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  1. #1
    Join Date
    Dec 2003
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    28

    New Furnace potential problem

    I'm going to get proposals on a new furnace installation, but I am concerned that my chimney cannot handle the modern furnaces. That is, I currently have a water heater and a furnace sharing the same clay lined brick chimney. I only want to replace the furnace, that is 20 years old, and the modern units I see online seem to have a fan in the vent system. Can these modern furnaces share a common chimney with a (no-vent-fan) water heater?

    If a fan vent furnace cannot share a chimney are there furnaces available without that fan that could solve my problem? If I have to give up some efficiency I can live with that.

    I'm trying to get prepared for the problems I am likely to hear when I get the proposals.

    Thanks.

  2. #2
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    2,602
    Regardless if it's a 80% AFUE or a 90+% AFUE, the chimney should be lined. Many city codes require it. The 80% furnaces can share the chimney liner with the W/H. The 90% furnace will vent elsewhere leaving the W/H on the (smaller sized) chimney liner by itself.

  3. #3
    Join Date
    Dec 2003
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    28
    Quote Originally Posted by George2 View Post
    Regardless if it's a 80% AFUE or a 90+% AFUE, the chimney should be lined. Many city codes require it. The 80% furnaces can share the chimney liner with the W/H. The 90% furnace will vent elsewhere leaving the W/H on the (smaller sized) chimney liner by itself.
    Thank you for the help.

    So far I have one proposal over the phone. That person said he didn't see any problem hooking a new furnace to the same chimney setup. He was talking about an 80% furnace and when I asked, "doesn't that have an induction motor for the vent and can that be used with my current setup". He said "shouldn't be a problem".

    I'm waiting for other proposals.

  4. #4
    Join Date
    May 2012
    Location
    Upper Michigan
    Posts
    3,588
    I would also get a price on a 90% and liner for water heater, if your in a colder climate it may be a better option.

  5. #5
    Join Date
    Sep 2003
    Posts
    1,665
    Anyone giving you an estimate over the phone is doing you a diservice,

    Wag, commonly refered to as Wild Ass Guesses in this business don't take into account
    all the factors that you have in your home.

    For Example is your chimney in the center of your home or completely on the outside wall.

    Has the chimney been inspected.

    Some other things to consider

    Where is your home located

    How long are you going to be in your home, Maybe a dual fuel setup would be more prudent for you, Maybe not

    This message board is a great place to get information......This is your thread......ask away

  6. #6
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,602
    A 80% furnace is going to be the least expensive. But, regardless, if you choose a 80% or a 90%, a liner should be installed. Or the other option, if money is no problem, is to install a 90% furnace and a side vent W/H....then chimney become a non issue.

    If you decide to go the liner route, and if the chimney is in need of tuck pointing (and comes out the roof) and/or is in overall bad shape, another option to consider is to have it torn down to below the roof line. The contractor then would patch the roof and run class "B" all the way to the appliance(s). This is not done (or thought of) enough in my opinion.

  7. #7
    Join Date
    Dec 2003
    Posts
    28
    I looked at one of the new Lennox 80% units and it clearly says a clay lined masonry chimney shared with a water heater is okay.

  8. #8
    Join Date
    Nov 2011
    Location
    St. Louis
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    3,220
    Quote Originally Posted by don87109 View Post
    I looked at one of the new Lennox 80% units and it clearly says a clay lined masonry chimney shared with a water heater is okay.
    Not around here. Absolutely no way, regardless of what you read. It's one of the first things I look at when replacing an older natural draft furnace. It's also the first thing an inspector will look at - and he had better see a UL tagged metal masonry collar sticking out of the wall and connecting to the flue. And then he's going to walk outside if he didn't spot it on the way in to look for a metal flue cap sitting on top of the chimney.

    The newer, more efficient furnaces do not burn as hot, the exhaust gasses will condense more readily and create a lot more moisture.

    Mositure+acidic exhaust gases+masonry=big time trouble. Do you really want an extremely heavy and weakened structure on top of your home?

  9. #9
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,140
    When you change the fuel type or efficiency of a heater you must conduct a Level II inspection of the chimney to ascertain the suitability of that chimney for the intended purpose. I can assure you a proper inspection will reveal the need for a listed liner for whatever is venting into this chimney. They ALL have defects and/ or damage such that they can NOT perform its intended function. Many mfrs. now flat out require it regardless of what an inspector says. A liner will size the chimney to the attached appliances, contain flue gases and moisture and provide a quick draft unlike a cold wet masonry flue.

    A 90% appliance can NOT be common vented with an atmospherically vented appliance so it will be vented out elsewhere with PVC or polypropylene venting either sidewall or vertically.

