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

Posted by on February 5, 2007, 11:48 pm

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!!!

Posted by Jeffrey Lebowski on February 6, 2007, 1:46 am

> 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!!!

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.

--

Posted by on February 6, 2007, 2:59 am

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?

> 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.

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.

> Can anyone help me out or has this already been done before somewhere
> else?

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

> HeatPump variables

COP at outdoor temps...

> Outside temperature
> Cost of electricity
> Natural Gas furnace variables
> Cost of fuel
> efficiency of unit

(Gas content, heating value per cf)

> Are these the main variables needed? Where can I find the necessary
> formulas?

In my head...

> 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!!!

Start by getting me the info...

Posted by Jeffrey Lebowski on February 6, 2007, 11:41 am

> > HeatPump variables

> COP at outdoor temps...

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.

--

Posted by on February 6, 2007, 12:14 pm

> > > HeatPump variables

> > COP at outdoor temps...

> 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.

#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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