After running calculations on my furnace upgrade with inconclusive results I figure I would look at how much my AC upgrade really saved. yes my use was a LOT less in 2013 vs. 2012, but it was a lot cooler also.

Old AC: 1999 Carrier 12SEER 3 ton.

New AC: Thermalzone 14.5SEER 2 ton.

Same furnace with 3 ton drive used both years, blower switched from high to low speed when AC replaced at the end of summer 2012.

First the usage data:

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2,000 CDD's @ 65f for 2013

2,430 CDD's @ 65f for 2012

2,700 CDD's @ 65f for 2011

Hi Bob,

Have you looked up your CDDs?  What base would you use and was the house set temp the same for all 3 years?  If you used a set back (up) then you will need to select a time related average number.

Electric use is even more difficult to sort out as to what went to AC and what would have been normal for the house, but there is still a baseline to be determined.  A quick guess would be to subtract 350 from every column.

You said "After running calculations on my furnace upgrade with inconclusive results"  actually, my take on that discussion was the results were conclusive, the new more efficient system used more energy than the older less efficient one it replaced.  The "why" question was what was inconclusive.

BTU per HDD or CDD is still a good indicator of direction and a reasonable indicator of magnitude.


My post got garbled somehow, wasn't able to edit the original post. CDD's for each year are listed in 2nd post. I was thinking on the lines of 500KWH as "base use" per month. We don't use our AC at all during the winter.

On the Furnace replacement I was thinking that too, until somebody mentioned that heat loss isn't linear to Delta T. No further calculations/formulas were provided. Wind Chills/Additional use by Water Heater are other variables.

My mistake, I work from a printout of the post and only printed the original post.  The CDD I pulled up vary a little from yours, not sure where is best to find them.  I used and they have a good explanation on selecting the base number:

Lots more at that web site plus some posts and answers.

I limited the CDD to April through Oct. so it excluded a few and initially used a base of 75°.  Few people turn on their ac when it is 75° outside, but no one turns it on at 65°.

I simply used KWH per CDD for comparison. 

2011 = 3.1 kwh/cdd

2012 = 3.7       "

2013 = 3.0       "

I'm not sure what you expected, but 2013 doesn't look much better than 2011 based upon energy used per CDD.  I'll check the other base numbers but I don't expect much of a change.  I also used the 350 kwh offset as 500 seems way too high.  Electric use for other than ac actually goes down during the summer, less lighting, less water heating, less cooking inside, and so on.

As for the comment that heat loss is not linear with delta T, doesn't alter the relationship that they move in the same direction, the greater the ▲T the greater the heat loss.  Using HDD or CDD, short of hourly calculations, is still good for what you are comparing.


65f - 75f outdoor temps, we normally leave the windows open if humidity isn't too high. House is closed up between 55f-65f, with no heat running. Heat goes on below 55 outdoors. We leave the house closed up at night during the summer to prevent humidity build up, even when it's below 70. Internal gains and heat penetrating into the house from the days sun will cause the AC to run when it's 75 indoors/70 outdoors. In the morning the AC doesn't run much at 75/70.

For people who never open their windows, it isn't uncommon to run the AC at 70 degree outdoor temps. Yeah it's a waste to run AC when its a beautiful day outside, but that's another discussion..

Interesting that you mention more water heating/cooking use during winter. Both of those are gas in our house. Should I be factoring those into my winter base for DTh use? Lights are LED or CFL, so not much use there.


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