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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Same city, same HDD's.

He's about 10% more than me for 2012 and 2013. 16% more for 2014. It's looking more like the new furnace did NOT increase consumption, and the weather was the major factor. Do you use 65f as a base temp for commercial buildings? I've seen many commercial buildings running AC down to 50f outdoor temps. Sometimes we even have to install low ambient kits to keep pressures in check when outdoor temps are low.

While you continue to contemplate the use, here is a test you can play with.  Virtually all homes will have a neutral zone (NPP) where openings above that level will leak air out and openings below it will leak air in.  If you find an opening with almost no air movement on a calm cold day, it is probably close to the neutral height.

Ranch if I remember, so crack open the bottom of a window and feel for air flow.  Then switch to the top.  Any opening you create above the NPP will move it up and any openings you create below it will move it down.

Once you have established an opening that is close to neutral, and hopefully away from any supply air flow, cycle the furnace on to see if operating that fan increases or decreases the air flow through the neutral opening.  If it does, then there is an imbalance in the supply/return or some duct leakage that is affecting heat loss.

It takes a little practice to get used to this, but it does provide a manual way (without a pressure gauge) to estimate what is happening.


Cold calm days are a rarity in Oklahoma. The top 1/2 of our windows do not open, I would need to find another "high side" to use. Wouldn't return duct leakage show up in the temp difference between the return grill and blower compartment of the furnace?

Correct on the return leakage, but supply leakage or pressure imbalance is simply lost air.  Test the bottom of a window and see if you are getting a little or a lot of infiltration.  Tape a piece of tissue to the bottom of that window to act as a weather vane and then cycle the furnace blower on and see if you can detect a change in air flow through that window.

If the in-flow increases, then an imbalance or supply leak to the outside is causing more infiltration.  If it decreases, then an imbalance is pulling in excess air thus reducing the infiltration from the window.


Collected some more local use data:

last 12m 772.93 (77.391);  30% more compared to my use
previous 664.13 (54.919);  32% more compared to my use
previous 25-36 m. 714.27 (59.581) 25% more compared to my use

Looking more like WEATHER was the major factor. Makes me question the accuracy of HDD's, the generic 65f degree base temp and/or the lack of wind chill are the top suspects.. The BTU/HDD ratios start making more sense with a 55f - 60f degree base temp. I don't think the 65f base temp properly accounts for solar/internal gains of a typical residence. Determining an ACCURATE base temp is the challenge...

And because it is challenging they choose to use one consistent number.  Doesn't make it right, just makes it easier and more consistent.

When you install a newer, smaller, more efficient heating system and the results LOOK like they went the wrong way, that is when you start adjusting (truing, fudging) the numbers to match the real world.  Part of that process involves understanding where these numbers come from and you have done that.

Early in this thread you mentioned colder temperatures affecting your DHW heating costs.  I recently played with the cost of hot water for a class I'm taking and it seem it is an area where I have always taken one of those standard allowances without digging out the real number.  It would be reasonable to assume your incoming cold water has gotten a bit colder and does so only during the colder months.  Estimate your water use per month and calculate a 5° or 10° added load.  It certainly is costing something.


I know incoming water temps are VERY significant when sizing a tankless water heater. I checked it last weekend when temps were extreme, had 48f degree water out of the tap after running 10 minutes. IIRC summer water was about 75f.

Hello Bob,

Rather than heating dergree days (HDD), I much prefer to work with average monthly temperatures.  The average temperatures can be easily determined for each specific billing period.  The plot below shows your energy use before and after installing the high efficient furnace.  Based on this, it does not appear that you are getting significant energy savings with the new furnace.


The data would line up better if I had the actual meter read dates. The dates you provided appear to be based on the effective billing date, not the actual dates that the meter was read (The number of days does not agree with the read dates).  I have attached  the Excel file that I used to generate this graph using the daily temperatures for Moore, Oklahoma.


Meter is read on the 10th or the nearest weekday to the 10th. My personal "experiment shows that 90%+ furnaces really aren't worth the cost for our area unless you can buy one at a steal or  rebates cover most of the difference. I gave $250 for the furnace (New in box, but a 2004 model), my main concern was to downsize for comfort purposes. A 10% difference in gas bill from a furnace change out is hard to detect when there are 2 winters so different from each other.

I looked more into the "average monthly temp" method, it appears to be more "repeatable" than the HDD method. There is no "base temp" calculation to be concerned about. The extra water heater gas is somewhat "built in" because it would be roughly the same at each average monthly average temp.

Could the 2012 and 2013 data use different coloured lines instead of both being blue?

Could other users data be somehow put in the chart for comparison? Would it provide any useful info?

How about using wind chill numbers instead off actual temps? Is average monthly wind chill data even available?

Another data share:

Last 12 month history: $888.77 365 97.002 (51% more than mine)
Previous 13-24 month history: $729.20 365 67.121 (63% more than mine)
Previous 25-36 month history: $826.67 366 76.542 (42% more than mine)

More user data came in, here's what I have so far:

Last 12 month history: $917.68 365 86.053 (34% more than mine)
Previous 13-24 month history: $679.65 365 51.202 (24% more than mine)
Previous 25-36 month history: $743.24 366 56.841 (20% more than mine)

Last 12 month history: $888.77 365 97.002 (51% more than mine)
Previous 13-24 month history: $729.20 365 67.121 (63% more than mine)
Previous 25-36 month history: $826.67 366 76.542 (42% more than mine)

last 12m 772.93 (77.391);  (30% more compared to my use)
previous 664.13 (54.919);  (32% more compared to my use)
previous 25-36 m. 714.27 (59.581) (25% more compared to my use)

The more data I collect, the more I'm convinced that errors in calculating for weather/user differences exceeds my savings/costs for the new furnace. I was looking for a 10% savings, it's simply not there. At this point I'm calling the 90% furnace "myth busted", it doesn't save enough to be significant.


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