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  1. #14
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    Quote Originally Posted by sharp View Post
    but now think the "optimum" is much higher than zero. FYI. 11 Seer HP, 68% Nat Gas furnace, 70 cent gas (right now) and 8.2 cent electricity. Any thoughts are appreciated.
    At your posted rates, and gas efficiency.

    Any time your heat pump's COP drops below 2.4, gas heat is cheaper.
    On an 11 SEER heat pump. that probably puts you right around 40 as a balance point.

    However. I suspect that your furnace steady state efficiency is higher then you posted.
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  2. #15
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    Quote Originally Posted by dan sw fl View Post

    Dual Fuel Set-Ups

    1

    Do you believe that a decline in Nat. Gas wholesale price will eventually create less demand for heat pumps?

    In my area, yes. Electric is taking a sharp price hike in Jan of 2010.
    In areas where electric is generated primary by NG, no.


    2

    Will customers recognize that a change in Economic balance point should be evaluated?

    Some will, some won't.

    3

    ARE customers actually seeing an associated decline in RESIDENTIALl Nat Gas rates?

    Area specific.

    Until a greater amount of the general population becomes more concerned/involved with controlling their energy cost. Little to no change.
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  3. #16
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    Quote Originally Posted by beenthere View Post
    At your posted rates, and gas efficiency.

    Any time your heat pump's COP drops below 2.4, gas heat is cheaper.
    On an 11 SEER heat pump. that probably puts you right around 40 as a balance point.

    However. I suspect that your furnace steady state efficiency is higher then you posted.
    Thanks, Beenthere. My Furnace is a ~1990 Lennox and it is very reliable, and since I have a 2005 Lennox HP and can't cost justify a new furnace given the HP takes care of most of my heating needs.

    Regarding your balance point calculation - Wow! I wouldn't have every guessed that .70 cent gas would be cheaper than ~.082 Cent electricity. I figured the HP efficiency would be maintained until at least 25 degrees. BTY (and I'm bragging), my 3 ton Lennox HP provides around 80% of my heating needs for a 2300 sq. ft. house for about $80 month/average during the winter. My nat. gas furnace, before I bought the heat pump, was averageing ~ $350/Month to heat my house, but that was when gas was $1.40 or so.

    Again, thanks for the help. I'm going to raise my balance point and take advantage of this cheap gas for a while.

  4. #17
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    Quote Originally Posted by sharp View Post
    Thanks, Beenthere. My Furnace is a ~1990 Lennox and it is very reliable, and since I have a 2005 Lennox HP and can't cost justify a new furnace given the HP takes care of most of my heating needs.

    Regarding your balance point calculation - Wow! I wouldn't have every guessed that .70 cent gas would be cheaper than ~.082 Cent electricity. I figured the HP efficiency would be maintained until at least 25 degrees. BTY (and I'm bragging), my 3 ton Lennox HP provides around 80% of my heating needs for a 2300 sq. ft. house for about $80 month/average during the winter. My nat. gas furnace, before I bought the heat pump, was averageing ~ $350/Month to heat my house, but that was when gas was $1.40 or so.

    Again, thanks for the help. I'm going to raise my balance point and take advantage of this cheap gas for a while.
    Don't make any changes until you see, and understand, the math.

    Compare the cost of 1 million btu's of heat.

    Insert your own furnace efficiency and/or heat pump C.O.P. for a more accurate cost comparison.

    80% efficient gas furnace, gas at $.70 per therm
    (1,000,000 / 100,000) x .7 / .8
    = $8.75

    Heat pump w/electricity at $.082 per kw-hr delivered price, C.O.P. = 3.25 at 35F ambient
    (1,000,000 / 3414) x .082 / 3.25
    = $7.39

    In the above example, the heat pump is 15% cheaper for the same 1 million btu's.

    As ambient temps increase from the 35F example, the C.O.P. increases and the heat pump is even cheaper to run.

    The C.O.P. in the example is taken from published data for my 3-ton Goodman heat pump.

    Get the C.O.P. for your heat pump at various ambient temps (as well as furnace efficiency - 68% sounds wrong) and calculate your own economic balance point temperature.

    Take care.

  5. #18
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    Quote Originally Posted by gary_g View Post
    Don't make any changes until you see, and understand, the math.

    Compare the cost of 1 million btu's of heat.

    Insert your own furnace efficiency and/or heat pump C.O.P. for a more accurate cost comparison.

    80% efficient gas furnace, gas at $.70 per therm
    (1,000,000 / 100,000) x .7 / .8
    = $8.75

    Heat pump w/electricity at $.082 per kw-hr delivered price, C.O.P. = 3.25 at 35F ambient
    (1,000,000 / 3414) x .082 / 3.25
    = $7.39

    In the above example, the heat pump is 15% cheaper for the same 1 million btu's.

    As ambient temps increase from the 35F example, the C.O.P. increases and the heat pump is even cheaper to run.

    The C.O.P. in the example is taken from published data for my 3-ton Goodman heat pump.

    Get the C.O.P. for your heat pump at various ambient temps (as well as furnace efficiency - 68% sounds wrong) and calculate your own economic balance point temperature.

