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  1. #1
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    Jun 2008
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    heat pump vs forced air gas

    I had a Fujitsu minisplit heat pump installed. I also have forced-air gas heat.
    Below some temperature, from what I understand, it'll cost more to have the heat pump helping to heat the house, than to have the forced air gas furnace do all of the heating.

    Does anybody know what that temp is?

    Thanks
    Laura

  2. #2
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    Quote Originally Posted by plarian View Post
    I had a Fujitsu minisplit heat pump installed. I also have forced-air gas heat.
    Below some temperature, from what I understand, it'll cost more to have the heat pump helping to heat the house, than to have the forced air gas furnace do all of the heating.

    Does anybody know what that temp is?

    Thanks
    Laura
    Depends on your rates for natural gas and electricity, as well as gas furnace efficiency and heat pump C.O.P. at various ambient temps.

    What are your rates for gas ($ per therm) and electricity (cents per kw-hr)?

    What is your furnace efficiency (80%?)?

    What is the model number of the heat pump?

  3. #3
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    Sep 2005
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    Depends on a lot of details, too many to make an accurate online answer. Generally HP's tend to loose efficiency to an uncomfortable level around freezing outside. Some work fine at lower outdoor temps.

    I would watch it (feel the amount of heat it produces) at different temps for a while... then you will have a good idea of what your installation's limitations are.

    Might also keep a scratch pad close and write down a few notes...
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  4. #4
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    Sep 2006
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    Quote Originally Posted by plarian View Post
    I had a Fujitsu minisplit heat pump installed. I also have forced-air gas heat.
    Below some temperature, from what I understand, it'll cost more to have the heat pump helping to heat the house, than to have the forced air gas furnace do all of the heating.

    Does anybody know what that temp is?

    Thanks
    Laura
    Laura,

    It does not get cold enough in D.C. to make your heat pump less efficient than your gas furnace. You have a very efficient model so let it run. Now, it will loose heating capacity as the outdoor temps drops and thusno t produce as much heat, but it will not be drawing as much current to run either (the COP will decline but it would go below 1.00).

    May want to set your furnace temp just a notch below the heat pump and when the heat pump can't maintain the temp in the house the gas furnace will then kick in and make up the difference. Spec's on your heat pump are about 11,000 BTUs at 17*F.

  5. #5
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    Sep 2002
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    For the record, efficiency and capacity are not the same.

    Heatpumps are always more efficient than furnaces but capacity drops off rapidly as it gets colder; they only become uneconomical to operate when electric aux heat is required.

    Modern 13+ SEER units have COPs ranging between 2-3 at 15-32F, and COPs of 3-4 above freezing.

    Right now natural gas is obscenely cheap relative to electricity, so the break even point could be quite high.

    The terms: "efficiency, operating cost, and capacity" should never be used interchangeably.

    I would run the mini-split and furnace simultaneously - Set mini-split two degrees higher than the furnace; you'll only burn gas when the heatpump needs a little help.

    In extremely cold weather (Below 10-15F), shut the heatpump down and burn gas.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  6. #6
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    Mar 2008
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    Baltimore MD and Ridgebury PA
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    Quote Originally Posted by amd View Post
    For the record, efficiency and capacity are not the same.

    Heatpumps are always more efficient than furnaces but capacity drops off rapidly as it gets colder; they only become uneconomical to operate when electric aux heat is required.

    Modern 13+ SEER units have COPs ranging between 2-3 at 15-32F, and COPs of 3-4 above freezing.

    Right now natural gas is obscenely cheap relative to electricity, so the break even point could be quite high.

    The terms: "efficiency, operating cost, and capacity" should never be used interchangeably.

    I would run the mini-split and furnace simultaneously - Set mini-split two degrees higher than the furnace; you'll only burn gas when the heatpump needs a little help.

    In extremely cold weather (Below 10-15F), shut the heatpump down and burn gas.
    I can't argue with most of what you said as you are quite correct. Except the statement that I colored blue. If your capacity drops then by definition your efficiency will drop. It takes the same amount of energy to operate the compressor at 17F as it does at 47F yet the capacity is less. Therefore the efficiency drops as well. Your following paragraph mentions this very fact when it references the COP.

    I would have to do the math to be sure but given the cost of NG vs electricity in the DC market it is likely that you're better off with the NG at a temperature closer to freezing. Perhaps 25-30F. It would certainly help to know if you have a 80% efficient furnace or 90%+ efficient furnace.

