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Calculating Heat Loss Using Past Usage

JofG Member Posts: 3
(Re-posting here. Inadvertently posted in wrong forum)

I am in the process of converting from oil to to natural gas for home heating and hot water (I live near Philadelphia). I have decided to go with a high efficiency mod/con boiler but have received wildly divergent estimates for sizing from potential contractors (59k to 211k!). I have attempted to estimate the heat loss but am not confident in my estimates because I am missing accurate insulation information.

It hit me that I should be able to do better than estimate the heat loss like this by finding my heat loss from actual usage. I know exactly how much oil I have used and the number of degree days for this period. I can reasonably estimate the efficiency of my old oil boiler (Columbia FT-6165C) and make a guess at how much of the usage is for hot water (I have a summer/winter hookup). Worst case I can ignore the hot water and get an upper bound estimate.

Listed below is the information from the spreadsheet I created for this. I have never done anything like this before and want to know if I am missing something? Also am I correct in assessing that this is actually better than a traditional heat loss calculation since it is measuring my actual usage?

Thanks to anyone who can shed some light on this.


Measured Heat Loss

The tank was full on 12/30/2010 and then again on 3/5/2013. So the oil delivered between these dates is exactly what we used.

Total Gallons Used..............2,921

BTU's Per Gallon.............140,000

Gross BTU's.............408,884,000

Boiler Efficiency.....................0.82 (estimate)

Net BTU's.................335,284,880

% BTU's For HW...................0.05 (wild estimate!)

Heating BTU's..........318,520,636

Degree Days.....................11,206

Degree Hours.................268,946

BTU's Per Hr Per Degree...1,184

Delta T For Coldest Hour........54 (68 - 14)

Max Heat Loss Hour........63,954


  • heatpro02920
    heatpro02920 Member Posts: 991

    That could be a good start but a real heat loss done by the installing contractor is the way to go...

    How many sq ft is the house?

    How old is the house?

    Updated windows and when?

    Drafty or pretty tight?

    anything out of the norm, a lot of big windows, cathedral ceilings, irregularly high ceilings, ect?

    Finding the insulation factors is pretty easy, the attic is easiest, then with the outside walls you can usually remove a small piece of base molding and make a small hole behind it and check out whats in there, I ussually take a light switch cover off and send in the camera over the box where the cover will cover any hole you make... If you have 2x6 construction and there is pink stuff in there with a house built in the last 20 years you know you are r19-r21 area, 2x4 construction in the last 30 years r11-r15.

    I have a friend of mine that is also a contractor that figures out his heat losses with the existing systems, he puts a timer on the unit for 24 hours, then takes the outdoor temp, indoor temp, and figures the btus by the amount of time the unit ran and the units firing rate, I like to do a manual J personally...

    I did a building calc today, 1980sq ft, 2 floor colonial, built in 98, with a good amount of glass {2 sets of glass french doors in the rear, huge single entry door with glass sides, 2 bay windows in the front, 4 windows in each bedroom, even had a couple windows in bathrooms and closets...} - my heat loss {using 0* outside temp and 70 inside} came to 47K BTU. I sold them a Weil McLain CGs3 they currently have a 140K BTU oil fired Burnham. But I put my name on it, he was really concerned about such a small boiler but I told him I don't want this boiler to shut off when its 0 degrees out, that will mean I did my job 100% and hes getting the most for his money...
  • JofG
    JofG Member Posts: 3
    Still Wondering

    if my usage based approach is correct and complete. Your comment that it is a "good start" confuses me. As I have thought about this it seems to me that this approach (assuming reliable usage data) is actually better than the traditional heat loss calculation because we are examining exactly how the house behaves rather than estimating how it should behave given its characteristics.

    As a novice at this though, I am wondering if I am missing something (either theoretical or practical) in my calculations that would invalidate my conclusion. If not it hits me that the calculations indicate that my house, regardless of (or my precisely because of) its square footage, number of windows, exposed walls, insulation values etc., loses 1,184 BTU's per hour per degree. And the only assumptions embedded in that value are the efficiency of the current boiler, the proportion of the heat being used by hot water and I suppose the validity of using 65 as the degree day reference point. But even then, I could adjust the boiler efficiency to 1.0 and the water usage to 0 and get an upper bound of energy usage. (Doing this raises the heat loss to 1,520 BTU's per hour per degree and 82,097 for my max hour load).

    So my original question still stands, "Am I missing something here?"

    Also, I am looking at the Lochinvar Knight series of mod/con boilers with a matching 40 gallon indirect water heater. If my calculation of approximately 64k for heat load is correct, do you think that the KBN081 (input of 80K) is big enough or is this cutting it too close? The next bigger model is the KBN106 (input of 105K).

    Thanks for your thoughts.
  • JohnHenry_2
    JohnHenry_2 Member Posts: 70
    edited March 2013
    your approach

    does account for passive solar heating which sounds to me to be good for overall yearly energy use but will probably skew your actual heat loss numbers down from what they actually are, especially at night.
    The bitterness of poor quality remains long after the sweetness of low price is forgotten.
  • JStar
    JStar Member Posts: 2,752

    I work around Philadelphia. I can offer you an eatimate and proper heat loss of your home. We use ACCA approved Manual J software that doesn't leave us to assume anything. That's the only way to know what numbers you truly need to look for.
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