95% efficiency unit vs 80% efficiency unit

Discussion in 'HVAC Heating & Cooling' started by northeastguy78, Jan 6, 2011.

  1. northeastguy78

    northeastguy78 New Member

    Messages:
    16
    Location:
    New Jersey
    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. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    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.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,003
    Location:
    New England
    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.
  4. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    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. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,003
    Location:
    New England
    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.
  6. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    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.
  7. northeastguy78

    northeastguy78 New Member

    Messages:
    16
    Location:
    New Jersey
    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. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    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. Ian Gills

    Ian Gills Senior Robin Hood Guy

    Messages:
    2,777
    Location:
    USA
    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. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    DEC 09 2.jpg

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

    Rich B DIY Senior Member

    Messages:
    283
    Location:
    New Jersey
    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. Ian Gills

    Ian Gills Senior Robin Hood Guy

    Messages:
    2,777
    Location:
    USA
    Is that a tree house Ballvalve?
  13. northeastguy78

    northeastguy78 New Member

    Messages:
    16
    Location:
    New Jersey
    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. Rich B

    Rich B DIY Senior Member

    Messages:
    283
    Location:
    New Jersey
    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. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california


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

    I'll try and post another opposite view.
    Last edited: Jan 17, 2011
  16. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Another cool day. The house has radiant heat. And then summertime fun in California, outdoor radiant heat, taken from the front yard.

    Attached Files:

    Last edited: Jan 17, 2011
  17. Dana

    Dana In the trades

    Messages:
    2,839
    Location:
    01609
    In most of the US adding a humidifier is usually a bad idea- a mold-producing band-aid "solution-probem" adding modest amount of increased comfort for a leaky building envelope. The more humidity you add to the air, the more condensation/frost you get along the exfiltration path(s), leading to higher rot potential over time, and higher interior air mold spore counts (particularly during the spring.)

    The better solution to dry winter air is to tighten the house up to the point where it stays above 30% RH in winter, and use mechanical ventialation (particularly exhuast fans in bathrooms and kitchens) to expell any concentrations of indoor air humidity/pollution. If it's so tight that it stays above 40% RH all winter (difficult to achieve with retrofit air-sealing in most homes in 6000+ heating degree-day climates) heat recovery ventilation (HRV or ERV) under dehumidistat control, or continous low cfm exhuast ventilation in bathrooms/kitchens would be in order. If the air isn't leaking through wall cavites or ceiling leaks, it isn't creating localized condensation and long-term accumulation of humidity on the exit paths. (Vapor barriers & vapor retarders seem to have captured the popular imagination, but most condensating/mold related moisture problems in buildings are from air leaks, not vapor permeation through walls.)

    The most important places to concentrate the effort are the basement and the attic, to quell the stack effect. Air leaks in between are subject to the wind, but the stack-effect works 24/7, creating suctino pressures to the tune of ~4 pascals for every 10' of height. Stopping the inflow at the bottom, and the outflow a the top are key. Besides the obvious window & door weatherstripping, dryer vents, flue dampers, etc, foam-sealing the foundation sill and band joist is usually the single largest overlooked air leak in most homes (it's typically bigger than an entire home's worth of window & door weatherstipping). Attic hatch weather stripping, while important, is usually a fraction of the air leakage from recessed lights, plumbing & electrical penetrations (particularly plumbing chases where vent stack follow through to the roof), flue & chimney chases (use sheet metal air dams), and any balloon-framing/partition wall framing with leaky or absent top plates. Fixing the bigger air leaks are typically easiest, an provide the most benefit. After fixing the obvious, it often takes pressurizing/depressurizing the house with windo fans (or calibrated blower doors) to find & fix the myriad smaller leaks.

    After any round of air-sealing, check for backdraft potential on any atmospheric-drafted combustion equipment.

    As for the original question, with a gas-fired burner the cold end of the heat exchanger has to stay- under ~90F on the fire/exhaust-gas side to get 95% combustion efficiency. Assuming 65F air on the return plenum that's not too tough to do as long as the air flow stays high enough. With a 25-40F delta-T on the heat exchanger it means the exit air will usually be well-under 130F, often under 115F. With an 80% system it doesn't much matter- you can run the thing in "scorched-air" mode and it can still hit low-80s for raw combustion efficiency. Running it hotter there's a bigger delta-T between the air & fire sides, and the heat exchangers can be more compact too. Many 80% furnaces control the blower with snap-disc type thermostats on the heat exchanger to guarantee a minimum average exhuast temp to prevent condensation damage to the heat exchanger, whereas 95% furnace controls need to set a max temp to maximize condensation, and minimize exhaust temp to well below the operational range of plastic vent pipe.

    Duct design affects the total air flow, and will affect exit temps. A 2-stage 95% burner with a 2-speed (or variable speed) fan can still deliver warmer air at the register than a bang-bang controlled single-speed high-flow 80% unit- it depends on both the furnace and the duct design. Anying above 110F is usually pretty comfortable, but under 100F (body temperature) the wind-chill temp at the register becomes relevant. (Low speed air can still feel pretty cozy even at 85F, but would have a chilling effect at most air-handlers' high-settings.) 95% 2-stage units are counting on running at low-temp low-fire low-speed most of the time to achieve that AFUE.
  18. Rich B

    Rich B DIY Senior Member

    Messages:
    283
    Location:
    New Jersey
    My bypass Aprilaire 600 humidifier install made it more comfortable to me in my house. No mold issues and no other issues other than having to change the pad. I also see the evidence that it is better in one piece of furniture that has been in my living room for many years. It does not shrink as much as it used to.....and I know that is true because it used to be easy to see a ceramic tile insert that would start to warp upwards from the wood shrinkage. It does not do that any more.....
    My higher efficiency furnace also made it much more comfortable and is getting a real workout this winter. If I feel cold I just turn the t-stat up and 72 seems to be just about right. I also played with the single stage t-stat anticipator adjustment and heat is more even now. Eventually I will install the 2 stage t-stat but need to run a new piece of t-stat wire with more conductors. It is a short run and easy to do....

    Trying to tighten up an old house poorly built with poor insualtion would be a rediculously expensive and complicated project. I'd sooner burn it down and start over but that is not a good plan either.......I'll live with it....I have for 40 years.....LOL
  19. Dana

    Dana In the trades

    Messages:
    2,839
    Location:
    01609
    Sometimes yes, but usually not. Find an insulation contractor that specializes in air-sealing- the first 90% of the cost-effective reductions in air infiltration is typically 10% of the cost of doing them all, and it WILL make a difference. Most places can be air-sealed to 3ACH/50 (air changes per hour at 50 pascals pressure) without breaking the bank or ripping the house apart. Getting it down to 1ACH/50 is when it gets ridiculous.

    The mold issues related to humid air exfiltraion may actually be there, but not necessarily inside of conditioned space. I may show up as rot a decade or two, or "sick house syndrome" in as little as two years. Just 'cuz it made it 40 years prior to adding the humidifier isn't necessarily predictive of future performance. Air sealing to the point where the humidifier never needs to run to keep it ~30% RH @ 70F in winter is a safer approach. (And unlike a humidifier, air sealing usually has sub 5-year payback on energy cost savings- which is far better than insulation, or Wall Street. ;-) )
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