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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Not a snowflake in sight down here, expecting temps in the 60's today. First real break from the record cold we've had in awhile.

Furnace size could be the issue, the current 44k furnace runs about 2/3 of the time 10min on/5min off when temps are at our 18f design temp. Old 88K furnace was reverse, 5on/ 10off. Maybe oversized furnaces use less gas overall?

Wind chill not being accounted for is a big factor, not sure why they don't use it for HDD..

Using a lower base temperature makes the numbers "look better" for colder weather. The ratio of HDD's/DTH used changes as a base temperature drops.

Playing with the "base temp" setting in order to to make the numbers "work". I'm having to drop it all the way down to 45f to get 665HDD for Feb 10 2012 to Feb 10 2013 and 1270HDD for Feb 10 2013 to Feb 10 2014. I suspect the house's real "base temp" is somewhere in between 45 and 65, just not sure where. It appears getting it right is a fairly large factor in determining how much to offset energy use due to differences in weather. Using a record mild (2013) and record cold (2014) winter back to back isn't helping matters any, especially when trying to establish a case for installing more efficient furnace. Seems like difference in weather is such a huge factor compared to the paltry 10% difference in furnace AFUE.

Sorry to be so slow, but my numbers have been all over the place so having to fine tune almost every input.  So far I'm using 1.1 dekatherm per month for the other uses.  Early numbers are looking like about 1,200 btus per HDD, that's in late 2013.  What we would like to see is a constant btu/HDD for each furnace and hopefully the newer one is some level lower than the old one.

As for using a very low base for determining the HDD I would not go more than 10°, base 60.  Whatever you choose you need to keep it the same for all years to eliminate that variable. 

I still haven't played with Ben's spreadsheet, but will do that and see how he is handling the variable billing dates.  Once I understand this issue better I will want to tackle the oil delivery issues here in NE where long periods can pass between deliveries and some homes will purchase spot deliveries of less than a full tank.  I'm thinking I will need to tie the projected oil consumption to the HDD, thus I need more confidence the HDD #s have some degree of accuracy.  Others I have talked to are less confident in HDD #s.  I may need Ben's help on this.

I'll be back


Being able to use the 10th of each month for meter read date is much more accurate than the 1st. Meter isn't always read the 10th, but it's within a few days.

It appears base temp is HUGE factor in the numbers. 60f is a reasonable number, I've not seen the heat run when it's warmer than 60f outdoors. It will run a little at 55f if it's in the morning. 55f in the evening the system won't run.

It would be a lot easier to compare if the winters didn't go from record warm in 2013 to record cold in 2014.

HDD numbers with 55 degree base temp:

Jan   Feb 

409   429  2012

553   397  2013

638   641  2014

Really wish there was an app/spreadsheet that would just let you put the numbers in rather than manually calculating it. A cool feature would be to take several years of data and have it calculate the correct base temp. How many commercial buildings (especially restaurants) have you seen using AC when it's 50f or less outdoors? 65f base temp won't be accurate, especially for a moderate climate/winter.

The factor that levels out the swing in the numbers between record warm and cold should be the btu/HDD.  I got through Ben's paper once, but need to actually practice with it.  His spreadsheet is next.

Sorting out the billing is difficult.  I'm going to check out Ben's recommendation for HDD to see how well it squares up the HDD per billing cycle.

How well air sealed and insulated would you describe your home? 


Air sealing isn't bad but could be better. I got the "big leaks" that I could feel by hand, but have never done a blower door or used a thermal imaging camera. House is 1600sqft built in 1999. You see the HDD and DTH data, not sure what is "normal" for a house my size.

Furnace in garage, supply/return ductwork in attic. Ductwork has been taped/mastic.  Furnace runs about 2/3 of the time on our 18f design day. Never had a power bill over 1,000KWH last summer, 2 ton AC will keep in 75f indoors until outdoor temps go above our 98f design temp.

