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Thread: 95% efficiency unit vs 80% efficiency unit

  1. #16
    General Engineering Contractor ballvalve's Avatar
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    Another cool day. The house has radiant heat. And then summertime fun in California, outdoor radiant heat, taken from the front yard.
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    Last edited by ballvalve; 01-17-2011 at 01:21 AM.

  2. #17
    In the trades Dana's Avatar
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    Quote Originally Posted by Rich B View Post
    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.......
    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.

  3. #18
    DIY Senior Member Rich B's Avatar
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    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

  4. #19
    In the trades Dana's Avatar
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    Quote Originally Posted by Rich B View Post
    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
    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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