Cost efficiency of installed dual-system (air source heat pump and natural gas furnace)

I currently have a natural gas furnace and a air-source heat pump. Since the effiecency of a heat pump varies on a number of factors, I would like to find out at what outside temperature does it become more COST EFFICIENT to switch over to my natural gas furnace vs air-source heat pump? I am pretty sure I'm going to have to create a spreadsheet to calculate this. That way when costs change, this would allow the user to know how it effects the outside temperature.
Can anyone help me out or has this already been done before somewhere else?
HeatPump variables HSPF Size of unit (in what measurement?) Outside temperature Inside temperature Cost of electricity
Natural Gas furnace variables Cost of fuel efficiency of unit
Are these the main variables needed? Where can I find the necessary formulas? I really appreciate any help that can be given. If I can figure this out I may be able to put up an online spreadsheet or something. Thank you!!!
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Not all heat pumps are created equal...really need to look at the heat ratings chart for the particular unit.
Given, capacity as well as cop of any heat pump will drop off where you have lowered outside temps--what's not given is the effect of humidity....which can form frost that will eventually impeded mass airflow....result is a greatly reduced btu output until a defrost cycle is initiated and finally terminates....
Units having a fairly coarse fin spacing and the newer "spine type" coil construction appear ( at least they do to ) to perform better in the above respect.
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Would need utility rates...
What's your Natural gas cost per therm? (It would be nice to have the actual heat content of the gas in your area) Would need to know your furnace's efficiency?
What's your Electric cost per kwh? Would also need to know your heat pumps COP at different outdoor temps?

If you want to get really technical yes... But for the most part, there would have to be some big changes in rates to make a big difference.

Yes, would need the infomation that I asked for above.

COP at outdoor temps...

(Gas content, heating value per cf)

In my head...

Start by getting me the info...
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Typical 13 seer unit can be found here :
http://tinyurl.com/265clq
Pg 18 charts operation under various outdoor temps ranging from -10F on up alongside btu capacity and unit electrical power input.
Like I say though--if you throw high (or low) humidity into the picture then all bets are off.
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#1 he didn't list his SEER, not that all are equal. But that guide is a joke... I like how they MIS-LEAD people with this statement...
"All units provide high operating efficiencies of 13 SEER, 3.3 COP or greater"
They ASSUME that nobody uses their heat pumps below 47 degrees!! As that is their lowest rated unit at 47 degrees. They FORGET to say... "this unit's COP at temperatures below 17 degrees is under 2.2"
As that is the highest of their available sizes all the way down to 2.0 for their 2 ton unit.
That's what I call BULLSHIT marketing.
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Isn't it all?......................
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I'd need to see a couple of pics of your tits!
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try here.... http://www.aridirectory.org/index.html

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