How to determine HDD's and "savings" from furnace upgrade

Last fall I upgraded my furnace from a 88,000BTU 80% to a 44,000BTU 90%. My gas bills are at record highs, although I'm sure some of it can be attributed to the record cold weather we've been having in Moore Oklahoma. I've talked to friends/neighbours/co-workers and they have said theirs have "gone up some" but won't give specifics.

Where do you find HDD data and how to you calculate the increased "load" from colder weather? I downloaded the HDD file using a 60 degree base temperature. Heat doesn't run when it's above 60 outdoors.

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Hi Bob,

There are many links for determining HDD.  This one allows you to compare different years.  Unfortunately each heating season spans two different years so some manual work is necessary.

Once you have your HDD for each month, then calculate the energy used per HDD to see if your new more efficient system is using less or not.  That's a start.

Your bills will not track one for one with your HDD.  If it is 10° colder outside, that would be the same as increasing your thermostat by 10°.  Increasing my house temp from 70° to 80° increases my fuel consumption by 45%.  So a cold winter can have an unusually large affect on fuel use.

What major city near you would have the same temperature range.


Oklahoma City would be the closest major city. I never realized a 10 degree difference was equal to a 45% increase in fuel use!!

If you pull your HDD for 2012 and 2013 and compare them you will see a huge change, 35% more total HDD in 2013.  One of the difficulties with the billing numbers is the charge date, it is in the middle of the month.  You will need to adjust the numbers in one way or another.  As long as you are consistent the resulting comparison will illustrate the improvement, but not the exact number.  If you search harder they will have the HDD per day where you can then match your charge month.  I will have to play a little more to decide which which method is easiest.

The 45% number is from a 10° shift in the average monthly temperature.  But it does track a thermostat and that is why a setback yields a significant savings.  I won't drift.

The other problem with the anticipated savings is a 90% furnace only operates at that efficiency during a steady state.  Does your new furnace modulate down to a lower BTU?


Furnace is not a mod unit, just a basic 44k 90% single stage. I don't think you can even buy a mod under 60k. Billing cycle is about the 10th of the month. We do 20 degree setback (70 to 50) from 8:30 - 3:30, only on the coldest days does it get below 60 during the day. I've never seen if drop all the way down to 50 in the house, no matter how cold it was outside. What source are you using for HDD Data? Jan use is about the same even though it was 42% colder, I'm good with that. Feb use going from 7.9DTU to 12.9DTU (60% increase) while HDD went from 598HDD to 719HDD (20% increase) is what concerns me. But as you stated it could be attributed to billing cycle differences.

I will say the 44k furnace has made a HUGE difference in comfort. I can no longer tell if it's running unless it's really quiet in the house (rare occurrence with a 4yr & 7yr old kids in the house) and I'm listening for it. With the 88k furnace you could FEEL when it was running, even if you weren't able to hear it.

How do you input the HDD/gas use not being 1:1 in energy calculations?

Hi, Bob.  I have a spreadsheet and a rather lengthy explanation on how it works here:

I like to use Wolfram Alpha for HDD and CDD data, because it can handle any arbitrary time period, and its interpolations of any missing data points are very accurate in my experience.  Links are in the article.

Using actual billing cycle dates makes things a lot easier. 789HDD 2014 vs. 533HDD 2013.  .016DTH per HDD for 2014, .015 DTH per HDD for 2013. Using 60 degree "base temp".
Jan bill was 787HDD for 2014, 705 for 2013. gas use the same 11 DTH for both time periods. .014DTH per HDD for 2014 .015DTH per HDD for 2013

Using ANNUAL data I'm at .018DTH per HDD for both years.

Really close, doesn’t look like the new furnace makes much difference on the bills. I use a 60 degree "base temp" since heater doesn't seem to run unless it's colder than 60 outdoors. Doesn’t look like the new furnace made a lick of difference in the bill when a 60 degree base temp is used. Using the default 65 degrees base temp equates to a LOSS with the new furnace, going below 60f base temp shows an improvement for the new furnace.

