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  1. #14
    Join Date
    Jul 2012
    Posts
    32
    To bring this back, here is what I am looking at:

    2750/sf

    Windows: Original (1976) 2 pane. Are leaky and will be replacing in the next few years
    N:80.25/sf E: 50/sf S: 99.5/sf W: 12/sf

    Shading probably around 10-15% Ceilings are 8ft most of the house with about 400sf at 12'

    Have a poured concrete basement with a crawl space.

    Insulation: I would guess R6-10 in the walls. Attic will be about R60 once I blow more in within the month

    Blower Door was around 3300-3500 I will be making some improvements soon to hopefully get that down a bit.

    Tedkidd, using the site you gave me I am still around 76k for the heat load. Given this info, should I go 60k or 80k furnace? Thanks!!

  2. #15
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Do you have your gas bill from the coldest month? Does it show the average daily temperature. Take the Therms used, multiply it by 100,000, divide it by 24, divide by the number of days in that billing cycle, divide by the ratio of the average daily temeprature and the desing temperature, then divide by the efficiency of the current furnace.

    I personally think it's even more accurate than the manual J because its measuring complete real world conditons, not any estimates. But there's not a "fudge factor" and on a 2 story home, the stack effect accelerates as it gets colder. Plus it doesn't subtract out days where it's over 65F and your actually cooling. But I think it gives a good estimate overall. The numbers don't lie. If you used 250 Therms (which is a lot for most people) and the average temp is 33F with a 0F design, you were at around 50% average capacity. That's an average consumption of 34k BTU's. IF you had enough thermal mass, you could have heated the home with just 34k BTU's. That's why goethermal works soem well on highly insulated new construction with fulol basement and lots of thermal mass. It just runs around hte clock at 30-40k BTU output and the tmeprature might swing 1F or so, but it just chugs along wihtout heat strips. A load calculation might call for 60k BTU furnace. But it's rarely needed. Also consider all the internal heat loads. A larger home easily consumes 0.6-0.8kw. That's another almost 3000 BTU's on energy, nearly all becomming heat indoors. Also add heat gained from showers as well.

    My home for example. 3200sqft. You'd expect that it might need at least 100k BTU. Using my gas bills, the calculated consumption is just 61k BTU's with 2 oversized 92% furnaces totalling 180k BTU's. I've since downsized on to 60k (the smallest available in that model) and downstairs I'm debating dual fuel, geothermal or a conventional system.

    Remember heat distribution matters as well. An oversized furnace won't heat as evenly and will consume more fuel to satisfy the same comfort level.

    THe pro's on here will tell you that they've probably never seen a undersized furnace. That's because in reality even a larger home really only needs a realatively small amount of heat.


    In summary, if in doubt, or borderline, go smaller.

  3. #16
    Join Date
    Jul 2012
    Posts
    32
    Unfortunately, this is a new house for me, and I am switching from electric to natural gas. I don't have any previous data to compare from.

  4. #17
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    If you're at 76k now and plan on making improvements, then I'd go with the 60k. Larger homes are more tolerant to being slightly undersized since they have a lot more mass. They don't swing very quickly.

    In my old 1300sqft home, I woudl often be heating in the morning when it dipped under 55F and AC in the afternoon when over 65F on the same day if it was sunny in May and October. But in my new home that's 2-1/2X larger, on those same days, the indoor temps will stay pretty constant and not vary by more than 1-2F. SO rather than heating bills being $30, $50, $80, $140, $90, $60, $30... it's more like $0, $60, $180, $300, $200, $80, $20. It ramps up really fast.

  5. #18
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,301
    Also keep in mind manual j is based on peak conditions. It does not take into account the effects of thermal mass.

  6. #19
    Join Date
    Jul 2012
    Posts
    32
    Quote Originally Posted by 54regcab View Post
    Also keep in mind manual j is based on peak conditions. It does not take into account the effects of thermal mass.
    Meaning that it won't take as much heat to keep my house at a temp as it would to get it to that temp from something lower?

  7. #20
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by jmon22 View Post
    Meaning that it won't take as much heat to keep my house at a temp as it would to get it to that temp from something lower?
    Yes, that's true in part. But Manual J doesn't factor in the need to raise the temperature of a space. It does however have a certain amount of error built-in to compensate for less than perfect installations.

    In thermal mass, he's referring to energy stored within building materials in a home. Older homes for example might lack insulation, but often use real brick and stone for loadbearing walls, plaster instead of sheetrock, and solid wood floors and subfloors, heavy ceramic tiles and thick thinset and cement board material. Those materials store more energy than modern materials. Because of that, the home will heat up and cool off very slowly, so the building can "ride through" the coldest outdoor tmerpatures. With a enough thermal mass, you design temrpeature becomes not the lowest daily temperature, but the lowest rolling 12, 24 or even 48 hour average temperature.

    An anologies. A empty refrigerator will cool off quickly, but if you unplug it, it warms up quickly s well. Fill it with 200lbs of food, and it will cool off and warm up much more slowly. The food stores energy... or in thsi case absorbs energy, meaning that if the heat loss through hte insulation around the fridge enclosure is the same, it will take longer for it to change temperature.

    The heat loss to a home is based on R value and is deendant on outdoor temperature (less solar and interior source heat gain) is also why (I have a hard time explaining this to my father in law) a 60k BTU furnace running for 40 minutes uses the exact same amount of gas as a 120k BTU furnace running for 20 minutes. Actually, because of larger temeprature swings, it uses more. In reality, the smaller furnace would probably run for 39.5 minutes.

