(206) 949-5683, Top Rated Plumber, Seattle
Page 1 of 2 12 LastLast
Results 1 to 15 of 19

Thread: 95% efficiency unit vs 80% efficiency unit

  1. #1
    DIY Junior Member
    Join Date
    Jun 2009
    Location
    New Jersey
    Posts
    16

    Default 95% efficiency unit vs 80% efficiency unit

    Hi folks,

    Just a general question for you. Is it true, that the 95% efficiency furnace will produce warmer heat coming out of the registery than a unit that only does 80% efficiency?

    In my previous home, I had a 95% efficiency unit installed and the air coming out of the registery was very hot. Since we moved, this unit is only 80% efficiency, is air coming out is only luke warm....

    please let me know

    thanks

  2. #2
    DIY Senior Member zl700's Avatar
    Join Date
    Aug 2009
    Location
    Texas
    Posts
    235

    Default

    The two are not related in fact, high efficiency furnaces typically have a lesser temp rise. The differences you are feeling is ducting distribution and fan speeds.
    If Payback is so important to you, why are you not driving a Toyota Corolla?

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,395

    Default

    If you also have central air conditioning, the fan speed may be set to promote that which typically wants a higher flow rate than heating. It doesn't always change based on the mode, you have to do that yourself. Many times, you can adjust the fan speed to be slower which allows the air to absorb more heat as it goes through the heat exchanger. You must keep it above the minimum for the system's heat output or you risk overheating the exchanger. The manual should show you your valid options.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    General Engineering Contractor ballvalve's Avatar
    Join Date
    Dec 2009
    Location
    northfork, california
    Posts
    3,259

    Default

    If the ducts and fan speed were the same you would get air theoretically 15% warmer. You are putting 15% of your heat in the garbage can in this house.

    If our GOV banned 80% rigs, the cost of the 95% units would drop dramatically.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,395

    Default

    The efficiency of the unit has NOTHING to do with how hot the exiting air is...it is all dependent on the heat exchanger design, the fan speed, and the distance and state of the ductwork before it hits the register. If the ducts run through unheated space and are uninsulated, that may be the primary reason the exit temp is cooler. Have you actually measured the temperature at the register? Just like a cooling breeze in the summer feels cool, it will feel cool in the winter, too if you can't slow it down a bit.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    DIY Senior Member zl700's Avatar
    Join Date
    Aug 2009
    Location
    Texas
    Posts
    235

    Default

    Ball valve

    Actually if you compare similar size furnace from same manufacturer you will quickly notice higher recommended temp rise on 80%ers in order to maintain higher flue temps to avoid condensation of ht exh and flue. Thus 90%ers which there is a desire to condense have lower temp rise allowances meaning more cfm's across ht exh, resulting in cooler discharge temps.
    If Payback is so important to you, why are you not driving a Toyota Corolla?

  7. #7
    DIY Junior Member
    Join Date
    Jun 2009
    Location
    New Jersey
    Posts
    16

    Default

    yes, the 80% out puts temp at the register at 131, returns 72 when themostat is set at 74 (winter). The temp rise is 59, as this is when fan speed is set at med-low.

  8. #8
    General Engineering Contractor ballvalve's Avatar
    Join Date
    Dec 2009
    Location
    northfork, california
    Posts
    3,259

    Default

    Quote Originally Posted by jadnashua View Post
    The efficiency of the unit has NOTHING to do with how hot the exiting air is...it is all dependent on the heat exchanger design, the fan speed, and the distance and state of the ductwork before it hits the register. If the ducts run through unheated space and are uninsulated, that may be the primary reason the exit temp is cooler. Have you actually measured the temperature at the register? Just like a cooling breeze in the summer feels cool, it will feel cool in the winter, too if you can't slow it down a bit.
    I think thats what I said. In reverse. Anyway, its nuts to toss 15% to 20% of our precious natural gas. This is ONE place we need some regulations to make the MFG's go all condensing.

  9. #9
    Senior Robin Hood Guy Ian Gills's Avatar
    Join Date
    Jul 2007
    Location
    USA
    Posts
    2,781

    Default

    Yep, which is why I bought another 80 the last time round.

    High efficiencies are still too expensive to make the investment worthwhile. At least for people that don't intend to die in the houses they currently live in.

    It's hard to fix a Trane.

    Which is why I went for a Bryant 80 variable speed instead (I'm willing to pay for peace and quiet).

    I'm surprised you need heating in California.

