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Calculating savings with degree days

NYplumberNYplumber Member Posts: 482
Good afternoon wallies,

Trying to calculate a clients savings with their new system.

Off the bat I see savings of +8% with the cold weather. I have ccf usage, thermal factors and dates. Fuel is NG both before and after upgrade.

Help would be appreciated.

Thanks in advance.


  • Jamie HallJamie Hall Member Posts: 6,653
    The number you are looking for

    is ccf/day/degree-day.  Depending on what you have for input, there are a number of different ways to calculate it.


    You may find -- as I have -- that there is a tremendous amount of variation (in my case a good deal of it is wind -- but not all).  You may also find that it isn't quite the beautiful constant relationship that we have always been led to believe it should be (mine isn't -- the building uses a good bit more fuel per degree-day on cold days than it does on warmer ones, and I haven't figured out why yet).

    There's also variation from your client possibly playing games with the thermostat?

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • GordyGordy Member Posts: 7,111
    edited January 2014
    Btus per degree days sf

    Is what I have been using in a two year daily calculation in my own home. I personally have found in my situation that it's pretty much linear +\- 1/2 to 1 degree day per sf. Through out the OA temperature range. Exceptions in those ranges of deviation in my case would be the usual solar gain, wind over 15mph, and ventilation through exhaust fans or masonry fireplace usage.

    In my case with my home the number is 9.5 HDD per sf. I have taken that number in the past, and extrapolated what my btu usage would be in a 24 hour design day scenario which until this last cold snap that happend was hypothetical. It proved to be right on when I actually did experience a 24 hour period of design conditions.

    I also just today took the total heat call time for the year, and did a calculation off my gas bill years usage to date deducting for my water heater, and came right in line reasonably,with daily calculations.

    It leads me to believe that what we all know is true that heat loss calculating software is padded.

    So from my own experience I have faith in that method IF you have an accurate way to know the actual burn time of the boiler, and the btu output in the 24 hour period With a mod/con this will be difficult with varying firing rates. With a CI boiler it's much easier.

    Then to do it all for a past customer makes it even more of a challenge.

    I think I would give them the formula it's not real difficult as long as the inputs are not garbage let them calculate, and report back.

    Personally I think boiler manufacturers could benefit by having that info in the boilers software would not be hard to implement, and the owner could see the benefits have it log daily weekly monthly yearly, and resettable, and log usage for the life of the boiler. Just a thought.
  • gennadygennady Member Posts: 677

    In my understanding properly sized boiler will run non stop at design degree hour. So having heat loss done and knowing how much BTU is lost at these conditions, I multiply total hours of degree day per year. I get these at , then I get fuel usage per year based on those numbers. Then i deduct hot water fuel usage from customer bills, and get annual savings with new equipment.
    Gennady Tsakh

    Absolute Mechanical Co. Inc.
  • GordyGordy Member Posts: 7,111

    So you offer a projection to the customer. Any feedback as to how your projected savings applied to real life?
  • gennadygennady Member Posts: 677
    savings projections

    I usually under estimate savings. Real life savings sometimes surprise me as well. There are more factors affecting savings other then just HDD calculations. And those factors are really impossible to predict. Main factor is ability of the boiler to follow load without overshooting. Proper set up of PID, where access is available, becomes essential. Another factors in modulating condensing boilers is avoiding boiler stopping and short cycling. Just my 2 cents.
    Gennady Tsakh

    Absolute Mechanical Co. Inc.
  • chapchap70chapchap70 Member Posts: 130
    Jamie, if you are still watching this thread...

    I don't know the type of building you are referring to or how you calculate fuel used per degree day but if it is commercial with high night time set backs, you may get better results in your calculations if you change the baseline for degree days from 65 to a lower number.

    An example for calculating oil deliveries; A new account autobody repair shop keeps the building at 59 instead of 69 of the average home. The average temperature for a time period is 51 degrees. In calculating degree days, a K factor (With oil it is degree days per gallon used.) is determined based on 14 degree days per day instead of 4 degree days per day. The determined 65 base K Factor is of course too high. In the next time period, the average temperature drops to 30 degrees. If the oil dispatcher does not correctly account for the lower building temp/incorrect K Factor, the tank runs out of oil.

    Conversely, if the K factor is determined when the average temperature is 30 degrees, and the next period temp rises to 51, the second oil delivery will be way less than what was projected.
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