Bob, while we continue to sort out the rate of fuel consumption it seems clear so far that your results are not as expected, so lets add into the discussion, why that might be.  We will eventually have to go there anyway.

Leaky ducts, even though you say they have been sealed, have they been tested for leakage to the outside.  Being in the attic (outside the thermal envelope) there is a greater than average risk.  A disconnect or panned return can have a huge effect, especially with the longer run times associated with the smaller furnace.

Imbalances that are creating pressure differences that can drive warm air out and pull cold air in are also factors that longer run times would create an increase in heat loss.

Longer run times also produce a more uniform temperature throughout the house.  A difficult factor to evaluate but raising the temperature next to the exterior walls and windows does increase the heat loss.  Moving air also has the same effect.  During off cycles (which were longer) the house cools from the outside in.  As walls and windows get colder the house loses less heat.  Increasing the run time on the fan increases the heat loss.


I've considered leaky ducts as an issue, but wouldn't summer KWH be increased also? I'm @ 800sqft per ton for cooling, and that simply won't do 75 indoor with on a 98f design day without at least decent ductwork.

More even temps and longer run times could be a factor, discharge air is 25 degrees cooler than the old furnace. With the old furnace the blower had to be on HIGH speed for a 130f discharge temp. New furnace 105f discharge temp on LOW speed. The old furnace rarely made it all the way to 130f before the thermostat would satisfy and shut it down, don't think a cycle ever went longer than 5 mins.

Bob, OPower does this for the customer automatically. I would think that your utility's usage analysis program should be doing this for you, but maybe not, since you pose the question. I'm sure energyvanguard would run the numbers for you, gratis, since the're the experts on hdd's. The main thing that the repairman say about condensing furnaces is oboy, another service call. They're known to cost more than they save! Mine's leaking condensate from the collector box as we speak, tho' that's an easy five hour repair. enjoy, t

Part of me wonders if the condensing furnaces are more prone to damage from short cycling/oversizing. Customers that buy the condensing furnaces are sold on the "bigger is better" mentality. Rarely do I find mid/high level equipment that isn't oversized. My 2ton 14.5SEER condenser sat in the warehouse 3YEARS before I bought it. My 44k furnace was made in 2004, still brand new in the box. I didn't specifically want a condensing furnace, but for $250 I couldn't pass it up.

You ask "not sure what is "normal" for a house my size." Hard for me to be accurate as we will run over 8,000 HDD this year, but your Dekatherms are 1,000,000 btus or about 10 gallons of my oil.  Converting it to my thinking you are running 280 to 510 gallons of oil (heating only) at less than half my HDD.  That doesn't sound very good.  If others are in the same region maybe they will chime in.

A correction on my earlier numbers, I was thinking therm and not Dekatherm so the guess was way off.  Below I subtracted 1.1 DTH per month to account for baseline use.

For 2013

Jan 2013 to Dec 2013 = 3086 HDD,  12 months Fuel use was about 51 DTH or 510 Therms.

That's 510 x 100,000 btus divided by 3086 HDD = 16,500 btus/HDD with the new furnace.

For 2012

Jan 2012 to Dec 2012 = 2092 HDD,  12 months Fuel use was about 28 DTH or 280 Therms.

That's 280 x 100,000 btus divided by 2092 HDD = 13,400 btus/HDD with the old furnace.

So, not only is your usage too high, but it got worse when you went to the smaller more efficient unit and that may be giving us a clue.  There was something wrong before the new unit was installed.   

You said "Furnace in garage, supply/return ductwork in attic."  That is a potential major source of air leakage.  Do some smoke testing around that furnace and I bet you will find some major leaks.  There or the ducts/panned ducts in the attic.  Air leakage would make fuel use worse with longer run times.

Related to distribution losses, do you have multiple returns or one central return?

How well Insulated are those ducts in the attic?  Were they sealed before the insulation was added?

BTW, I cranked the numbers every way I could to align the starting dates with the HDD and it makes no difference.  The important number is btu per HDD and that can come from the total, close enough.



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