    Finally the larger the equipment, the more ductwork you need and more opportunity for air leaks.


    The ultimate home for example, might be a concrete home with insulated forms, plus spray foam betwee the stud cavities. You could match of beat the overall R value with a stick frame home with double studs and foam board over the sheathing, but the high thermal mass wold allow you to size equipment based on average daily temeprature. So where I am the design temp might be 0F in winter, but the average for that period would be 6F. That's a difference of almost 10% is sizing for heating. For cooling it's even more since temeprature swing in summer are greater. The deisn in summer is 93F when I am but average daily temp is only maybe 84F. I've cut my convective heat gain in 1/2 and total peak cooling load by maybe 1/3. SO where I would have needed a 3 ton system, now i can use a 2 ton system.


    Soem on this board would like ot see small scale residential chilled water equipment become more popular for this reason. You could store enough chilled water to shut your system off for utility time of day demand pricing.

    Remember 1 ton of cooling can be defined by the amount of energy needed to melt 1 ton of ice. It will also take that much energy to cool about 5 tons of concrete by 5F. That's about a 210 sqft slab, 4" thick.

  8. #21
    Join Date
    Jul 2012
    Posts
    32
    Makes sense. I think I will try to get away with the 60k, seal my ducts really well, and get the house where it needs to be to make it work. Worst case would be it is a few degrees behind the thermostat a handful of the coldest nights of the year. I doubt it will ever have trouble keeping up during the day. I have been agonizing over this (stupid I know) but I do not want to make a mistake I will regret for 15-20 years whether I go too small or too large.

  9. #22
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,732
    And how you manage temperature will impact on comfort, and air temperature requirements.

    http://bit.ly/comfortcalculator

    If you set your thermostat back 8 degrees when you go to work, when will all the surfaces in your house be the at their COLDEST POINT?

    When you walk in the door.

    The comfort calculator helps show you'll need much higher air temperatures to accomplish the same level of comfort when your surfaces are 60 than when they are 68. And that's not factoring in the greater airflow required for that bigger furnace (which is also a comfort penalty in the heating phase).

    So if you want high bills and an uncomfortable house, put in a big "high efficiency" furnace and use an aggressive setback strategy.

    If you want low bills and a comfortable house, put in a tiny furnace and do not adjust the thermostat! Think of your house as a giant flywheel. A car tire you are rolling with a twig.

    Do NOT expect recovery. Better not let the "tire" stop or fall over, because that twig ain't gonna get it upright and rolling again.

    The difference is do you want a house that you heat up, or do you want a heated house?
    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.

  10. #23
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,301
    +1, correctly sized equipment that runs continuously will result in lower overall energy costs than an oversized furnace with a setback strategy. The large temp swings caused by oversized furnaces cost you more than you think. By getting blasts of hot air raising the temperatures 3-4 degrees each cycle, you actually increase heat loss quite a bit.

  11. #24
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295
    Quote Originally Posted by tedkidd View Post
    And how you manage temperature will impact on comfort, and air temperature requirements.

    http://bit.ly/comfortcalculator

    If you set your thermostat back 8 degrees when you go to work, when will all the surfaces in your house be the at their COLDEST POINT?

    When you walk in the door.

    The comfort calculator helps show you'll need much higher air temperatures to accomplish the same level of comfort when your surfaces are 60 than when they are 68. And that's not factoring in the greater airflow required for that bigger furnace (which is also a comfort penalty in the heating phase).

    [/B]
    NICE tool ... really emphasizes the COMFORT impact of radiant temperature.


    C ___ F
    15.56 -- 60
    16.11 -- 61
    16.67 -- 62
    17.22 -- 63
    17.78 -- 64
    18.33 -- 65
    18.89 -- 66
    19.44 -- 67
    20.00 -- 68
    20.56 -- 69
    21.11 -- 70
    21.67 -- 71
    22.22 -- 72
    22.78 -- 73
    23.33 -- 74
    23.89 -- 75
    24.44 -- 76
    25.00 -- 77
    25.56 -- 78
    26.11 -- 79
    26.67 -- 80
    27.22 -- 81
    27.78 -- 82
    28.33 -- 83
    28.89 -- 84
    29.44 -- 85
    30.00 -- 86

    FYI... Centigrade to Farenheit
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  12. #25
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295
    Quote Originally Posted by jmon22 View Post
    To bring this back, here is what I am looking at:

    2750/sf

    Windows: Original (1976) 2 pane. Are leaky and will be replacing in the next few years N:80.25/sf E: 50/sf S: 99.5/sf W: 12/sf

    Blower Door was around 3300-3500.
    I will be making some improvements soon to hopefully get that down a bit.
    _ Please translate 3300 - 3500 to ACH.
    Looks like you may have 15,000 BTU/hr HEATing penalty (loose to tight in Manual J abridged) due to EXCESSIVE Infiltration.

    a.k.a. seems like 70,000 could be reduced to < 60,000 BTU/HR
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  13. #26
    Join Date
    Jul 2012
    Posts
    32
    Quote Originally Posted by dan sw fl View Post
    _ Please translate 3300 - 3500 to ACH.
    Looks like you may have 15,000 BTU/hr HEATing penalty (loose to tight in Manual J abridged) due to EXCESSIVE Infiltration.

    a.k.a. seems like 70,000 could be reduced to < 60,000 BTU/HR
    I agree. House needs tightened up and I am going to be remedying that. Going with the 60k will give me a goal to shoot towards. If I have to put an extra blanket on during the coldest nights of the year in the meantime, then so be it.

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