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  1. #1
    Join Date
    Jan 2013
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    9

    What size furnace do I need? 60,000 BTUH or 80,000 BTUH?

    I would greatly apprecaite some feedback on the correct furnace size for my installation? House in Seattle is 1994, 3205 square feet, 3 story, including a walk out basement, R19 wall insulation, R26 in ceiling, dual pane windows, and the basement is always cooler than the rest of the house.

    I have been presented with two furnace options, both of which are 96%variable speed 2 stage furnaces: first option is a 60k BTUH furnace @ 58k hi / 37.7k low output, or second option is an 80k BTUH furnace @ 78k hi/50k low output.

    My BTUH total heat loss is 48k, using 72 degree indoor temperature and 23 degree outdoor. Does the 60k unit leave me with enough extra BTUH in reserve? My concern is, what if I let my indoor temperature at night drop to 64 degrees on a real cold night, will the 60k furnace have enough BTUH in reserve to heat my house back up fast enough to a cozy 72 in the morning?

    Do I go with the 60k or 80k unit?

    Thanks.

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
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    6,296
    Quote Originally Posted by VFRguy View Post
    I would greatly apprecaite some feedback on the correct furnace size for my installation?

    House in Seattle is 1994, 3205 square feet, 3 story, including a walk out basement, R19 wall insulation, R26 in ceiling, dual pane windows,

    I have been presented with two furnace options, both of which are 96%variable speed 2 stage furnaces:
    ___ first option is a 60k BTUH furnace @ 58k hi / 37.7k low output, or
    second option is an 80k BTUH furnace @ 78k hi/ 50k low output.

    My BTUH total heat loss is 48k, using 72 degree indoor temperature and 23 degree outdoor. Does the 60k unit leave me with enough extra BTUH in reserve?

    My concern is, what if I let my indoor temperature at night drop to 64 degrees on a real cold night, will the 60k furnace have enough BTUH
    in reserve to heat my house back up fast enough to a cozy 72 in the morning?

    Thanks.
    60/ 57/ 37.7 K bTU/Hr furnace will provide the most Comfort.

    Heat load will normally be about 35,000 BTU/hr given that Seattle does not go below 38'F very often (34'F in comparison to design dT of 49'F)
    You would likely find that Low Stage heat on 60K furnace would be sufficient more than 2/3rd of the time.

    Set-back recovery might be "learned" as 6 hours IF the temperature actually dropped to 67'F.
    Therefore, the 'recovery mode' would start at 1 am for a 72'F - 7 am setpoint.
    Thinking about something that might happen _once every other year_ is not the method I would use to select a furnace.
    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

  3. #3
    Join Date
    Mar 2006
    Location
    The South
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    Assuming the accuracy of a good load calc, the 60 K model should be just fine.

    Dealer may want to evaluate basement and add additional supply(s) for more BTUs. You mentioned basement was always cooler which is quite typical.

    What if anything are you doing about AC? With the 60 K furnace model that has a 3 ton rated blower, you would be limited to no larger than a 3 ton AC condenser.

    It should be noted though the importance of having a good two stage thermostat that will control furnace staging rather than a timer on the control board. Otherwise you will always be on high stage unnecessarily. Not a detail to overlook.

    A pet peeve of mine for homeowners to spend extra $$$ on a nice two stage var speed furnace and then for a dealer to place the wrong and cheaper thermostat that shortchanges the best operation of that furnace.

    IMO
    Good Luck!

  4. #4
    Join Date
    Aug 2010
    Location
    West Monroe, LA
    Posts
    1,535
    Quote Originally Posted by tigerdunes View Post
    Assuming the accuracy of a good load calc, the 60 K model should be just fine.

    Dealer may want to evaluate basement and add additional supply(s) for more BTUs. You mentioned basement was always cooler which is quite typical.

    What if anything are you doing about AC? With the 60 K furnace model that has a 3 ton rated blower, you would be limited to no larger than a 3 ton AC condenser.

    It should be noted though the importance of having a good two stage thermostat that will control furnace staging rather than a timer on the control board. Otherwise you will always be on high stage unnecessarily. Not a detail to overlook.

    A pet peeve of mine for homeowners to spend extra $$$ on a nice two stage var speed furnace and then for a dealer to place the wrong and cheaper thermostat that shortchanges the best operation of that furnace.

    IMO
    Good Luck!
    A great point as the OP would need to investigate. As I agree he or she will
    Be limited to what size furnace they can install if the a/c is over (3) tons. Good catch.

    To the OP you will need to find out what size a/c unit you have and go from there as tigerdunes pointed out you might be asking about the 60k furnace for no reason if the a/c unit is bigger then (3) ton. The furnace will have to have enough air flow for the a/c unit to work right if over (3) tons on a/c!

    As far as your load numbers go if correct I would Chosse the 60k in 96% furnace and you should be fine. You will need a good t-stat to control the staging of the heat as I agree a timer is not the best way. If your a/c unit is bigger then (3) tons you will have to look at 80k furnace but still will need a good t-stat to control heating stages instead of timers.

