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  1. #14
    Join Date
    Jul 2006
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    Southern Ca, Elkton Md
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    7,572
    Quote Originally Posted by jechow View Post
    My home originally had a furnace. I replaced the furance with a duel fuel HP system. The installer is saying that the reduced output temperature at the registers (as descirbed above) is due to my undersized ducts.

    Does this make sense? If this is true and we assume that the HP system is working properly, where did the heat energy from the A-coil go?
    The size of the duct is going to decrease the amount of volume.

    If the insulation value is not there you will lose temp.

    Without looking at the unit, I would say you still have a problem. You stated 66 deg supply air temp, Thats low depending your current conditions.
    "Correct Installation is the Key"

    .1 has killed more HX then Rush Limbaugh

    What is your TESP?

  2. #15
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,048
    The supply temp isn't low because of small ducts. Unless the HP is sutting off on HPS.
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  3. #16
    Join Date
    Feb 2007
    Location
    Bellingham, WA
    Posts
    115
    Quote Originally Posted by beenthere View Post
    The supply temp isn't low because of small ducts. Unless the HP is sutting off on HPS.
    Beenthere. What do you mean by "unless the HP is shutting off on HPS"? What is HPS?

  4. #17
    Join Date
    Jul 2006
    Location
    Southern Ca, Elkton Md
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    Quote Originally Posted by jechow View Post
    Beenthere. What do you mean by "unless the HP is shutting off on HPS"? What is HPS?
    Unless the duct work is so small it restricts the airflow and raises the head pressure and shuts off on a saftey switch.

    HPS, high pressure switch.
    "Correct Installation is the Key"

    .1 has killed more HX then Rush Limbaugh

    What is your TESP?

  5. #18
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,048
    Quote Originally Posted by weber View Post
    Unless the duct work is so small it restricts the airflow and raises the head pressure and shuts off on a saftey switch.

    HPS, high pressure switch.
    What weber said.
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  6. #19
    Join Date
    Feb 2007
    Location
    Bellingham, WA
    Posts
    115
    What I observed is that the output temperature is 66F with the HP working and

    1. The air temperature entering the outdoor coil is 40F;
    2. The air temperature entering the indoor coil is 70F; and
    3. The CFM is 1400

    Thus, it could not be the HPS as the HP remained on. If the saftey switch did turn off the HP, the furnace should come on and from what I have observed, the furnance has no trouble getting the output temperature about 80F.

    What else might be the problem?

  7. #20
    Join Date
    Jun 2007
    Location
    The Twilight Zone
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    2,964
    Quote Originally Posted by jechow View Post
    What I observed is that the output temperature is 66F with the HP working and

    1. The air temperature entering the outdoor coil is 40F;
    2. The air temperature entering the indoor coil is 70F; and
    3. The CFM is 1400

    Thus, it could not be the HPS as the HP remained on. If the saftey switch did turn off the HP, the furnace should come on and from what I have observed, the furnance has no trouble getting the output temperature about 80F.

    What else might be the problem?
    "Shophound" is correct with 93 degrees:

    BTU = CFM x 1.08 x Delta T

    Delta T = 34,400 / (1400 x 1.08) = 23 degrees.

    So 70F + 23F = 93F at the air handler exit.

    66F output temp is basically room temp. Your compressor isn't doing squat.

    Good luck.

  8. #21
    Join Date
    Jun 2007
    Location
    The Twilight Zone
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    Quote Originally Posted by jechow View Post
    ....and from what I have observed, the furnance has no trouble getting the output temperature about 80F.
    The output temp from the furnace should be much higher than 80F - more like 120F minimum.

    You may have a duct return that is sucking in cold air.

    How are you measuring your temperatures?

  9. #22
    Join Date
    Feb 2007
    Location
    Bellingham, WA
    Posts
    115
    Quote Originally Posted by gary_g View Post
    The output temp from the furnace should be much higher than 80F - more like 120F minimum.

    You may have a duct return that is sucking in cold air.

    How are you measuring your temperatures?

    Yes, I do have a 4" pipe that goes from the side of my house to the main duct return. It was there when I had my gas furnace and when I had my HP and new gas furnace installed the 4" pipe remained.

    I am measuring my temperatures using an indoor/outdoor thermometer with an outdoor sensor.

    When you provided the formula above, what is the "1.08" factor for?

    Thanks

  10. #23
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,048
    The HP is NOT working right.
    Call your installing back.

    It sounds like you have a 2 stage running in first stage with the fan running at second stage speed.
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  11. #24
    Join Date
    Jun 2007
    Location
    The Twilight Zone
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    Quote Originally Posted by jechow View Post
    Yes, I do have a 4" pipe that goes from the side of my house to the main duct return. It was there when I had my gas furnace and when I had my HP and new gas furnace installed the 4" pipe remained.

    I am measuring my temperatures using an indoor/outdoor thermometer with an outdoor sensor.

    When you provided the formula above, what is the "1.08" factor for?

    Thanks
    The "1.08" is a factor that gets all the units straight in the calc.

    I don't think you are getting accurate temps with your indoor/outdoor thermometer. The response time is too slow and its not meant for the application that you are using it for.

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