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  1. #14
    Join Date
    Feb 2009
    Location
    Portland OR
    Posts
    1,845
    Where you are a modulating furnace would probably be better. I put in both depending on the house, the less sealed and less efficient a house is the more a modulating furnace will help because the heat load is always ever changing and never holding the heat in.
    If it is a really tight insulated house then a two stage furnace should be fine because the house will hold its heat in and it doesn't need constant adjusting that a modulating furnace will. In the tight house the furnace will mostly run in 1st stage and then run in 2nd stage when the house needs to heat up or doors/windows get opened.

    As for the sizes, most manufacturers only make 60K BTU as their smallest size in high efficiency furnaces because of the modulation and staging capacities.

  2. #15
    Join Date
    Aug 2011
    Location
    Eastern Shore, MD
    Posts
    798
    modulating furnaces modulate everything....but the length of the heat exchanger....which is important!

    you can make a single stage condensing furnace truly more efficient (btus in vs btus out) than a modulating furnace any day of the week. key phrase "you can make"

    oversizing is not as big a deal as ppl make it out to be. yea itll burn through parts faster (we are talking 10 years vs 13...not significant) but even as it short cycles...the only efficiency loss is to heat the heat exchanger...which will still radiate back into the home....modulating furnaces cause shorter than needed flame length in the heat exchanger....IE condensation before the secondary......bad for numerous reasons....

    Comfort- yea nothing better....

    btu vs btu? single stage condensing all day.

  3. #16
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,635
    Quote Originally Posted by Gross View Post
    modulating furnaces modulate everything....but the length of the heat exchanger....which is important!

    you can make a single stage condensing furnace truly more efficient (btus in vs btus out) than a modulating furnace any day of the week. key phrase "you can make"

    oversizing is not as big a deal as ppl make it out to be. yea itll burn through parts faster (we are talking 10 years vs 13...not significant) but even as it short cycles...the only efficiency loss is to heat the heat exchanger...which will still radiate back into the home....modulating furnaces cause shorter than needed flame length in the heat exchanger....IE condensation before the secondary......bad for numerous reasons....

    Comfort- yea nothing better....

    btu vs btu? single stage condensing all day.
    "Aaah, so what if you burn 20% more energy, gas is cheap. Aaah, so what if you short cycle and burn out your heat exchanger, it's better than it rusting out (Hey, go back to 1993, ok?). Aaah, so what if it's noisy and you have temperature imbalance, go sit next to your furnace you'll be fine...."

    You are joking right? If you are, the sarcasm is humorous but could be more clear.

    If not:

    Not sure what btu's in btu's out means, but most homeowners want $ in to go down.

    If you don't get energy bills you have no idea what you are talking about. You need to get the energy bills, then track some usage after replacement or any claims made are not only a waste of peoples time, they're possibly harmful.

    If you can't back it up with results, your prescription is malpractice.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  4. #17
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I'f he's serious, I'm not sure he understands the whole smooth line vs. squiggly line on a temperature graph and how the area under the curve represents energy used. Mod furnace do run just a little more efficiently since they run at a even temperature.

    But, with a smaller furnace, the savings will be smaller, so it might not be worht it. It depends on the value added to the homeowner for slightly more comfort.


    As for condensation in the primary heat exchanger on low fire. I'm pretty sure the mfg was careful to make sure that the exhaust gasses remain above the condensation point even at the minimum temp rise (highest CFM setting) and lowest return air temp. The same goes for a 2 stage 80% furnace.

  5. #18
    Join Date
    Aug 2011
    Location
    Eastern Shore, MD
    Posts
    798
    No, dead serious. Take a combustion analysis course. The flames in low stage on two stage furnaces and almost always on modulating furnaces don't scrub the walls of the heat exchanger well. You get less than optimal heat transfer.

    Most ppl think natural gas is best at 3.5" and at that pressure, yea a mod furnace will win. You also end up with super low stack temps, super low co, and very high oxygen % in the flue. That equals poor performance

    Don't believe me? Clock your meter...see how many btus ur putting into the furnace, find your true cfm and see hOw many btus ur putting into the air. Off the shelf.... Ur looking at 75% maybe.

    AFUE strikes again!

  6. #19
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,635
    Harikrishna to you... You drank the cool aid.

    I know about that course. Jim Davis NCI class? Yeah, mix inside and outside air to tweak efficiency, max gas pressure, etc... right? Some good stuff but misses the point that furnaces are installed in houses which exist in continuously varying conditions, most of which don't ever come close to design.