  10. #10
    General Engineering Contractor ballvalve's Avatar
    Join Date
    Dec 2009
    Location
    northfork, california
    Posts
    3,259

    Default

    Name:  DEC 09 2.jpg
Views: 967
Size:  70.8 KB

    Gets a bit chilly at home in California from time to time.

  11. #11
    DIY Senior Member Rich B's Avatar
    Join Date
    Sep 2008
    Location
    New Jersey
    Posts
    282

    Default

    I installed a Rheem 93% efficiency condensing gas furnace a few years ago. It replaced a unit that was about 20 years old and no more than an 80% efficiency rated model. The newer unit is 2 stage......has a low fire and a high fire burner and also the fan speeds change with the burner output. NO doubt whatsoever it saves money and is warmer.......I saw about a 20% lower gas bill the first year I had it......and it IS warmer at the heat registers.

    The efficiency rating has nothing to do with the heat temperature though. I did the heat rise measurments instructed in the install manual. It was perfect and I did not need to change the fan speeds to adjust that temp. rise. The high efficiency is simply from not letting wasted heat go up the chimney.......I have luke warm air going out a side wall thru pvc now instead of hot gas going up a chimney. Most of the heat energy gets into the house instead of going out the chimney....
    I love the unit and am going to install a second one in another unit I have that still has an older low efficiency furnace.....

  12. #12
    Senior Robin Hood Guy Ian Gills's Avatar
    Join Date
    Jul 2007
    Location
    USA
    Posts
    2,781

    Default

    Is that a tree house Ballvalve?

  13. #13
    DIY Junior Member
    Join Date
    Jun 2009
    Location
    New Jersey
    Posts
    16

    Default

    Quote Originally Posted by Rich B View Post
    I installed a Rheem 93% efficiency condensing gas furnace a few years ago. It replaced a unit that was about 20 years old and no more than an 80% efficiency rated model. The newer unit is 2 stage......has a low fire and a high fire burner and also the fan speeds change with the burner output. NO doubt whatsoever it saves money and is warmer.......I saw about a 20% lower gas bill the first year I had it......and it IS warmer at the heat registers.

    The efficiency rating has nothing to do with the heat temperature though. I did the heat rise measurments instructed in the install manual. It was perfect and I did not need to change the fan speeds to adjust that temp. rise. The high efficiency is simply from not letting wasted heat go up the chimney.......I have luke warm air going out a side wall thru pvc now instead of hot gas going up a chimney. Most of the heat energy gets into the house instead of going out the chimney....
    I love the unit and am going to install a second one in another unit I have that still has an older low efficiency furnace.....
    I see. perhaps the town home that I sold (95% efficiency) unit had warmer air coming out at the registry had to do with more heat energy gets into the house like the way yours work. Our current home ( A SINGLE HOUSE) gets luke warm air coming into our registry figured it's due to 80% efficiency unit.

    curious, what was your temp rise when you did the measurement? Mine was at 59 degrees which is right smack in the middle of the unit manual 40~70.

  14. #14
    DIY Senior Member Rich B's Avatar
    Join Date
    Sep 2008
    Location
    New Jersey
    Posts
    282

    Default

    I think my temp rise was right in that area.....60 degrees. ?? It's been a while....

    My house is old.....has sheet metal ductwork that is not insulated. I turn the heat down during the day and at night to save $$$. It fairly quickly warms up once I turn the heat up.....but hot air heat in an older poorly insulated home is not great. I added a humidifier when I installed the furnace and between the 2 items it is better but I sure wish I had a fireplace or pellet or wood stove.......

  15. #15
    General Engineering Contractor ballvalve's Avatar
    Join Date
    Dec 2009
    Location
    northfork, california
    Posts
    3,259

    Default

    IAN: Is that a tree house Ballvalve?


    Not a tree house, but bolted to bedrock just above two waterfalls.

    I'll try and post another opposite view.
    Last edited by ballvalve; 01-17-2011 at 01:11 AM.

Similar Threads

  1. Replies: 4
    Last Post: 06-23-2010, 05:57 PM
  2. efficiency in this application
    By Thatguy in forum HVAC Heating & Cooling
    Replies: 8
    Last Post: 12-26-2009, 09:29 AM
  3. efficiency puzzle
    By dstrutto in forum Boiler Forum
    Replies: 0
    Last Post: 12-04-2009, 09:56 AM
  4. The cost of efficiency
    By bmccabe in forum HVAC Heating & Cooling
    Replies: 5
    Last Post: 07-21-2008, 08:05 AM
  5. The cost of efficiency
    By bmccabe in forum Remodel Forum & Blog
    Replies: 9
    Last Post: 07-02-2008, 05:16 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •