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  1. #1
    Join Date
    Oct 2005
    Posts
    96
    I live in Kansas City and are winters are relatively mild. We do get some temperature in Jan/Feb that get near the zero mark but usually only last for a couple of days at a time. I have decided on a heat pump. Is adding a high efficiency furnace (90%+) overkill or are there benefits to doing both.

  2. #2
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    Have you looked into geothermal,, high initial cost but very efficient and economical in the long run. Planning on staying there a while?

  3. #3
    Join Date
    Oct 2005
    Posts
    68
    I agree. Geothermal is the best for efficiency but it is very expensive. If you dont want to go with Geothermal then a heat pump with a gas furnace is not overkill. The gas furnace will operate the same way a normal air handler with electric heat would.

  4. #4
    Join Date
    Oct 2005
    Posts
    96
    Should I save the money and go with a 80% versus in lieu of the high efficiency furnace that cost extra $$

  5. #5
    Join Date
    Oct 2005
    Posts
    68
    Absolutely not. You just dropped your efficiency by 10%. Go with high efficint equipment it will pay for its self in the long run. Do whatever is best for your budget.

  6. #6
    Join Date
    Oct 2005
    Location
    Southern Michigan
    Posts
    24
    Originally posted by wayner211
    Absolutely not. You just dropped your efficiency by 10%. Go with high efficint equipment it will pay for its self in the long run. Do whatever is best for your budget.
    I am in Michigan, and am considering the same type of system (gas/heat pump). BUT, how do you KNOW that a 90% will pay off over an 80% when the heat pump is supplying a large amount of the annual BTU requirement used to justify a 90% over an 80%?

    Is there a way to quantify how much of the annual heating requirement will be handled by the heat pump and how much will be switched over to the gas backup furnace? Perhaps number of days below a certain temperature?

    I also noted another "curve ball" on the Trane web site, where they state that their electrically-assisted heat pump will keep on supplying BTUs after the electrics switch on, but a gas-assisted heat pump switches off when the gas fires up. I suppose there is a good reason for this, but it also seems that some of the efficiency of the heat pump is lost and the decision point to go gas vs electric may be affected.

    KJ, confused in Michigan...

  7. #7
    Join Date
    Oct 2005
    Posts
    96

    Wink

    Thanks KJ. This was really my initial question. You just worded it better!

  8. #8
    Join Date
    May 2004
    Location
    Rapid City, SD
    Posts
    7,415
    You can't (or shouldn't) run the furnace and HP at the same time. That much temp going across the coil will make the pressures skyrocket and the poor compressor will have it's tongue hanging out.

    The electrical backup though is typically after the coil, so that heat just goes straight into the duct system, not across the coil.

    Heat pumps will produce heat into some very cold temps (0 deg and well below). But when you run a gas furnace you need to shut the HP off around or slightly above the balance point of your house (where it looses more heat than the HP can produce).


  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,323
    Originally posted by ksspartan
    I live in Kansas City and are winters are relatively mild. We do get some temperature in Jan/Feb that get near the zero mark but usually only last for a couple of days at a time. I have decided on a heat pump. Is adding a high efficiency furnace (90%+) overkill or are there benefits to doing both.
    With K.C. Winter electric rates at ~$0.04/kW
    adding another system would NOT provide a benefit.

    Specific Energy Cost Analysis would likely determine an economic balance point which concludes
    ONLY use a Heat Pump

    Approximate Energy Co$t ...

    100,000 BTUh / Therm
    _ 3,413 _ BTU /kW
    29.30 kW
    $0.04_ per kw
    $1.172 per Therm at < ~ 20'F or when Aux Heat is On.

    2.0 ~ C.O.P. for heat pump at 20'F
    $0.586 Effective rate per therm using Heat Pump
    when Aux Heat is NOT on.

    Actual $ rate at < heat pump capacity
    balance point ( i.e. 33'F ) is
    in the $0.59 - $1.17 range.

    Heat cost doesn't get any better than that.

    Gas rate is $_ .___ per therm
    (~2.5 * Electric for > 30'F )

    Heat pump model and gas rates would make above
    calc more specific to your area.
    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
    6,323
    Originally posted by kjones
    Originally posted by wayner211
    Absolutely not. You just dropped your efficiency by 10%. Go with high efficint equipment it will pay for its self in the long run. Do whatever is best for your budget.
    I am in Michigan, and am considering the same type of system (gas/heat pump). BUT, how do you KNOW that a 90% will pay off over an 80% when the heat pump is supplying a large amount of the annual BTU requirement used to justify a 90% over an 80%?

    Is there a way to quantify how much of the annual heating requirement will be handled by the heat pump and how much will be switched over to the gas backup furnace? Perhaps number of days below a certain temperature?
    Use of local Temperaature Bin Data and a complete energy cost analysis may justify use of a heat pump/gas combo in Michigan where electric cost is in the $0.10/kW range.

    Needed:
    1. Heat pump model

    2. Balance point from ACCA Manual J load calc

    3. Temperature Bin Data

    4. Gas Cost per therm
    a. November, 2005 $_.__
    b. Janaury, 2006 $_.__

    5. Electric cost ( $0.11 ) $0.__

    6. Use 80% Efficient Furnace
    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
    Oct 2005
    Posts
    96
    Dan,

    I plan to use a gas dual heat system. My question is do I spend the extra $ to buy a 90% furnace or just stay with a 80%. I understand that the 10% additional efficiency is great but is it worth the extra money. Can you walk me through that calculation. Like you I am also a HVAC designer and have to prove things with numbers

    Thanks for you time.

  12. #12
    Join Date
    Oct 2005
    Posts
    45
    KSpaprtan

    You and I are both in the same boat -- and in the same town. i will tell you - that when I was making the decesion -- I looked at the number of days where the average temperature was below 30 degrees in KC.

    The toal number of days that gets below 32 degrees on avg. is around 100. Bu thte numbe rof nights dropping below 20 is only aboy 50 or so -- thats about 15 of the nights. If my furnace is only going to run an avg of 15% of the time -- there is no way a higher efficiency furnace would ever pay for itself. YOu would be better off -- putting the money you would put into higher efficiency furnace and stepping up to the next level on the heat pump.

    I love my Bryant system I just had installed in July. My A/c bill dropped immensely and the system is ultra quiet.

    [Edited by hamons on 10-15-2005 at 08:14 AM]

  13. #13
    Join Date
    Mar 2004
    Posts
    1,042
    For Kansas City, 80% is plenty. It's not much colder there than in Atlanta, and my dual fuel system with an 80% furnace uses very little gas at all. At those electric rates you still really should consider a heat pump with electric backup (no gas).

    Based on the weather in, say, Detroit, the heat pump isn't going to see much action in December, January, and Feburary. Average highs for those three months are 40 or below. With that kind of weather, a 90% furnace is going to be worthwhile for a dual fuel system- especially in a larger house with relatively more demand for heat. With gas prices rising and little prospect for them to ever decline significantly again, if I were in Michigan and planning to stay somewhere for 10+ years, I would get dual fuel and the 90% and actually not even try to do a hard cost analysis. You also get the benefit of using outdoor air for combustion with most 90% furnaces (two pipes instead of one), which will keep the house more comfortable when the furnace is running by not sucking in cold, dry air through every crack in the house to support the combustion.

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