The real question is how to determine an accurate base temp? Is it based on a 24hr average? Day high? Night low? Often on mild days the heat will run in the morning, shut off when we leave for work, and won't come back on until the next morning when the furnace recovers from night setback.

Thanks Ben, I will definitely play with that.

Bob, interesting to process actual data on a conversion to a more efficient system.  Do you use the gas for anything else in the home, a base line amount.  I noticed your consumption never went to zero.

Also interesting that the numbers get worse if you push the set temp up and better if it goes down.  I will need to think on that.

As for the base temp I have had some discussions on that as well.  My concern was similar in that the house seemed to hold very well with much lower temps than a 5° difference would suggest.  Having watched those cycles I have noticed that wind makes/made a huge difference.  Improvements are helping.  But, if you have a tight well insulated home, those miscellaneous heat sources can account for much more than 5°.

Other than adjusting your real consumption to match your calculated needs, and varying the base number in the process, I have no way to determine a different number.  In your own home it is ok to play, but when producing audits it is important that other auditors end up with the same results, thus base 65.


I forgot to include other use!! We have water heater, dryer, stove, grill (used summer only), and gas fireplace starter (rarely used). Summer use is roughly 1.4DTH/month. 1.4 x 12 = 16.8DTH per year. Taking 17DTH off of annual use number makes things look REALLY bad for the furnace replacement. 64 -17= 47DTH for furnace for 2014, 41 - 17= 24DTH for 2013.

Furnace gobbled up almost TWICE as much gas in 2014 vs. 2013. Using the 65f standard I'm at 4360 HDD in 2014 vs. 3090 in 2013. 0.00776 DTH per HDD in 2013. vs. 0.0108DTH per HDD in 2014. 40% MORE fuel per HDD in 2014 vs. 2013. Either I made a BIG mistake "upgrading" my furnace, or my calculations are off somewhere.

I just don't see where the "waste heat" is going on the new furnace. The new furnace exhausts 120 degree air through a 1.5" PVC pipe, the old furnace exhausted 300+ degree air through a 4" pipe. The old furnace has FOUR 22k burners, the new one TWO 22k burners. Going by the fuel use calculations the new furnace is running 4x as much as the old one did last year !!

The water heater and dryer may be using more gas due to the lower ground water temps, but I'm not sure it would make that much difference.

Is using the 65f degree standard good enough? Or should it be "calibrated' to the actual building's load?

@Bob, How do you input the HDD/gas use not being 1:1 in energy calculations?

I haven't reviewed the spreadsheet as yet, but you can adjust your gas use by averaging the use from the three lowest monthly bill and subtract that from each month or x12 from the year. 

I will have the manually generated numbers soon so we will be able to see if they are in line with the calculations you have.


I didn't calculate the "offset" of there not being a 1:1 ratio of HDD and heat loss. The only way I can figure to compensate is change the 65f base temperature. Could the industry standard of 65f be that far off? Why is 65f used for heating AND cooling when ashrae standard temperature is 70f heat/75f cool? Shouldn't base temperature also have a 5 degree spread?

I believe the ASHRAE 70/75 also adjusts for internal gain, they just specify those temps and the HDD/CDD are from an adjusted database.

HDD is a fickle number, example:

They use the average of the days high and low temperature, however, it may have been an extremely cold day for 23 hours with a brief appearance of the sun for just one hour.  Or the reverse.

On a windy day with a leaky home (even a normal home) the actual heat loss can be double what the delta T tells us (wag).  Maybe we should adjust for the wind chill factor because it really matters.

Much of what we do in energy auditing is related to consistency as opposed to accuracy, a fact that is coming back to bite us in that real results are failing to match calculated results.  For the question you are trying to answer, as long as you use the same base numbers for the different years your conclusions should be correct.  The size of the furnace may be off, but often there is only a broad choice there.

Busy day with snow to plow so will be later that I get more numbers.



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