    Take care.
    Thanks Gary. the math is simple - much simpler than I realized. I'll see if I can find a COP curve for my Lennox HP and perform the calcs. This is a big help.

  6. #19
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    Quote Originally Posted by sharp View Post
    Thanks Gary. the math is simple - much simpler than I realized. I'll see if I can find a COP curve for my Lennox HP and perform the calcs. This is a big help.
    OK - don't forget to check on your furnace efficiency rating. 68% sounds low.

    This is why I like to show the math so that people can see how the costs are calculated, instead of just stating what is cheaper or more expensive to run.

    Take care.

  7. #20
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    Quote Originally Posted by gary_g View Post
    Don't make any changes until you see, and understand, the math.

    Compare the cost of 1 million btu's of heat.

    Insert your own furnace efficiency and/or heat pump C.O.P. for a more accurate cost comparison.

    80% efficient gas furnace, gas at $.70 per therm
    (1,000,000 / 100,000) x .7 / .8
    = $8.75

    Heat pump w/electricity at $.082 per kw-hr delivered price, C.O.P. = 3.25 at 35F ambient
    (1,000,000 / 3414) x .082 / 3.25
    = $7.39

    In the above example, the heat pump is 15% cheaper for the same 1 million btu's.

    As ambient temps increase from the 35F example, the C.O.P. increases and the heat pump is even cheaper to run.


    Take care.
    11 SEER heat pumps usually don't make 3.5 COP unless outdoor temps are above 47. Ands some not even at 65 outdoor temp.

    So its probably a bad idea to use your Heat pumps COP as an example. It can tend to grossly mislead people.
    Specially, when they have an older heat pump.
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  8. #21
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    Homeowners really need to be aware of the efficiency of the HP unit they own or hope to buy. That's easier to do if the manufacturer makes the full specs publicly available, like Amana/Goodman does. As beenthere pointed out last winter, the calculations also need to factor in defrost cycles. Here's the link: http://hvac-talk.com/vbb/showthread....defrost+cycles

    With an Amana ASZ16 (HSPF 9.5) + an 80% furnace, my economic balance point is currently 25 degrees with total gas costs of 1.10 a therm and electricity at 12 cents a KWH. That's at low stage. At high stage, the cutoff is about 5 degrees, which is lower than I will ever see here in Northern California. So folks need to keep quite a bit in mind: the HP's HSPF rating, BTU output and COPs at each OD temp interval (actually two different sets of data if the HP is two-stage), the backup furnace's AFUE, plus defrost cycles, gas cost, and KWH cost. If you want to reduce overall energy consumption (no matter the cost), it's worth doing. If all you want is to reduce heating costs (no matter what), it's still worth doing in order to determine exactly how much cost efficiency one can squeeze out of your HP by setting the cutoff temp correctly. If all this seems just too much to deal with, and you don't mind paying a bit too much each winter for the next 20 years, then leave the factory cutoff settings as they are. It's your choice.

  9. #22
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    Question EER ?

    Quote Originally Posted by beenthere View Post
    11 SEER heat pumps usually don't make 3.5 COP unless outdoor temps are above 47. Ands some not even at 65 outdoor temp.

    So its probably a bad idea to use your Heat pumps COP as an example. It can tend to grossly mislead people.
    Specially, when they have an older heat pump.
    Wouldn't an 11 SEER HP be more like
    SEER 11
    EER 9.2 [ ? ]
    COP 2.696 at > 40'F ?
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  10. #23
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    2.6 to 2.8 at 40.
    Many fall off quickly, below 40. So his COP BP could be 35. or, 38. Depending on his coil match up, and CFM.

    But, when you throw in the cost of defrost. 40 may be the idea BP for the rates he posted.
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  11. #24
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    Quote Originally Posted by beenthere View Post
    11 SEER heat pumps usually don't make 3.5 COP unless outdoor temps are above 47. Ands some not even at 65 outdoor temp.

    So its probably a bad idea to use your Heat pumps COP as an example. It can tend to grossly mislead people.
    Specially, when they have an older heat pump.
    That's why I said:

    Quote Originally Posted by gary_g View Post
    Don't make any changes until you see, and understand, the math.

    Insert your own furnace efficiency and/or heat pump C.O.P. for a more accurate cost comparison.

    Get the C.O.P. for your heat pump at various ambient temps (as well as furnace efficiency - 68% sounds wrong) and calculate your own economic balance point temperature.
    The point is to show the OP how the cost calculation is made so he understands it.

    He learns nothing by just stating at which C.O.P. to switch over to gas.

    You should read my entire post before jumping on me.

  12. #25
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    Quote Originally Posted by gary_g View Post
    That's why I said:



    The point is to show the OP how the cost calculation is made so he understands it.

    He learns nothing by just stating at which C.O.P. to switch over to gas.

    You should read my entire post before jumping on me.
    Read your entire post.

    Using your 14 SEER heat pump's COP as an example. Still makes people jump to a bad conclusion.
    Specially, when their heat pump won't reach that COP on a 65 day.

    Next time. Just post the formula, using a COP of 2, They will still get the point, and understand the math.
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