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Thread: Sizing question

  1. #1
    Join Date
    Aug 2004
    Posts
    91
    If I have a furnace sized for the coldest day of the year, it will be oversized for the majority of the year. ie: We have maybe 10 days/year at 30F but most winter days are closer to 45F. So, would your design temp be 30, 45 or somewhere between the two?

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,248

    Question LOCATION ?

    Originally posted by martik
    ... have maybe 10 days/year at 30F but most winter days are closer to 45F.
    Design Temp may be about 35'F.
    Location ?

    A.S.H.R.A.E. data and statistical ( < 1 or 2% of time) procedure determines the Design Temperatures

    Other furnace sizing factors:
    LOCATION
    House SIZE
    Number of stories
    House Age
    Insulation values: ceiling, wall & floor
    Window sizes, orientation and U-value
    Infiltration
    Efficiency: output capacity versus actual heating delivered
    Aside: Current size furnace

    Perform ACCA Manual J calc or provide additional details so that reasonable "ball-park" guidance may be provided.

    USE "HVAC Calc" available on right side of tab above.

    OR see you local HVAC PRO if you will be buying
    a new furnace soon.
    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
    Nov 2002
    Location
    VA
    Posts
    296

    Have you considered a Heat Pump?

    martik

    With winter extremes so mild, I'm have to assume at your summer extremes are high. Are you in a humid climate (A.K.A. green grass)? Either way sizing for your cooling load may drive the equation for the air handling system. However without qualifying location no meaningful answers can be given.

    That said, it might be to your decided advantage to consider a Heat Pump instead of a fossil fuel heating source. They are at their best in the temperature range you're indicating. Given the current fuel prices electrical may have a decided long-term advantage since it isn't exclusively dependent upon somewhat questionable sources and depleting national reserves.

    Please Note: I’m not pro/con either way on equipment fuel type. In fact natural gas furnaces have a decidedly longer design life (tried, true and proven) than refrigeration equipment. It’s just that the realities of fossil fuel have changed over time and we all need to consider the finite nature of that resource.

    Sometimes speaking with local contractors who deal with your issues on a professional basis has much merit. Ask each of them to explain their recommendations in terms of initial investment and operating cost verses comfort provided...

    God Bless...

  4. #4
    Join Date
    Aug 2004
    Posts
    91
    Thanks for the replies.

    I am aware of the factors involved in a heat loss calc. I was just curious how you determined the design temps and it appears you don't base it on the LOWEST temp of the winter.

    My contractor advised that a heat pump does not last long enough to justify the savings.

    Gas has already increased 20% this year and probably will continue to rise so I have considered Zone heating with baseboard heaters. Electricity is only 6cents/KWH here.

  5. #5
    Join Date
    Oct 2004
    Location
    Metro ATL
    Posts
    454
    Ask for several opinions. One contractors opinion isn't gospel. Here in ATL heat pumps work great when designed and installed correctly in conjunction with a secondary fuel.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,765
    Design temp is from manual J.

    Give your area, and we can look it up for you.

    If you size for 45°, the 10 days that its 30° out, your house will be 55 inside.

    Your contractor may not like heat pumps.
    But if they were as bad as he says, no one would use them.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  7. #7
    Join Date
    Aug 2004
    Posts
    91
    Vancouver BC, Canada

    Another question: If a 100k btu furnace runs approx 1 min on, 3 mins off during an average winter day could one
    assume it is oversized by 300%?


  8. #8
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,248

    Thumbs up Load Calc

    Originally posted by martik
    Vancouver BC, Canada

    Another question: If a 100k btu furnace runs approx 1 min on, 3 mins off during an average winter day could one
    assume it is oversized by 300%?
    80,000 BTUh Output ?

    Night time operation ?

    It might be 20% to 50% oversized.
    50,000 to 70,000 BTUh might be in the ball park.

    Also, the T-stat may not be working properly.

    You need to look into an Efficient heat pump
    to save lots of $$$$
    based on those LOW electric rates.

    Buy 10-year warranty on Parts & Labor for a few hundred $.

    Sounds like your house is 1,600 + square feet.
    TRANE model 2TWX4042 might be about right ...
    __ Windows u-value and areas are needed.

    However, a SPECIFIC ACCA Manual J (or other)
    load calc is Required.

    [Edited by dan sw fl on 10-12-2005 at 05:28 AM]
    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
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,765
    97 1/2% design winter is 19° summer 77
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  10. #10
    Join Date
    Jul 2004
    Location
    Kent, WA
    Posts
    129
    I was surprised by that number, but you're right. Here in Seattle, our weather is very similar, but the design temp is 26° F, seven degrees higher. Must really get cold on those few times it gets cold.

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