    If you want hypersimplified answers to complex interactions, don't be surprised when results are much different than you expect.

    Tuning a single stage at steady state and saying "see how much we're saving" is bunk. Houses don't exist on race tracks where you can keep equipment at full load continuously. Life ain't Nascar. Load changes and your ability to maintain steady state at your tuned output is a pipe dream. We live at design temp about 1% of the time.

    Clocking meters for 5 minutes to prove a point, really? In the end, what is your annual spend per square foot. I want to know the whole year, not 5 minutes under laboratory conditions.
    Last edited by tedkidd; 08-05-2011 at 07:17 PM.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  7. #20
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,579
    alright, I don't know much about modulating gas in residential, where as in commercial we modulate to make a supply air temp setpoint. Am I wrong to think this point.

    is super long or almost constant runtimes for heat a good thing, doesn't it make a low humidity issue. I have this issue in my own home on the coldest day high stage on my heat pump runs near constant, and I have a dried out house.

    I thing it's great for cooling where we want to dry the house out, but wouldn't a properly ballanced 2 stage really be the best.

    bottom line I don't want to have to add a humidifier. And I also ask this because I am not thrilled with my heatpump in my house and will be putting in a gas furnace.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  8. #21
    Join Date
    Aug 2011
    Posts
    568

    Red face

    Quote Originally Posted by tedkidd View Post
    Harikrishna to you... You drank the cool aid.

    I know about that course. Jim Davis NCI class? Yeah, mix inside and outside air to tweak efficiency, max gas pressure, etc... right? Some good stuff but misses the point that furnaces are installed in houses which exist in continuously varying conditions, most of which don't ever come close to design.

    If you want hypersimplified answers to complex interactions, don't be surprised when results are much different than you expect.

    Tuning a single stage at steady state and saying "see how much we're saving" is bunk. Houses don't exist on race tracks where you can keep equipment at full load continuously. Life ain't Nascar. Load changes and your ability to maintain steady state at your tuned output is a pipe dream. We live at design temp about 1% of the time.
    most people who have 4 wheel drive may need the 4 wheel drive a few times a year, but when they need it on that snowey day, they have that capability. it's the same thing with central a/c, on that super hot day people want the capability to bring that temp to 72 on a 102 degree day,

  9. #22
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Quote Originally Posted by Southern Mech View Post
    alright, I don't know much about modulating gas in residential, where as in commercial we modulate to make a supply air temp setpoint. Am I wrong to think this point.

    is super long or almost constant runtimes for heat a good thing, doesn't it make a low humidity issue. I have this issue in my own home on the coldest day high stage on my heat pump runs near constant, and I have a dried out house.

    I thing it's great for cooling where we want to dry the house out, but wouldn't a properly ballanced 2 stage really be the best.

    bottom line I don't want to have to add a humidifier. And I also ask this because I am not thrilled with my heatpump in my house and will be putting in a gas furnace.
    Run time of a furnace or heat pump does not dry out a house. Duct and envelope leakage does.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  10. #23
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,579
    Quote Originally Posted by Shophound View Post
    Run time of a furnace or heat pump does not dry out a house. Duct and envelope leakage does.
    t

    no offense and prove me otherwise, but if you have an air moving system that runs constantly, it will dry out the air. Don't even try and give me that mumbo jumbo. the tighter the struture the worse it will be. if my house leaks like a sieve and its 40% humidity out I would never drop below 40 then right?, my ducts dont leak, sealed with mastic my self. why does my house with no humidification run down to 15% humidity in winter? 1900ft Cape cod on 1st flr. 700 second 3 ton XL19i down 1.5 ton XL14 up full 8ft wrap around porch covered and shaded. I know I am at full capacity with HP and strip heat, cold nights HP runs constantly and strips on and off. forced air heating dries out the structure, how can you say what you did?
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  11. #24
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361

    "We have here a failure to communicate."

    First of all, we must deal only with either heating or cooling at a time.
    We pick heating.
    1. Start with the house open and the same temperature and humidity inside and outside.
    2. Close the doors and windows 100% perfect seal.
    3. Add heat in the house. (Assume that the stucture and contents do not absorb or release water.)
    4. The temperature will increase and the humidity in pounds of water will remain unchanged. This is called ABSOLUTE HUMIDITY.
    5. However as the temperature increases, the warm air has a greater capacity to hold water. The water in the air compared to what it could hold falls. This ratio is called RELATIVE HUMIDITY. The air is not actually drier as it has exactly the same moisture that it started with. The air FEELS drier because the ratio of the actual water content to the maximum possible water content (RELATIVE HUMIDITY) is less than it was when the air was colder and capable of holding much less water.
    6. It FEELS drier because our bodies (and other stuff in the house) will release water into the heated air. This makes our skin dry and uncomfortable.
    7. Notice that running time is not a factor in this illustration.

