condensation

Discussion in 'HVAC Heating & Cooling' started by hitmehhh, Feb 22, 2008.

  1. hitmehhh

    hitmehhh New Member

    Messages:
    6
    I recently had an 80% effic. furnace istalled. I now have condensation periodically dripping from flue at a joint at the botttom. Installer says flue should accomidate new furnace. The only thing I am wondering is. I turn the thermostat down to 60 degrees at night. Then when the furnace is turned back up in the morning It stays on for at least a 25 minutes to get the house temp up.Could this condensation be caused by the lack of cycling during the night which would not keep the flue warm? Condensation problem I have only noticed in morning.
  2. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    Yes and if your chimney flue is not lined the acids in the condensation could eat through the chimney.
  3. hitmehhh

    hitmehhh New Member

    Messages:
    6
    I don't have a chimney My house was built in the 50's and has a 7" transite stack.Common vent with hot water heater. So you think the lack of cycling at night could be causing this? Last night, I only turned it down a few degrees , and it was 15 degrees out. This morning, No condensation
  4. BigLou

    BigLou New Member

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    138
    WOW 80% efficient unit. It should be illegal to sell a unit with efficiency that low. The condensation is being caused by the flue gasses dropping below 140. Sounds like you need a smaller or an insulated flue

    lou
  5. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    C'mon Lou, that's a bit of a stretch, no? Illegal? My car gets 35 MPG. Does that mean large sedans that get 25 MPG should be illegal? Pickups not hauling loads should not be allowed to drive because they get 17 MPG?

    I can get an 80% furnace for X. I can get a 95% efficient model from the same brand at 2.26 times the cost of the 80% model. Both have similar warranties.

    What are you really saving? Say I need 40000 BTU for my home. The 80% will need 50K in to get 40K out. The 95% will need 42100. So you save 7900 BTU every hour with the 95% instead of the 80. I pay $1.30/Therm here for gas.

    So for every hour that furnace runs, a 95% one is 10 cents cheaper. Takes a LOT of hours to pay off the difference.

    Look at it this way:

    Last year I used about 516 therms for heating my home. If I had a 95% efficient furnace instead of the 80% one I have, I would've used about 435 therms. $105 a year difference for me. Not huge.

    Now, I understand that a larger home would need a larger BTU, and thus the differences would be larger between the 80% and 95%. But my point is that it's not a huge difference, percentage wise.
  6. BigLou

    BigLou New Member

    Messages:
    138
    Nate,
    There is more to the equation then raw BTU's in/out. Those ratings are under a fabricated set of test conditions that should never exist in actual usage. They use a delta T thats to to large to exist in most applications, would cause internal condensation and destroy the unit. The efficiency of a standard <90% efficient unit is never whats stated. The unit is also unable to modulate so its going to short cycle and loose efficiency there.
    A >90% efficiency unit will almost always have the ability to modulate. Allowing the unit to avoid short cycles and increasing seasonal efficiency.

    You stated that you will save about $105 a year over a 20 year life span I see some units that are 40+ years old. That equates to a $2100 savings assuming that fuel prices never increase in that time period I am pretty sure thats the same or less then the price difference between the two units. I will make another stretch and say that fuel prices are going to increase in the next 20 years and the savings may be more then double or triple that. 20 years ago gas was about $0.36 per therm so thats 360 % increase.

    I also believe in protecting the environment. You will save 81 Therms per year, Currently there are about 50 million houses that heat with natural gas. If half of those were to save the same 81 therms thats 2,025,000,000 therms of gas saved and not contributing to global warming. Many places mandate higher efficiency equipment, do a google search on the United Kingdoms policy.

    I could also argue that if the only thing manufactured was high efficiency equipment that the prices would drop substantially. The environment aside, a good portion of our natural gas is imported I would not be a stretch to say that importing less gas from unstable regions would A) help the economy by sending less money over seas B) help with national security by making us more self reliant. Setting the environment, the economy and national security aside. Natural gas is a feed stock for some many other processes that we depend on it seems foolish to burn it. So many of our plastics, medicines, and chemicals depends on natural gas (and oil) for their production. No more plastics in 100 years because we burned all the feedstock for its production.

    I installed a 98% efficient boiler and took an 84% combustion <50% seasonal efficient unit out of use. You said the 15% difference is not huge but I know I would sure like to pay 15% less or make 15% more. I think its a sizeable enough difference.
    I wish I was able to get 17 mpg in my pick up truck, I average around 12 mpg even less when I am plowing thats why I only drive my truck when I absolutly have to.
    Lou

    http://home-and-garden.webshots.com/album/561572191MXzZYj?vhost=home-and-garden
    Last edited: Feb 23, 2008
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,023
    Location:
    New England
    To keep from eating up the insides of your shared flue, you might have to opt for a SS liner, which will set you back more than a few bucks.
  8. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    I think many would agree that most new units today would not last 40 years, or anywhere close. They don't make them like they used to. :D I've seen 40-45 year old units as well.

    As of 2002, only 15% of our natural gas was imported. And 95% of that from Canada. So our Natural gas supply has little to do w/ unstable regions, FWIW.

    I knew the efficiency was rated seasonally, but I didn't think it really varied much. I guess I assumed it was 80% +/- a few percent during the season. Now I know differently.

    15% a sizeable difference? I think that just depends on one's opinion of what payback period is worth it.
  9. BigLou

    BigLou New Member

    Messages:
    138
    Didn't know that. learn something new everyday. Its nice having inteligent (this was spelled wrong) people who I can debate/disagree with and not have it turn in to an out right pissing match. I this we are both a bit more knowledgeable from this

    Lou
    Last edited: Feb 24, 2008
  10. Cookie

    Cookie .

    Messages:
    5,658
    Location:
    .
    Lou, from a readers point of view, you got that right; it is alot, nicer to read. People want to read the material, so they can contribute, share, learn, question, debate, talk, and spitting really confuses things, it never is entertaining and turns most people right off. You, could not had been anymore right if you tried.
    Last edited: Feb 23, 2008
  11. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Absolutely! We both learned something.
  12. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    I wonder what the efficiency of my "Sears 10" gun type oil furnace is?

    After 10 years... I finally got around to opening up the ports last year and it was actually quite clean inside.

    As part of a dual fuel system it only runs a few times a year.

    --
    Back on topic.

    I've thought about adding a heat exchanger as part of the chimney, but I could not figure out a way of preventing water from condensing and even freezing.

    So the moral of the story is to keep the chimney warm.

    I wonder if it is possible to run a smaller pipe up the middle to better insulate the exhaust air?

    Edit: Making the chimney smaller could cause the furnace to force exhaust gases down the water heater.
    This means that it would be better to separate the two and run two smaller pipes up the chimney if possible.
    Last edited: Feb 25, 2008
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