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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Benchmarking is the path to understanding opportunity.  It is growing rapidly in commercial, eventually it will make its way to residential.  It exists in some places already.  

I created this tool to get a rough idea of how "energy obese" homes are in my area:

We download daily degree-day data from and then total by meter reading periods.  Subtract the summer DHW/kitchen loads and then convert to BTU per degree-day per square foot.

Update: Did a mid month evaluation. Meter reading on the 24th is 1673, reading on the 11th was 1644. About 3DTH. 245HDD for the same time frame based in 65f. 3DTH/245 = .0122 DTH per HDD. So far it appears the milder weather we have been having recently is in fact showing less gas use per HDD. Either heat loss isn't linear compared to outdoor temps, or I'm way off on the 65f base temp.

With a 10 degree outdoor temp I decided to do some duct testing.  43.5C and the supply plenum right above the A-coil, 42.6C at the furthest register. About 1 degree C difference (thermometers only read in C). Similar results on the return, about a 1 degree difference return grill to blower compartment in furnace. Doesn't look like it's a duct issue causing the increased consumption with the smaller furnace.

My thermostat is a "cycles per hour" style and there is a MUCH smaller temp swing than the old furnace had. Old furnace was about a 4 degree swing, new is 2 degrees. I can change the CPH from 4 to 2 and get the 4 degree swing back. The thermostat is more sensitive than most, it responds very quickly to changes in air temp. Other thermometers in the house show less than a 1 degree change between cycles, but seem to be more affected by wall temperature or simply have a slower response time.


With great interest, I've been watching from the sidelines as this mystery play out.

Many knowledgable and experienced pros have contributed theories and it appears a resolution, if indeed one exists within the grasp of extant building science (I'm ruling out the supernatural, for now) will come as a surprise.

I believe Mark Twain said words to this effect "It ain't the things you don't know that cause problems, it's the things you think you know, that just ain't so."

We're all firm believers in the scientific method, and I have begun to suspect that somewhere in this seemingly intractable dilemma, a fundamental premise that has been filed under "things we know", just ain't so.

I shall retreat once again to the sidelines and await further developments.

Best wishes.

I'm HEAVILY questioning the use of the correct "base temp", the 65 degrees "one size fits most" is a tough sell. Since the winters are so different getting this right is key, but determining the correct temp is another matter. If the winters were more similar, base temp would matter less.

If someone described a horse race to you, and later they showed you a picture, you would conclude from that picture you know all there is to know about horse racing?  

When someone showed you a video, how would that make you feel?  Would you feel you suddenly knew everything about horse racing?  There were not fine and important points missing leading you to bad conclusions?  

Then, if you attended a race, would you conclude yourself NOW a horse racing expert? 

When I meet subject matter experts I open myself to the idea all my ignorant presumption are probably wrong.  Since I've adopted this mindset, my rate of learning has increased geometrically.  My schemas change all the time, and that no longer makes me uncomfortable.  In fact I have come to enjoy it.  Ignorance is not stupidity, unless the ignorant hold tight to false beliefs.  

So your snapshot of the return shows say, 20c and supply is 43 c, and you are losing about 10% just in sensible (no duct leakage, no envelope leakage, etc...), I'm not sure how you could so easily come to that conclusion.  Uninformed gut feel?  So, where does your gut tell you your big opportunity is then?   How do you feel about that gut, since it told you changing the furnace would save buckets and that hasn't happened?  

Until you do complete diagnostics and complete measurement, you must assume your loss is 20-50% due to all the bad things about your duct (location, leakage, etc...)

Steve Waclo is a thought surgeon with a laser scalpel.  There is simply too much that we KNOW we don't know about your home.  We need the movie, not the snapshot:

The furnace replacement was more for comfort reasons than energy savings. The old 88k furnace would run you out of the house if it ran more than 5 minutes.

I agree that those measurements do not indicate a problem.  The only aspect not covered would be supply duct leakage to the outside.  That would not affect air temperatures.

Here is an article related to adjusting the delivered volume of gas based upon temperature.  No idea how this might affect your dilemmaJ, but the implication that a warm adjustment goes in one direction and a cold adjustment another, combined with you comparing a record warm year with a record cold year makes me wonder.

Fine tuning the HDD based upon you personal knowledge is acceptable, and you did say you added another gas appliance.

Hot water is a major user of gas and that use would be expected to increase as temperatures dropped, both by the colder incoming water and by the nature of taking longer showers. 

Where is the water heater located?

So what do we know so far?

1.  Longer run times produce warmer exterior surface temperatures and increase heat loss. 

2.  Basic numbers for fuel use per HDD, at base 65, show a significant increase in heating fuel use with the new more efficient furnace, contrary to what was expected.

3.  Return duct size is less than the supply which increases supply and house pressures.  If there is fan induced leakage, that would increase with longer run times.  Some benefit would be gained by a smaller fan.

4.  Much of the heating system is located outside the thermal envelope, furnace is in the garage and supply and return ducts are in the attic.


ALL of the heating system is located outside of the thermal envelope except the registers on the ceiling. Return is (2) 12" = 226sq in. Supply is (2) 10" + (2) 8" = 257sq in. Is the 30sq in significant enough to make a difference? Average supply runs is long than return runs. All ductwork issues existed with previous system also, is it possible that the ductwork is now oversized (never seen that before)?

I've entertained a smaller fan considering I have too much flow for the 2 ton AC anyway. I'm running about 950CFM on the lowest speed with a 36f degree temp rise. Furnace is rated for 25f-55f rise so I've got some wiggle room. One options is to replace the motor with an Evergreen, but they are costly. Swapping the 1150rpm motor for an 850rpm is another option. I've even considered going to with backward inclined blower wheel, less flow at the same RPM. Might even "redneck it" and partially block one side of the blower housing.

Today have been in the mid teens degrees most of the day. Furnace runs 3x what it did yesterday when the temps were in the mid 30's. Cycles yesterday were about 7min out of 30, today 20min out of 30. Design temp is 18f for our area. Seems like the furnace is running MUCH harder at the colder  temps.

Water heater is in the closet next to the furnace.

I managed to collect some use data from another person that lives nearby, it looks like his use trend is VERY similar to mine:

Last 12 month history: $734.81 365 69.414
Previous 13-24 month history: $617.79 365 46.773
Previous 25-36 month history: $691.04 366 57.141

Looks to me like the 65 base temp is either wrong or heat loss isn't linear to Delta T. Then again it could have simply been windier and HDD's don't take wind into account.

The basic equations don't reflect an increased heat loss at a greater delta T, but as you noted, other factors come into play, increased energy to heat water, wind, longer run times for the furnace and so on.  In essence, these other factors do indicate a non-linear relationship, but it is difficult to see it adding up to what your numbers are showing.  Run your neighbors numbers to see if he is getting a uniform fuel use per HDD.



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