    I will stop here and Shophound can explain why the lack of a perfect seal in a real world house affects our experience and thinking.
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  12. #25
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Quote Originally Posted by Southern Mech View Post
    t

    no offense and prove me otherwise, but if you have an air moving system that runs constantly, it will dry out the air.
    Explain to me, then, how a device (a gas furnace or electric heat and/or heat pump) that has no other ability than to change air temperature can also alter its absolute moisture content.


    Don't even try and give me that mumbo jumbo. the tighter the struture the worse it will be.
    Not sure what "mumbo jumbo" you're referring to. You must first understand what you oppose before you can hope to form a solid stance against it. And then, what often happens, you find what you're reacting against isn't what you thought it was.

    if my house leaks like a sieve and its 40% humidity out I would never drop below 40 then right?
    Let's try some basic psychrometrics here, shall we?


    • Outdoor air condition: 30^F @65%RH = 21^F dew point @15.7 grains of moisture per pound of dry air
    • Indoor air condition: for the sake of this illustration let's say it's the same as outdoors, minus frozen pipes and other cold weather disasters

    Start furnace, allow house to warm to 70 degrees dry bulb. No moisture added or removed from structure. House is unoccupied. Nobody showering, bathing, cooking, breathing. No pets, nothing. No plants. No moisture adding characteristics at all. It's just a structure being heated by the furnace. After house stabilizes at 70^F dry bulb, the result is:


    • Outdoor air condition: same as above
    • Indoor air condition: 70^F dry bulb, 14%RH, 21^F dew point @15.7 grains of moisture per pound of dry air

    "Ah HA!" you say. "I got you! I mean, look at that RH...14%!!! See? The furnace DID dry out the air!"

    No, the furnace did no such thing. All it did was add sensible heat content to the air. Period. This is basic psychrometrics.


    my ducts dont leak, sealed with mastic my self. why does my house with no humidification run down to 15% humidity in winter?
    See above. Plus, this nasty little building science geeky thing called "stack effect". Check out buildingscience.com and do a search on that term.

    BTW, when it's 30 degrees at 65%RH outdoors, my house does not need supplemental humidification, whether the furnace runs or not. Not only because my ductwork is fairly tight (it's not perfect, none are), but I have a good handle on my home's stack effect. RH in my house in winter seldom drops below 40% at air temps between 68 and 72 degrees F. Until you experience this first hand, you doubt that it is possible. It's because you've lived with the crappy alternative for too long, now.


    1900ft Cape cod on 1st flr. 700 second 3 ton XL19i down 1.5 ton XL14 up full 8ft wrap around porch covered and shaded. I know I am at full capacity with HP and strip heat, cold nights HP runs constantly and strips on and off. forced air heating dries out the structure, how can you say what you did?
    Willis Carrier backs me up.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #26
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,804
    Quote Originally Posted by Southern Mech View Post
    t

    no offense and prove me otherwise, but if you have an air moving system that runs constantly, it will dry out the air. Don't even try and give me that mumbo jumbo. the tighter the struture the worse it will be. if my house leaks like a sieve and its 40% humidity out I would never drop below 40 then right?, my ducts dont leak, sealed with mastic my self. why does my house with no humidification run down to 15% humidity in winter? 1900ft Cape cod on 1st flr. 700 second 3 ton XL19i down 1.5 ton XL14 up full 8ft wrap around porch covered and shaded. I know I am at full capacity with HP and strip heat, cold nights HP runs constantly and strips on and off. forced air heating dries out the structure, how can you say what you did?
    It is not possible to destroy moisture. Its one of those laws of physics of things.

    So no matter how long the furnace or a heat pump runs. The warm/hot air is not drying out the house. The lack of moisture is/will be from a source of drier air entering the house.

    If the return air is at 70F and 40%RH, and the furnace/heat pump rises that airs temp to 120F, its RH will be 8.6%, but when that air's temp drops to 70F again, its RH will be 40% again. Since the grains per pound of air never changed. 43.6570 per pound weather at 70, or at 120.

    Either your duct work is leaking, or your house is not as tight as you think.

    While we may not be able to prove this to you over the internet. If you really want to know. You can test your duct work's leak rate with a duct blaster, and have a blower door test done on your house. I think if you did, you would be surprise how tight one or both of them aren't.
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