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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One more, this time use is LOWER than mine.

Last 12: $485.23/35.797 DTH (46% less than mine)
Previous 12: $279.60/11.784 DTH (73% less than mine)
Previous previous 12: $191.40/10.472 DTH (78% less than mine)

I'm getting details on this one, seems very low.

Sorry I'm slow Bob, I'm still back trying to decide if JW's mean temperature vs avg btus is giving us anything different to look at.

But your last set of data seems to indicate you improved in the last 12 month cycle as oppose to the two older time periods, although the other examples do not.

There are also 2 presumed improvements in efficiency here.  One is the "80% to 90% combustion upgrade" and the other is the "right size" match to reduce cycling losses.  If both of these were working as we would normally anticipate (or promise our customers) then you should be seeing more than a 10% improvement.  At present we are stuck having to accept the paper promises in lieu of being able to sort out the extreme seasonal variations we have had.  But keep crunching as either a solution will surface or more data will provide an answer.


I'm convinced that the new furnace is the more efficient 'box'. Flue on the old furnace was a 4" metal double wall pipe, new furnace uses a 1.5" PVC pipe. You can hear the water dripping from the condensate line as the new furnace runs, it's getting the latent heat that the old furnace sent up the flue.

I have changed the thermostat to 2CPH from 4CPH so the total BTU delivered per cycle/temp fluctuation will more closely match that of the old furnace. Unfortunately winter will be over before the results are in.

As far as selling customers go, comfort is key. You can never win selling based on efficiency numbers, the math just doesn't work. Gas is too cheap, the premium for the 90+% furnace can't be justified unless there are significant rebates involved or you can buy the 90+% on clearance. Downsizing the furnace will improve comfort due to the more even temps and quieter operation. Less drafts/"cold blow" at start of cycle due to the lower airflow rates. Smaller furnaces are also more "compatible" with ductwork that was marginally sized for their old furnace. Old furnaces were good with 100CFM per 10K BTU, 80% needs 130CFM, 90+% needs 150CFM.

One more graph for you to look at.  I revised the actual read dates as you suggested and added the dates for the past six months of reads to show how they compare with previous years' data.   

In this case, the new furnace does not appear to be significantly more efficient.   


Agreed, it looks as if the "more efficient" furnace is a farce.

Question, if you had not made the change and had gone through this deep cold cycle with your old furnace, do you think the results would have been better?

Reviewing James chart, no different than the btu vs HDD analysis, just a different way of displaying the data.  But that is what graphs do, help you visualize the information.

So, what ate up the savings from the improved combustion?


That is the $1M question. 10% is such a small savings it's difficult to detect. Part of me thinks downsizing the furnace caused duct losses to cause a greater impact on heat loss. 10% greater losses due to more time spent with ductwork warmer than attic temps. 10% could be higher wind chill factor or more water heater use.

I've never seen proof positive results of lower gas use for any furnace upgrade, but I’ve never cut a furnace size in HALF before. I've considered buying a 24VAC timer to log the number of hours the gas valve is open vs. HDD/Monthly temp data, but that will be a project for next winter.

AC system replacements have made PROVEN results in energy bills, but I suspect that has as much to do with the old unit not delivering rated capacity as it does improvement in SEER ratings.

When tell a customer it is time to consider a replacement, it is usually something running under 50% where just about anything they put in has to show up on the next bill.  But I hear it all of the time, "just put in a new high efficiency furnace".  They installed a new pipe line for NG from Canada about 10 years ago and dug up the nearby major city to pipe most of the streets.  That created a wave of new heating systems and for awhile much lower rates.  But as always, those rates have now gone up and I'm waiting to hear some long term results as to how those people are doing. 

I'm surprised we didn't see more comments from the rest of the membership, this is a tug on the rug many stand on.


I've only tracked one non comprehensive project, but it did have significant results.  All duct was within the envelope, the equipment was downsized making the duct more appropriate (static ranged .15-.4), and AC was downsized to a 2 stage hp.  

Savings was 35% greater than we expected.  If I recall correctly it was +/- $400 yr instead of $275.  

Trying to dope out the error all I could figure was comfort improvement leading to less thermostat interaction which allowed the communicating system to keep itself optimized, running very low and very long.  The comment that lead to that conclusion:

"Had to adjust thermostat on old system 3-4 times a day... Now if I touch it twice a week that's a lot.  House just seems to be always comfortable."

Comfort with the smaller system is MUCH better, and that was my primary reason for downsizing the furnace. For $250 I couldn't pass it up, an 80% normally costs more than that.

I would not paint all "high efficiency furnaces" as fraudulent based on this one instance.  It would be interesting to see before and after billing histories of other homes that have installed high efficiency furnaces.

I don't doubt that a 90% furnace wastes less heat than a 80%. Going from a 4" double wall vent pipe to a 1.5" PVC is proof enough for me. It's the lack of change in utility bills that I question. 10% really isn't that much change in efficiency considering the typical cost difference in the furnace. I would love to see a study where just the furnace was changed and it made significant reductions in the gas bill.


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