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Thread: Correct sizing

  1. #1
    Join Date
    Aug 2004
    Posts
    91
    I currently have a 120k btu 30 year old upflow gas furnace. The contractor did a heat load calc and came up with 30k for a 2400 sqft house on the northwest coast - design temp 20F.
    The house has been upgraded, R40 ceiling, double low-e argon windows and air sealed to 4.5 ACH. This seems too low to me but then my old furnace never ran more that 10-15mins/hr.
    I am also concerned that the blower on the new furnace may not be powerful enough. It's 3 speeds are rated between 800-1350CFM at .1 ESP

  2. #2
    Join Date
    Mar 2004
    Location
    Grottoes VA
    Posts
    5,856
    What is your ?

    30,000btu is 1,000 cfm and the blower pushes 1350cfm. That is okay. (On a A/C or H/P)

    Are you putting in a heat pump? Are you putting in A/C with new gas furnace? Will it be 2 stage? What is the heat gain? What is the heat loss? Does it get colder than 20deg? Are you zoning the system? What is the name of your dog?
    Karst means cave. So, I search for caves.

  3. #3
    Join Date
    Aug 2004
    Posts
    91
    No ac, no heatpump, no zones, no dog just 14 heat registers.

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,473
    Without a dog, I'd be worried about that sizing. Dogs put out heat to supplement the furnace's output.

    Actually, the 120K 30 year old probably has a blower similar to today's 50K furnaces! Nowdays, you won't find a 120K without a 4-5 ton blower but back then it wasn't unusual to find a big furnace that would move around 1200 CFM.

  5. #5
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    my 1974 elec furn moves 2100+ cfm! [ per label & as measured]

  6. #6
    Join Date
    Aug 2004
    Posts
    91
    I'd really appreciate a response about the sizing change from 120k to 30k

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,473
    If the heat loss says 30,000 BTU will do it, then get a furnace close to that size. You'll likely end up 20-30% larger as there aren't many furnaces out there that small anyway. You tightened up the house, insulated the he#$ out of it, great windows. You won't need much heat now.


  8. #8
    Join Date
    Aug 2004
    Posts
    91
    Thanks for your help. It's hard to believe that those few improvements would result in 1/4 the heating capacity!

  9. #9
    Join Date
    Jul 2004
    Posts
    322
    Did you really mean 4.5 ACH? With 8 foot ceilings that 86,400 cubic feet of leakage per hour? You could do wing design testing.

  10. #10
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    Originally posted by martik
    Thanks for your help. It's hard to believe that those few improvements would result in 1/4 the heating capacity!
    That furnace was probably 2X too big to begin with...
    How tall are you Private???!!!!

  11. #11
    Join Date
    Oct 2001
    Location
    Waterford Michigan
    Posts
    2,668
    They don't make a 30K but I wouldn't worry even if the unit were 100% oversized. Being heating only it won't make much difference in energy usage. Just make sure the blower can supply the necessary CFM of air to each and every room.

  12. #12
    Join Date
    Aug 2004
    Posts
    91
    Originally posted by fat bob
    Did you really mean 4.5 ACH? With 8 foot ceilings that 86,400 cubic feet of leakage per hour? You could do wing design testing.
    The energy test report says it was originally 10.85 ACH @50Pa which was equal to approx a 468 sq inch hole.
    After the upgrades it went down to 4.5

  13. #13
    Join Date
    Aug 2004
    Posts
    91
    Originally posted by johnl45
    They don't make a 30K but I wouldn't worry even if the unit were 100% oversized. Being heating only it won't make much difference in energy usage. Just make sure the blower can supply the necessary CFM of air to each and every room.
    The CFM on a 40k is between 800-1300 at .1 esp. As I said, I have 14 registers with a maximum run of 20ft and ducts of 8x14. The 4 in the basement are all closed off.
    I would need to go up to an 80k to get any increase in CFM
    which is between 1100 and 1800.

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