Not that it really matters, but the COPs discussed here seem way over the top.
The previous post by hvacrmedic would be more realistic and accurate.
Electric rates have marched upward while nat gas rates have remained constant and in most markets have actually decreased.
Depending on location and rates, DF is no longer the good option that it once was.
Data was taken from utility bill and entered in Excel each month.
Looked at NatGas usage in therms.
Used data from summer months to create average usage for hot water/cooking. This was subtracted from total therms during heating season.
From cost data created cost per kwh and cost per therm. This included all delivery charges, etc. Basically any charge on the bill that was based on per kwh or per therm was included. There is a fixed fee for electric service and a fixed fee for gas service. These were not included because no matter what I will always have gas AND NG service as I have gas cooking, HW, etc. and the fee was similar.
Using the above cost data, created a cost per million BTUs. for HP a COP of 3.5 was used. For NG, the furnace is 95%. That is the graph you see. So in summary the chart compares cost of creating BTUs.
Things to realize, prior to 2010 my service was based on Time of Use rates direct from utility (BGE). I was required to have TOU service. In summer 2010, thanks to dereg, I signed a 1 yr contract for fixed rate electric. The nice byproduct was no more TOU and this helped with my electric rates.
Funny thing about dereg, I also entered into my spreadsheet what I would have paid if I had signed contract for NG as opposed to buying from utility. Turns out that with the surprising drop in NG prices, the fixed contract for NG would have cost me more.
"The BTU output of a heatpump is not a constant value "
Yes that is true, but unless you had a fully instrumented house measuring usage and OD temp, one needs to use as "average" COP to have any hope of calculating. So takeing the average temp over a 5 month period and finding a COP for that temp is all one can do. For my area, the average temp was 39.1 to 41.0F depending on whos data you use.
In my case, electric rates have dropped since 2007, and switching to a fixed rate contract dropped them further. NatGas prices have also dropped.
Also, because of the fixed rates for electric (both utility and contract), electric prices don't have the wild swings that NatGas does (this can be seen earlier in graph).
I should restate this to be clear, the graph was created from cost data from the utility bills from 2007 thru 2011. From these costs, a "delivered" cost per kWh and a cost per therm was calculated. Really had nothing to do with the BTU's generated by my specific house/system. Then a cost per million BTUs was created and graphed.
How Prices Have Flucuated
Here's the complete graph from 2007 thru Jul 2011.
Note that this is Cost to Produce 1 Million BTUs using "delivered" rates from an actual utility bill
As you can see in 2008, duel fuel and HP seemed like a better solution.
Just realized, Spreadsheet does have avg temp for each month from utility bill. So if I had a way (formula or table) to convert avg temp to a COP. I could compare monthly instead of a single COP value.
Since you have access to COP data and you might have a favorite performer...
Do you have COP data for say
16 SEER Heat Pump (any vendor but lets keep SEER at 16 or should I say smallest SEER that gets a tax break as that is what drives consumers)
COP at outdoor of:
More data points the better, but with 4 to 5, I can create proformance curve/table and use this along with monthly avg temp to get better comparison on a monthly basis, as opposed using a single avg COP..
York YZF03613, with AHX42 air handler.
Excellent. I'll post back in a few days.
A couple of comments (Not Advice )
When we were looking at heat pumps, we also found that COPs, HSPFs and SEER rating were very confusing. A heat pump that has a great COP at 47F. might not be as good as a unit with lower COP when operated at lower temperatures.
Overall system COP in heating mode seemed more important than just the heat pump's quoted COP. If electric heaters have to come on part of time, the system COP changes considerably. But this type of COP information is not readily available.
Some may call it "volumetric efficiency" ??
It's really not that confusing when you take into account that those are three different methods of rating performance. HSPF means nothing IMO, COP is real performance. SEER, I've never seen the heating side of a heat pump rated in SEER.
Originally Posted by Freeagent
Heat Pumps, as we know, also do cooling and have a SEER rating. Just another term a heat pump buyer needs to understand. When we buy a heat pump in Canada, we can apply for a grant provided unit meets SEER, EER, and HSPF (for Region V) requirements. Latter is seldom in literature.
Originally Posted by beachtech
Regarding COP being real performance; There is COP of the heat pump and then there is COP of the overall system including backup heat. These varie with outside temperature. COPs at different temperatures, even for just the machine, are not usually in the sales brochures. Usually just the 47F figure.
Hope this helps explain why buying a heat pump is a learning experience for a first time buyer!