    Please keep us posted on the size of the a/c unit and what your dession you make.

  5. #5
    Join Date
    Mar 2006
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    The South
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    Thanks for the comment and support Duckman.

    Not certain if OP even plans on AC but he needs to think about this.

    One other thought and suggestion. I would recommend a good whole house media filter cabinet at time of your install. Changeout is very easy and normally filters last up to a year.

    IMO
    Good Luck!

  6. #6
    Join Date
    Aug 2010
    Location
    West Monroe, LA
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    Quote Originally Posted by tigerdunes View Post
    Thanks for the comment and support Duckman.

    Not certain if OP even plans on AC but he needs to think about this.

    One other thought and suggestion. I would recommend a good whole house media filter cabinet at time of your install. Changeout is very easy and normally filters last up to a year.

    IMO
    Good Luck!
    No need for thanks but you are welcome. That is what I love about this site/fourm, professinal people who take the time to help customers and techs with problems they face everyday.

    Also I agree with a 5" whole house media filter. A good way to protect the furnace and coil from dust, dirt, etc... along with helping to filter the air better to help with Indoor air Quality.

  7. #7
    Join Date
    Feb 2009
    Location
    Portland OR
    Posts
    1,994
    If you look at the actual CFM of many of the variable speed blowers on a 60K furnace they will provide airflow for up to 3.5 ton AC which covers a lot of houses in seattle/portland.

    I would put in the 60K furnace, I am working with a homeowner in Portland who is going through the same thing and the 60K really is the best option for them. Stop worrying about one day where the temperature might be 18 degrees once every few years and worry about operating efficiently and comfortably the rest of the time.
    Check out my YouTube channel - http://www.youtube.com/user/skyheating1 We have customer testimonials, product reviews and more!
    Like us on FACEBOOK if you like our advice here!

  8. #8
    Join Date
    Nov 2004
    Location
    SW FL
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    MIN 24'F MAX 91'F for the last 8,760 hours in Seattle

    http://www.wunderground.com/history/...q_statename=NA
    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

  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,296
    SKY HEATING KNOWS BEST.!

    OR
    ANY1 Who Cares
    to discover that Cooling Degree Days are ~200 and
    Heating Degree Days are ~ 5,000 ... depending on specific NW locale.

    OCTOBER TO MAY = HEATING

    30 days < 32'F
    _2 days > 90'F

    http://www.climate-zone.com/climate/...a-tac-airport/

    Seattle Sea-tac Airport
    Temperature Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
    Avg. Temperature 40.1 43.5 45.6 49.2 55.1 60.9 65.2 65.5 60.6 52.8 45.3 40.5 52.0
    Avg. Max Temperature 45.0 49.5 52.7 57.2 63.9 69.9 75.2 75.2 69.3 59.7 50.5 45.1 59.4
    Avg. Min Temperature 35.2 37.4 38.5 41.2 46.3 51.9 55.2 55.7 51.9 45.8 40.1 35.8 44.6


    Days with Max Temp of 90 F or Higher 0.0 0.0 0.0 0.0 < 0.5 < 0.5 1.0 1.0 < 0.5 0.0 0.0 0.0 2.0
    Days with Min Temp Below Freezing 10.0 6.0 3.0 < 0.5 0.0 0.0 0.0 0.0 0.0 < 0.5 3.0 9.0 30.0

    Seattle Sea-tac Airport
    Heating and Cooling Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
    Heating Degree Days 772 602 601 474 307 144 58.0 65.0 156 378 591 760 4908
    Cooling Degree Days 0.0 0.0 0.0 0.0 0.0 21.0 64.0 81.0 24.0 0.0 0.0 0.0 190
    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

  10. #10
    Join Date
    Jan 2013
    Posts
    9
    Feeling much better now in making the right decision with the right contractor. Thank you all for your responses! Great forum!!

  11. #11
    Join Date
    Mar 2006
    Location
    The South
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    VFR

    Pay attention to my recommendation on best thermostat selection and addition of media filter cabinet.

    IMO

  12. #12
    Join Date
    Jan 2013
    Posts
    9
    Quote Originally Posted by tigerdunes View Post
    VFR

    Pay attention to my recommendation on best thermostat selection and addition of media filter cabinet.

    IMO
    I consistently have been recommended the Honeywell VisionPro 8000. Is that the kind of thermostat you are referring to?

    I do have an electronic air filter, that the same thing as a media filter cabinet?

  13. #13
    Join Date
    Mar 2006
    Location
    The South
    Posts
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    There are many models within the 8000 series of Honeywell stats, only a few are true two stg thermostats that actually control the staging rather than the timer on the control board. You need to get the specific model number and perhaps a Honeywell thermostat expert can confirm that yours is a true two stage model. It is a big deal or else you will be running on high stage all the time when unnecessary.

    IMO

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