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  1. #1
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    Feb 2007
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    Bellingham, WA
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    Different duct designs

    How would the duct design differ between the following systems:

    1. A gas furnace only system
    2. A HP with a gas furnace

    Would there be any difference?

    Thanks.

  2. #2
    Join Date
    Feb 2003
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    Huntsville,AL
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    4,125
    1 may be smaller if not many nor big windows --
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  3. #3
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    Feb 2007
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    Bellingham, WA
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    115
    Do you have to be more careful about restrictive ducts (back pressure) for a HP system than a furnance only system?

  4. #4
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Quote Originally Posted by jechow View Post
    Do you have to be more careful about restrictive ducts (back pressure) for a HP system than a furnance only system?
    Heat pump versus gas furnace ,or gas furnace versus heat pump with a furnace as in your first post?

  5. #5
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    Feb 2007
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    Bellingham, WA
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    Quote Originally Posted by dash View Post
    Heat pump versus gas furnace ,or gas furnace versus heat pump with a furnace as in your first post?
    Sorry. Gas furnace vs. HP with gas furnace.

  6. #6
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    I don't see much difference there,as the heat pump and cooling coil are ,one and the same,assuming the gas furnace would have had cooling,and the heat pump coil will be dry,not wet so less static/resistance.

    No a heat pump versus gas furnace,or a gas furnace without cooling versus one with,there are differences,all to do will External Static Pressure.

    All this is covered in Man. D,from www.acca.org

  7. #7
    Join Date
    Jun 2007
    Location
    Metro St. Louis
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    181
    Quote Originally Posted by jechow View Post
    How would the duct design differ between the following systems:

    1. A gas furnace only system
    2. A HP with a gas furnace

    Would there be any difference?

    Thanks.
    If you're talking heat only - the amount of air required for a furnace is smaller because the temperature differential is higher. A heat pump will give you a 20-30 degree rise in output air temp. A gas furnace could give you a 50 degree rise.

    But, I'm guessing you're talking about Gas w/AC vs Gas w/HP. In both configurations, the baseline should be 400 CFM/ton. However, A/C is often slowed down to increase moisture removal. However, in heat mode, you lose efficiency as you lower the airflow (some pros here have said that the ideal CFM rate for heating with a HP is 450/ton).

  8. #8
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    450 per ton may be better for operating cost ,but not for comfort.

    Typically the cooling and heat pump fan speed choice will be the same speed.At the same speed the heat pump mode will move more air than in cooling.

  9. #9
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    Feb 2007
    Location
    Bellingham, WA
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    115
    Quote Originally Posted by Waterloo View Post
    If you're talking heat only - the amount of air required for a furnace is smaller because the temperature differential is higher. A heat pump will give you a 20-30 degree rise in output air temp. A gas furnace could give you a 50 degree rise.
    Are you saying for heating a home, a furnace can have more restrictive ducting than a HP. For example, the thermostat is set to 71F then the furnace has an easier time maintaining that temperature than a HP? (assume properly sized furnace and HP and ducting that may be over or undersized).

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    No.
    Undersized duct, isn't good for gas furnaces either. Casues premature failure of teh heat exchanger.
    Contractor locator map

    How-to-apply-for-Professional

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

  11. #11
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    From a comfort standpoint, undersized ducts would not be as offensive with a gas furnace.

  12. #12
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    Feb 2007
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    Bellingham, WA
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    Quote Originally Posted by bobb25 View Post
    From a comfort standpoint, undersized ducts would not be as offensive with a gas furnace.
    What happens when the ducts are undersized? If we assume that:

    1. The delta T for the furnace is 50F (across the heat exchange)
    2. The delta T for the HP is 25F (across the A-Coil)
    3. The thermostat is set to 71F and
    4. The indoor room temperature is 69F

    Wouldn't the HP just take longer to reach 71F in comparison to the furnace? If the ducting is undersized, it would just take even longer?

    For a 3-ton HP and a 80,000 BTUH 80% VS furnance, what should be the minimun size of the main plenum?

  13. #13
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    For the heat pump,it all depends on the outdoor temperature.

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