  7. #7
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    I can't argue with most of what you said as you are quite correct. Except the statement that I colored blue. If your capacity drops then by definition your efficiency will drop. It takes the same amount of energy to operate the compressor at 17F as it does at 47F yet the capacity is less. Therefore the efficiency drops as well. Your following paragraph mentions this very fact when it references the COP.
    Every heatpump performance spec sheet I've read shows that amp draw does drop off to an extent (not nearly as much as capacity does) as it gets colder outside. Yes, it's true that efficiency drops as the outdoor temp declines; however, even a COP of 2-2.5 is pretty damn good.

    With respect to the economic balance point, it really depends on the cost of electricity and gas.

    One year ago gas was much more expensive than it is today; for areas which rely mostly on coal and nuclear plants, I predict that electricity rates will rise much slower than those of gas.

    Why? Coal and uranium are more abundant than gas - most of the new gas wells (especially shale) have extremely high decline rates. (More wells must be drilled each year than the last to maintain supply)

    The US is the saudi arabia of coal.

    I hate reading statements (on other boards) along the lines of...

    "Below freezing there isn't enough heat in the air and the cop drops to 1"

    "Heatpumps are inefficient" (Usually this statement is made in reference to capacity by those who compare HPs sized for cooling to grossly oversized furnaces)

    "They're too complex" (You have a defrost board, a couple of check valves, outdoor metering device, and refersing valve. How complex can it be?!)

    "They stop working at 47F"
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  8. #8
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    Quote Originally Posted by platchford View Post
    It takes the same amount of energy to operate the compressor at 17F as it does at 47F.....
    From the Tech Operating Data for my 3 ton 14 SEER Goodman heat pump:

    Ambient--------System
    Temp----------Amps

    47F------------10.4
    17F------------7.9


    Take care

  9. #9
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    Quote Originally Posted by gary_g View Post
    From the Tech Operating Data for my 3 ton 14 SEER Goodman heat pump:

    Ambient--------System
    Temp----------Amps

    47F------------10.4
    17F------------7.9


    Take care
    Let's take it a step farther. Here are the spec's for an inverter (Laura has an inverter system too) VRV system at 3 tons nominal:

    Ambient--------------System-----------Outdoor------------COP
    Temp----------------Output------------Amps

    47F------------------42,400------------13.4---------------3.87

    17F------------------31,500------------11.3---------------3.40

    5.5F-----------------27,400------------10.0---------------3.33

    As can be seen there is a drop in output, power usage, and COP as the ambient temp drops, but there is still a lot of efficiency with the H/P. Only comparing the cost to operate both systems will tell you for sure, but my bet is on the HP to be the less expensive to operate at temps well below freezing.

  10. #10
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    Jun 2008
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    Quote Originally Posted by mchild View Post
    Laura,

    It does not get cold enough in D.C. to make your heat pump less efficient than your gas furnace.
    I live in New York state. It gets down to 0F often and sometimes colder in the winter.
    The electricity is 13.5 cents/kwh here, and natural gas is $1.66/therm.
    I have a Fujitsu 15RLQ minisplit.
    May want to set your furnace temp just a notch below the heat pump and when the heat pump can't maintain the temp in the house the gas furnace will then kick in and make up the difference.
    That's exactly what I'm doing. The heat pump can't heat the house by itself, it's just helping a bit for comfort.

    Laura

  11. #11
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    Quote Originally Posted by plarian View Post
    I live in New York state. It gets down to 0F often and sometimes colder in the winter.
    The electricity is 13.5 cents/kwh here, and natural gas is $1.66/therm.
    I have a Fujitsu 15RLQ minisplit.


    That's exactly what I'm doing. The heat pump can't heat the house by itself, it's just helping a bit for comfort.

    Laura
    Oh, then you need to move the D.C. area as it is not so cold. Not sure where I got the idea you lived in D.C.

    What is the efficiency of your furnace, 80%?

  12. #12
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    Quote Originally Posted by mchild View Post
    Oh, then you need to move the D.C. area as it is not so cold. Not sure where I got the idea you lived in D.C.

    What is the efficiency of your furnace, 80%?
    Doesn't matter. At her posted rates. Her heat pump is more efficient at a 3.3COP at 5.5 then a 95% efficient furnace is.
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  13. #13
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    Quote Originally Posted by beenthere View Post
    Doesn't matter. At her posted rates. Her heat pump is more efficient at a 3.3COP at 5.5 then a 95% efficient furnace is.
    bt,

    Those spec's weren't for her unit, bur rather, another inverter system with the intent of showing the relationship of falling temps, output BTU, and COP. I don't have the spec's on her unit and have no idea what COP she has at any temp.

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