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  1. #1

    BTUs rule of thumb

    Is there a rule of thumb for how many heating BTUs needed? My 17 year old Goodman gas pack which recently crapped out had 100,000 BTUs. The contractor told me that was too much for my 1640 sq ft home and is recommending a 60,000 BTU replacement. How could he determine that without coming inside. All he did was walk the perimeter of the house

  2. #2
    Join Date
    Jun 2004
    Location
    Howell, Michigan
    Posts
    16,158
    It's called a Manual J Load Calculation. It is impossible to do one on a walk around.........LOL

    I do agree that 100K is large for your home, unless it has no insulation, tons of windows and you are in Minnesota.

  3. #3
    Join Date
    Dec 2007
    Posts
    132
    The General rule in the Northeast part of the country is 35 btu's of heat per sqft ,some guys go 40 but that is on poorly insulated home, cooling load is 18 btu per sqft , so thats how he is doing it and it sounds like he is right your furnace was way to big for your house. SHHHH don't tell anybody

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,753
    He is going by pass experience.
    Many homes have grossly oversized furnaces in them.
    A Manual J would tell you better.
    If you want to know what size. Click the hvac-calc link, and do your own.

    In my area, very few houses need 30 btu's per sq ft(winter design 8*F). Which is about what size he is recomending.
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  5. #5
    Join Date
    Dec 2007
    Posts
    89

    Talking

    btu/h and the amount of air needed is based on size of the of ur house.

  6. #6
    Join Date
    Jul 2004
    Location
    Massachusetts
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    6,826
    Quote Originally Posted by Mrfixit39 View Post
    btu/h and the amount of air needed is based on size of the of ur house.
    That's a totally incorrect statement. If you open all the window in the home, open three access holes to the attic and leave the 3 fireplace dampers open, I'll bet the size of the house doesn't matter a hoot to the amount of heat needed.

    A Manual 'J' calculation as beenthere has stated is the ONLY way to determine what's needed to heat your home. When properly done, it takes into consideration all of the issues I listed in my example. Once the room-by-room Manual "J" is done, then it's easy to tell you how many Btu's you need in the home as well as how many CFM/Btu's you need in each room. Once you know that information, then the distribution system can be inspected to be sure it's in good condition, properly sealed and properly sized. The actual size of the home (square footage) has very little to do with the load calc. As a matter of fact, if a blower door test hasn't been done on the home to determine the leak rate, then even the Manual 'J' has an element of guess work in it but it's a long, long way better than putting back what's there or guessing you need XX Btu's per square foot.
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  7. #7
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
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    2,919
    Quote Originally Posted by skippedover View Post
    That's a totally incorrect statement. If you open all the window in the home, open three access holes to the attic and leave the 3 fireplace dampers open, I'll bet the size of the house doesn't matter a hoot to the amount of heat needed.

    A Manual 'J' calculation as beenthere has stated is the ONLY way to determine what's needed to heat your home. When properly done, it takes into consideration all of the issues I listed in my example. Once the room-by-room Manual "J" is done, then it's easy to tell you how many Btu's you need in the home as well as how many CFM/Btu's you need in each room. Once you know that information, then the distribution system can be inspected to be sure it's in good condition, properly sealed and properly sized. The actual size of the home (square footage) has very little to do with the load calc. As a matter of fact, if a blower door test hasn't been done on the home to determine the leak rate, then even the Manual 'J' has an element of guess work in it but it's a long, long way better than putting back what's there or guessing you need XX Btu's per square foot.
    AMEN!!!
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  8. #8
    Join Date
    Dec 2007
    Posts
    132
    I've had Manual j calcaulations get some call backs also the programs seem to undersize some heat loads and losses maybe its because of the climate I'm in but I,m not sold on it fully. You may think it is the bible but experince still play a big roll in any installtion, It works good for baseboard heat but warm air and cooling im not sold on it, I think most people like it because they have a sheet of paper to show the homeowner ,but bottom line is if the guy who's writing your checks ass is to cold or to hot he does not care about your manual j ,

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Quote Originally Posted by Dog View Post
    I've had Manual j calcaulations get some call backs also the programs seem to undersize some heat loads and losses maybe its because of the climate I'm in but I,m not sold on it fully. You may think it is the bible but experince still play a big roll in any installtion, It works good for baseboard heat but warm air and cooling im not sold on it, I think most people like it because they have a sheet of paper to show the homeowner ,but bottom line is if the guy who's writing your checks ass is to cold or to hot he does not care about your manual j ,
    The most common reason some think Man-J undersizes is that they undersize the duct system and lose btus.from your other post this may be the case.

    Don't get offended,search this site for more info., get Pro status to get in the Pro areas,where you can learn more about duct sizing.

    The second reason some think Man-J undersizes is they aren't familar with Man-S,which tells us for every degree of indoor design below 80 F ,we lose btus.

    Your own experience with non-ducted systems coming out correct with Man-J concurs with what I'm stating.

    Stick around ,lots to learn here.

  10. #10
    Join Date
    May 2007
    Location
    somewhere between heaven and hell
    Posts
    955
    That is exact rule of thumb I was taught the one dog mentioned if you don't do manuel J. But if I am correct when you do rule of thumb for heating it gives you the output not input. Correct me if I am wrong.

  11. #11
    Join Date
    Dec 2007
    Location
    Tucson Az
    Posts
    76
    Where gad packs considered 40% at an time? now they are all 80% correct?
    correctamundo ?

  12. #12
    Thank-You everyone for your responses. This thread and my previous thread has helped me make a sound decision on a replacement unit.

  13. #13
    Join Date
    Mar 2006
    Location
    Denver/Boulder
    Posts
    2,213
    Quote Originally Posted by rhunter15 View Post
    Thank-You everyone for your responses. This thread and my previous thread has helped me make a sound decision on a replacement unit.
    We are listening...
    What lies behind us and what lies before us are small matters compared to what lies within us.


    Two pressures, four temperatures = SUCCESS!


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