    If you orphan the water heater, it will need the liner as it can not vent properly on its own and will tend to backdraft. If you go the 90% route the best solution would be to replace the water heater with a power vented model and abandon the chimney.

    If you decide to common vent the new fan assisted 80% furnace with the water heater into a listed liner in the chimney, I recommend you have a pro certified by the National Comfort Institute modify the venting with spill switches. Otherwise, if the vent becomes blocked such as a birds nest, the furnace could happily vent out the water heater's draft hood and never trip the three safety devices in the furnace.

    Regardless whether you use this chimney or not, have it inspected because almost all chimneys suffer damage from the weather and require periodic repairs including crowns, repointing, flashing and water repellent application.

  10. #10
    Join Date
    Dec 2003
    Posts
    28
    Quote Originally Posted by hearthman View Post
    When you change the fuel type or efficiency of a heater you must conduct a Level II inspection of the chimney to ascertain the suitability of that chimney for the intended purpose. I can assure you a proper inspection will reveal the need for a listed liner for whatever is venting into this chimney. They ALL have defects and/ or damage such that they can NOT perform its intended function. Many mfrs. now flat out require it regardless of what an inspector says. A liner will size the chimney to the attached appliances, contain flue gases and moisture and provide a quick draft unlike a cold wet masonry flue.

    A 90% appliance can NOT be common vented with an atmospherically vented appliance so it will be vented out elsewhere with PVC or polypropylene venting either sidewall or vertically.

    If you orphan the water heater, it will need the liner as it can not vent properly on its own and will tend to backdraft. If you go the 90% route the best solution would be to replace the water heater with a power vented model and abandon the chimney.

    If you decide to common vent the new fan assisted 80% furnace with the water heater into a listed liner in the chimney, I recommend you have a pro certified by the National Comfort Institute modify the venting with spill switches. Otherwise, if the vent becomes blocked such as a birds nest, the furnace could happily vent out the water heater's draft hood and never trip the three safety devices in the furnace.

    Regardless whether you use this chimney or not, have it inspected because almost all chimneys suffer damage from the weather and require periodic repairs including crowns, repointing, flashing and water repellent application.
    I don't get it. I'm not saying to "change any flue type". I'm just saying the manufacturer says it's okay to use a clay-lined masonry chimney for an 80% furnace and common venting with a W.H. is allowed.

    BTW, I have looked at other manufacturers of 80% units and they also say clay lined masonry chimneys and common venting is okay.

  11. #11
    Join Date
    Dec 2003
    Posts
    28
    Quote Originally Posted by hurtinhvac View Post
    Not around here. Absolutely no way, regardless of what you read. It's one of the first things I look at when replacing an older natural draft furnace. It's also the first thing an inspector will look at - and he had better see a UL tagged metal masonry collar sticking out of the wall and connecting to the flue. And then he's going to walk outside if he didn't spot it on the way in to look for a metal flue cap sitting on top of the chimney.

    The newer, more efficient furnaces do not burn as hot, the exhaust gasses will condense more readily and create a lot more moisture.

    Mositure+acidic exhaust gases+masonry=big time trouble. Do you really want an extremely heavy and weakened structure on top of your home?
    Interesting that several manufactures installation instructions say clay-lined masonry chimneys are okay. Again, I'm talking about 80% units. I have not looked at the higher efficiency units.

  12. #12
    Join Date
    Aug 2009
    Location
    Beatrice, NE
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    This is a great thread and brings up a point of clairification I need.

    What is classed as a masonry chimney? I always thought it was either masonry or metal. As such a brick chimney with a tile liner is still masonry as is Trancite {asbestos/concrete pipe}. All the sizing tables that I have that show masonry also show fan assist as NR or not recommended. I always understood the reason for the ruling is that the cooler flue gases from 80% furnaces can't keep the mass of the chimney warm. So how does the tile liner lessen the mass, it actually increases it. The flue gases are still going to condense and run down the inside causing problems. So if it is not a metal chimney I don't use it for 80% furnaces unless its gets lined.

    Part II: Explain the orphaned water heater. The flue gases coming off of it are still 400-600 degrees, and it has room air, plus the heat off the pilot and hot water going up the chimney all the time, so unless it's a massive chimney what difference does it make? In 35 years I have never seen a problem caused by an orphaned water heater, no matter what chimney.

    Educate me!

  13. #13
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    Quote Originally Posted by don87109 View Post
    I looked at one of the new Lennox 80% units and it clearly says a clay lined masonry chimney shared with a water heater is okay.
    I suppose it depends on the area. Will it "work", probably/maybe. Recommended without a liner, no. Anyway, it's not code in my area.

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