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Heating degree days, Wrong?

A lot of commercial office building's HVAC systems are designed using degree days instead of BIN or an actual heat loss / gain load calculation.

Comments

  • RickRick Member Posts: 36
    Heating degrees days, not accurate for fuel use?

    OK right now where I am we usually get a littler over 6,000 HDD. As of today we have over 2,000 and so that means we should be 1/3 through the heating season right?
    I swear that the MIDDLE 2,000, I burn a lot more (I'm going to guess 50% or a bit more) of the yearly fuel amt. Yet I keep hearing that oil men use this as a pretty accurate means of determining oil deliveries. So is the dynamics of my heating needs different than most? At 60 it hardly ever comes on, 40 degrees my heat doesn't come on much, at 20 it comes on a fair amt and at 0 it runs a LOT. Doesn't seem to me that HDD are all that accurate. After all, it doesn't really address windchill or solar gain.
  • Don't forget the \"K\" factor

  • jpjp Member Posts: 1,935
    as you said,

    "oil guys consider it "pretty" accurate".

    its better if you can track your own HDD using hourly data.

    heating season is over, when its over.

    as far as when 'your' house needs heat, yep, no gneral rule. its up to the house itself.

    I think HDD is a pretty good yardstick to use.
  • MitchMitch Member Posts: 955
    from your post I think

    You are looking at an average 2000 in the 1st 3rd of the heating season, 2000 inthe second, 2000 in the third...it does not work that way.

    What your fuel supplier is doing is saying you (and these are TOTALLY random #s) used 6000 gal of oil for 6000 HDD. averaging 1 gal/HDD so if we had 600 HDD this week (cuz blimey it was cold) you used 600 gollons..we delivered 1000, and you have about 400 left..we should schedule a delivery.

    Make sense?

    mitch
  • Plumb BobPlumb Bob Member Posts: 97


    > OK right now where I am we usually get a littler

    > over 6,000 HDD. As of today we have over 2,000

    > and so that means we should be 1/3 through the

    > heating season right? I swear that the MIDDLE

    > 2,000, I burn a lot more (I'm going to guess 50%

    > or a bit more) of the yearly fuel amt. Yet I keep


    That's a pretty rough calculation. Everything is based on vague statements and guesses.

    If you want hard numbers you have to do better than that. Calculate your actual therms per actual degree-day, month by month. You'll see that the ratio varies some, but not as much as you say. Certainly you spend a lot more money in January and February than in other months, which feels bad, but calculations aren't based on feelings.
  • John KettermanJohn Ketterman Member Posts: 12


    > At 60 it hardly ever comes on, 40

    > degrees my heat doesn't come on much, at 20 it

    > comes on a fair amt and at 0 it runs a LOT.

    > Doesn't seem to me that HDD are all that

    > accurate.


    That's a really funny statement. Perhaps a refresher course in math would be useful, because what you describe is exactly what should happen if usage is proportional to HDD.
  • MitchMitch Member Posts: 955
    good point

    Do you understand what a degree day even is?
  • SinghSingh Member Posts: 41
    BIN

    Find software that uses the Bin method of heat calculations.
    Heating degrees days method is quick and easy but not accurate.

    On commercial jobs, engineers design using Bin or better yet the hour method.

    For the reason office buildings during the working hours have a peak load, with people, computers, equipment, ventilation. solar gain etc..
    The HDD method does not account for those gains or loads.

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  • Brad WhiteBrad White Member Posts: 2,393
    ???

    "A lot of commercial office building's HVAC systems are designed using degree days instead of BIN or an actual heat loss / gain load calculation."

    Mike- I have never used such a method to calculate H/C loads in any system. I might use the DD method to corroborate/triangulate a calculated heat loss and as a reference value only (beyond the energy consumption figures of course).

    Can you elaborate on how such a DD method would be used alone to determine the actual heat loss when the building is only on paper? Or is it a method much as I would use to cross-check an existing structure?

    Not a challenge, just seeking to learn what might be another way to do something I have never contemplated.

    As for Degree-Days and commercial work, I will offer that we commonly use a Base-50, not the Base-65 so often used in residential work. Not suprisingly, with tighter houses, that Base-65 should be coming down eventually. I can see easily Base-60 in retrofitted houses with higher internal gains. With good insolation, much lower. Maybe even Base-50! And how cool is that?
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"



    -Ernie White, my Dad
  • mtfallsmikeymtfallsmikey Member Posts: 765
    Weeeeeeel Brad,

    There was a new 170K s.f. building designed with degree days in mind at the site where I used to work. I questioned the project mgr. on this, and he said that was all that was needed...I went "huh"?. They were going to attempt to heat the OA feeding the AHU's with 2 Lochinvar gas boilers, by means of a veeery large coil in the OA intake shaft instead of using VAV's with reheats! A novel concept.
  • Brad WhiteBrad White Member Posts: 2,393
    I guess we both say

    Huh? :)

    Thanks, Mike. I can see the OA heating being a defined, calculable load, but overall the concept of using Degree-Days as a design target for heat loss is like driving looking in the rear view mirror.

    Heating the incoming OA as a replacement for reheat? I have no idea how that would serve terminal spaces.... oh well, not my design :P
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"



    -Ernie White, my Dad
  • chapchap70chapchap70 Member Posts: 147


    I believe that degree days came about somewhere around 1920 when oil companies used to calculate delivery dates with a slide rule.

    The degree day method is most accurate for homes where the people are living there all the time. A Secondary home gives oil companies fits because you have a bogus high K-Factor when the customer is away and the heat is turned down. The base degree day should be 55 instead of 65 in these cases but then the people go there for a couple of weeks unexpectedly and they call up screaming that they ran out of oil. The degree day method simply is not accurate in these cases and I believe oil companies should set their reserve close to 50% instead of 25% in these cases or use some sort of low level alarm.

    I did a spreadsheet reverse engineering K-Factors a while back. Using accounts I thought were secondary homes, I tried to set a base degree number based on my opinion that K-Factors remain almost constant but people change the thermostat settings or the "65" number is just wrong in some cases.

    One time when one of the secondary home customers ran out of oil, I was able to document that the K-Factor was close to constant using around a 55 degree number. The period where they ran out of oil, I believe the number was close to 67 using the same K-Factor. The K-Factors were erratic using a base 65 number especially in the spring and fall.
  • ConstantinConstantin Member Posts: 3,782
    Yup...

    As Brad points out: pay attention to the base.

    Some homes don't need any supplemental heating until it gets real cold out. I remember reading about a house in VT which has enough passive solar uptake and mass that it can freewheel through a multi-day blizzard and still not freeze up. It's in the insurance policy now that the place does not have to be heated in the winter.

    Besides, there are many more factor that account for rotten fuel consumption, such as a plant that is short cycling because it's totally oversized for the given load. It's but one reason why modulating boilers do as well as they do.
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