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  1. #1
    Join Date
    Nov 2012
    Posts
    6

    system size and ductwork

    Replacing oil furnace with heat pump, A contractor did manual j and it comes out to 2.5 ton, but the duct work he said supports only 2 ton, he proposed a lennox xp17 2 ton, with variable speed air handler(cbx32mv0 and 10kw(i think) strips. I plan to keep the temp at steady 67-71 in the winter. He says it will work fine, I am wondering if this sounds right? We have no room to add returns or supply easily. House is in Tacoma (near Seattle), WA.

  2. #2
    Join Date
    Feb 2008
    Location
    Altmar, New York, United States
    Posts
    4,825
    i would get a couple mor estimates with manual j's.

  3. #3
    Join Date
    Nov 2012
    Posts
    6
    I do have another, but he just looked at duct work and didn't say anything about it. This guy actually measured it.

  4. #4
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,326
    Quote Originally Posted by betmo View Post
    Replacing oil furnace with heat pump, A contractor did manual j and it comes out to 2.5 ton, but the duct work he said supports only 2 ton, he proposed a lennox xp17 2 ton, with variable speed air handler(cbx32mv0 and 10kw(i think) strips. I plan to keep the temp at steady 67-71 in the winter. He says it will work fine, I am wondering if this sounds right? We have no room to add returns or supply easily. House is in Tacoma (near Seattle), WA.
    We don't judge with sound but by NUMBERS?!!!

    Need R-value walls, ceiling

    U-value of windows, & sizes

    Infiltration ... average or loose

    1,600 sq ft ?
    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

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,326
    it sounds right to me, but that's just me listening to myself.

    What were your fuel costs for the
    last few years?
    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

  6. #6
    Join Date
    Nov 2012
    Posts
    6
    house is 1400sq ft. , my oil cost the last year was about $1600. My main question is if I get the 2 ton because the ducts can't support 2.5ton, what would happen? And if i got a 2.5ton and ducts can't handle it what then? I plan on doing more insulating in the next year(walls), attic well insulated now, have low e windows already. Being in the NW, cooling is not the concern, mainly heating.

  7. #7
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,385
    If you don't have enough ductwork you will pay to run a 2.5ton unit and only get 2 tons of cooling/heating out of it.

  8. #8
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,371
    Good chance the 2 ton is fine for 1400 sqft. A Manual J is only as accurate as the input data. A 2.5 ton would cost more to operate on undersized ductwork. While the XP17 is a bit short on heating btu's for a 2 ton, with a 10 kW staged heater package, you will have plenty of heat and one or both of the elements can come on if and when needed to supplement the heat pump.. What was the total heat loss as per Manual J?

  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,326
    Quote Originally Posted by betmo View Post
    house is 1400sq ft. , my oil cost the last year was about $1600. My main question is if I get the 2 ton because the ducts can't support 2.5ton, what would happen? And if i got a 2.5ton and ducts can't handle it what then? I plan on doing more insulating in the next year(walls), attic well insulated now, have low e windows already. Being in the NW, cooling is not the concern, mainly heating.
    What would happen?

    Aux Heat will run longer when it's < ~35'F.
    That might Not be much of an economic penalty in NW where eleCtric rates might be < $0.05 / kW-hr
    Going from $1,600 OIL to < $400 for heating is a HUGE heat pump benefit.

    Heat loss might be 20,000 BTU/hr at 20'F outside and 16,000 BTU/hr at 30'F.
    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
    Nov 2004
    Location
    SW FL
    Posts
    6,326
    Quote Originally Posted by betmo View Post
    house is 1400sq ft. , my oil cost the last year was about $1600. My main question is if I get the 2 ton because the ducts can't support 2.5ton, what would happen? And if i got a 2.5ton and ducts can't handle it what then? I plan on doing more insulating in the next year(walls), attic well insulated now, have low e windows already. Being in the NW, cooling is not the concern, mainly heating.
    What would happen?

    Aux Heat will run longer when it's < ~35'F.
    That might Not be much of an economic penalty in NW where electric rates are frequently < $0.05 / kW-hr
    Going from $1,600 OIL to < $400 for heating is a HUGE heat pump benefit.

    Heat loss might be 20,000 BTU/hr at 20'F outside and 16,000 BTU/hr at 30'F.
    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

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,366
    Do you currently have central A/C, if so what size is it.
    Contractor locator map

    How-to-apply-for-Professional

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

  12. #12
    Join Date
    Nov 2012
    Posts
    6
    No a/c, just oil furnace.
    My ducts start out with a 30x8 trunk, 1 6x8 supply off that then a 7x10 supply to a wall with register on both sides, trunk down to 25x7 with a 5x7 supply off that, trunk down to 20x7 and a 5x7 off that, down to 15x7 with a 5x7 supply off that, the trunk is then 10x7 to the final register about 15ft from furnace serving 2 rooms(supply in wall with register on both sides.. all registers are 12x6 4 returns about 14x7 each.

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,366
    2.5 tons might make some noise with the amount of supplies you have.
    Contractor locator map

    How-to-apply-for-Professional

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

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