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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Is your house a similar age/size? 4 occupants?

I think baseline use is closer to last summers numbers, it went up slightly due to replacing our electric stove with gas. Still, that's not enough to explain higher use.

Switching to a smaller more efficient AC unit saved about 25% on our use, but I haven't compared CDD's. If there were ductwork issues wouldn't it affect cooling numbers also?

Smoke testing would be easy to do since the system is in a closet, any brand/type that you prefer?

We have (2) 12" returns in the attic, flex duct.

65f "base" is at what indoor setpoint? Do you use 65f as a base temp for summer and winter even though set-points are typically 5 degrees apart?

They make small hand held smoke devices that use toy train smoke.  But incense sticks may work.

I noticed the slight jump in baseline, gas stove change explains it.

Whatever is happening was probably happening before the change so ac was and is still being affected, but no number changes. 

Are those flex ducts in the attic connected to ceiling registers or panned down to lower wall registers?  Quite often they will just open the top of a stud bay and then mount the register in the drywall, NJ style as I remember.

In any rooms with supply registers w/o a return, is there a path for return air.  A door undercut would need to be 1.5" free space min.  Or they could have installed transfer grills.  Usually they did neither, in which case the imbalance could be an issue.  Ideally you want larger returns than supply in every room.


Return grills in bedrooms are non-existent, but doors remain open 95% of the time. There is a 12x12" transfer grill for the master bedroom, supply duct is 8".

Flex ducts go direct to ceiling registers.

With all this ductwork discussion I'm going to have to do temp probe tests for supply/return temps at the return and supply registers. See how much heat I'm loosing between the furnace and registers. A 5 degrees loss in the attic would hurt more with 105f air (100f at register) vs. 130f air (125f at register).

I've also considered more gas use from the water heater due to lower incoming water temperatures. Lower cold water temperatures could also mean more gas used by the dryer. Not sure how significant these are considering summer base use is so low.

BTW, I use base 65, but I will probably change that once my renovations are complete.  My target for a modest "deep energy retrofit" (r-30 shell) is 200 gallons of fuel per year, 20 DTH for your fuel.

Check the r-value of that flex duct, it is typically 4, 6, or 8.  Its internal surface area is about 2 ft² per linear ft so it adds up.  And it loses more heat with longer run times.  Check both supply and return temp differences.

With just 2- 8" return ducts you have 100 in² total and that need to be de-rated for the flex.  Accounting for run length and all of the twists and turns could yield a 50% loss in return area.  And the grills themselves have a NFA of about 70%

We are down below 50 in² of return area and return area needs to be larger than supply, rarely is.  If you have 8- 6" supply ducts, that would be about 160 in², less if it is also flex.  But supply at double the return means you are pressurizing the house when the fan is running.  It also means the furnace in the garage is struggling to pull in enough return air and will draw outside air through any leaks.  The smoke test should spot those.  An IR inspection would also be interesting.

Your heat loss from the ducts will be less at the lower temperatures, but may be more due to longer run times.

Do some smoke testing or check the rentals to see if you can get an IR for the day or half day.  A window fan can provide some positive or negative pressure to help the IR.


I don't doubt that there is some heat loss/gain through the ductwork. You can feel the hot/cold blast when the blower kicks on at the start of a cycle if the system hasn't ran for awhile. Ducts are (2) 12" R6 for the returns. Supply is (2) 8" and (2) 10" off the plenum. The 10's" are split into (2) 8" near the end of the run. 6 supplies total.

Next time is gets below freezing I'll try a temp probe test with just the blower running and see how much drop there is from furthest return grill to furthest supply register. I know there is an "attic penalty", just not sure how much. Glad my equipment isn't up there also!!

20DTH savings per year would be substantial, what does it typically cost to have that done? What is typically the big thing that saves the most?

Air sealing and insulation and then I always thought a smaller more efficient furnace.  The smaller unit may take some rethinking.

I am adding 3.5" of rigid foam over the exterior (2x4) and new windows which the wife wants, and they are an improvement in looks and function.  Cape, so the slope on the second floor is getting additional rigid plus as much as I can get into the remaining attics.  My furnace died in mid winter so ended up being replaced before I could downsize.  Not happy with it so may replace it again after next winter and I see how it does. 

Essentially 6- 8" supplies and 2- 8" returns.  I'm sure you have seen the frequent comments on these forums about that being common and bad.  While your fan is running it would be like a large window fan exhausting at high speed, a wag, but some blower door numbers would confirm. 

My confidence is increasing that your problem existed before the swap and is increasing the heat loss with the longer run times.  The good news is a well balanced duct system will also reduce your summer use.

Up here most of us burn oil and it is expensive 4x what you pay per unit of heat.  I saw a state wide number of 900 gallons per oil heated home per year.  X $3.50 per gallon gives home owners lots of incentive to weatherize.  Thus my efforts for a deep retrofit.  Besides ironing out the details on how to accomplish the improvement, I end up with an affordable home to heat.


Hello Bob, it sounds like you have a great warehouse! Just wondered, will they ship to Pennsylvania? Thanks in advance for your reply. Regards, T

I used United Refrigeration (they are a chain, you may have one in your area) for the A/C, the furnace came from Craigslist.

We insulate our attics to r-60, but r-8 on our ducts is "a lot!" 

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.

Heat loss is more geometric than linear.  So imagine your loss when you add 40° rather than 10°.  That's what is happening to the not insubstantial and terribly insulated duct outside your thermal boundary.  And now that loss is 10 minutes of every 15 instead of 5 of every 15.  

You've never had duct or shell leakage testing?  Why do I feel there is a HERD of elephants in this room!  

Here's one approach to fixing your problem:

Heat loss is more geometric than linear.

So you cannot use a direct BTU/HDD formula like we have been attempting? What is the correct calculation to compensate for weather changes?

You are attempting to normalize?  I haven't the foggiest.  

I'd look at a few other actual houses, see how much they went up or down, and use that percentage.  Then I'd show range from non-normalized to normalized estimate.  

For example, my brother in law went from 750 to 450 annual therms after weatherization.  The next year was 530 - much colder.  So if you had a first year improvement in his second year, I'd seat of the pants adjust for that.  If you were projected to save 200 therms and saved 170, I'd call that on the very edge of accurate given the colder weather, then wait another year to see if that held up. 

You aren't going to land this "projected savings" ball in the hole, and most people are quite happy if you land on the green.  Getting within 20 feet of the pin makes them very happy, particularly when most programs don't deliver within 200 feet of the green: 

Looks like I'll need find more people willing to share their use data. Most seem to be very "I dunno" or hush-hush about it. Even people that gripe about how high their bills are reluctant to share usage data.

I was thinking that there should be some scientific way compensate for the colder winter in the use calculations.


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