. Install guides are easy to download and send off. Then call your local building department and ask them their opinion. They have the final say. It would be prudent to find out what they're going to say in advance.

    BTW, why go to all this trouble for a 90+? The ten percent savings over an 80+ is nice. But it isn’t that compelling, is it?

  9. #9
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,943
    That method of venting is not in any Goodman, York or Rheem/Ruud I/O manuals that I know of. In my area, if it ain't in the manufacturers I/O manual the inspectors will not pass it.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  10. #10
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    Robo,
    That's interesting. Given that these vents are positive pressure I would think that you could form decorative loops with the vent and it wouldn't matter as long as you didn't exceed the maximum equivalent length allowed and as long as you had proper drainage.

    But no matter. Except in the most extreme of circumstances I always bow to OEM guidelines. The last thing I need is some grease ball lawyer waiving an installation manual in my face.

    heetseeker,
    Check that install manual! Obviously some OEMs are cool with the idea and some aren't.

  11. #11
    Join Date
    Apr 2005
    Location
    illinois
    Posts
    257
    Irascible
    Thanks for all the info. I read it from start to finish and learned a lot. I absolutely agree that the 90% may never pay for itself. I also liked how you said that the 2 stage furnaces are not more efficient. If you read Jim Davis's {National Comfort Institute}posts he says you get much less efficiency with a two stage. I wanted to go to a 90% because we have slightly high radon readings and no air combustion vents for our two furnaces[ 119k btu and 71k btu for a house built in 1970]. I have not found a source that explains how the combustion vents work. I thought the 90% furnaces would decrease the negative pressure in the basement.
    I have been told that one vent should be on the floor with the end in a combustion bucket and that one vent should be at the ceiling. I live in the chicago area and when it gets cold I think that the vents would add a lot of cold air to the basement. I don't know how I could control the cold air. Not one service man or contractor has ever told me I need combustion vents so I don't think they know about them or want to deal with them.
    I don't like hearing that running a VS all the time will cause more air infiltration but does make some sense. Do you have some sources to further document this?
    As far as pulling a construction permit and an inspector goes our village doesn't require an inspection or a permit. Why, I don't know.

  12. #12
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    The floor/ceiling business is for getting combustion air to an 80+ furnace that sits in a closet or some other fairly closed space. You may or may not need them for your current 80+. It depends on how much open area the furnace has to draw from. If the open area is large enough then you're not require to have combustion vents.

    With a 90+ you would normally get the combustion air from the outside. But make sure that they do run a two pipe system. Cheaper 90+ installations will still draw combustion air from the space around the furnace and run a single pipe for exhaust to the outdoors. A two pipe system will relieve the negative pressure caused by the burning of fuels. But it won't stop pressurization caused by leaky ducts. Make sure the ducts are sealed up nicely.

    "Much less efficiency"? I'm not sure about that. I haven't been to the Jim Davis indoctrination seminar yet (though I plan on it). So I'm not sure what the reasoning is. My Trane book tells me that 1st and 2nd stages are within a percentage point of each other.

    As I hope was clear, the idea that running the fan could make things worse is purely a guess. It depends on how leaky the ducts are, where they leak to and where outside air infiltrates from. I don't know of specific documentation. But none is needed. Take a house with supply ducts in the attic. Disconnect them so that 100% of the supply air is lost to the attic. Now close the house up and punch a hole in the floor right above some standing water in the basement. Turn the fan on. The supply air will dump out of the attic through the roof vents. The negative pressure in the house will draw air through the hole in the floor above the pond scum. See what's happening? Now you're not losing 100% of your air to the attic. But many systems lose 20 or 30 percent. And you're not drawing air from a hole in the floor right above standing water. But you don't know where exactly it comes in from. It could come in fresh as a daisy or it could come in laced with all sorts of garbage.

    [Edited by Irascible on 05-31-2005 at 09:24 PM]

  13. #13
    Join Date
    Apr 2001
    Location
    san jose,ca.
    Posts
    5,285
    Hi john,from bigbird. Grease ball,i have`nt been called that in years.

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