I cannot find any conversion table that translates the BTU/hr heat loss in a Manual J report to the amount of fuel required to heat the building to the design temperature.  In this case I'm using propane and I have calculated the 92,000 BTU/gal using the actual BTU/Hr X Degree Days and end up requiring over 30K gallons of propane for a heating system.  This cannot be correct.  Any guidance would be greatly appreciated.

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  •              Estimated Fuel usage  formula .   Been using for years and has yet to be more than 5 - 10 % off .

                   ( Htg Load x 24 x degree days x correction factor )


                   AFUE x BTUh fuel content x design temp difference

      Correction factor for 3900 Degree days is .660

  • I do a heat loss/gain each "bid"  some times 3 ways.   I like to use fuel usage with a good program. If not use are going to be off.    find a person who is big big and middle age they will take more AC and less heat.   Find a young and fit is outside a lot, will not need as much. maybe 1/3.   If a cap is used in winter and 3-4 layers.  Not much heat is needed.     My 95 yr old mom wants 73-76  My wife wants 75-77   I work outside a lot and plus and look at 65-80 is good. 

     EDS auditor is $120 - 190 yr uses various online public sources.  its kinda close but only if you  twerk it.  For room to room I use writesoft.com.  

  • B-T-U

    The acronym stands for British Thermal Unit, which is the unit used to measure thermal (heat) energy. Specifically, it is the amount of energy needed to raise 1 pound of water 1°F at sea level.

    When used in relation to air conditioning systems or heaters, the measurement is expressed in terms of how many BTUs per hour the product can add or remove from the air.

    • Hi Folks

      Elite Software, in their suite of products (which also includes a full Manual J8 product) has a tool called "Energy Audit" which models energy use very nicely. It is kinda pricey though. I used it a lot when I was wearing a different hat. You might check that out.


  • Hi eric,

    Before we can reverse engineer that peak heat loss number from the manual J (MJ) report we need to know what they included when calculating it.  MJ is noted for over-estimating the appliance size, so has the 51,000 btus/hr number already been inflated (I suspect so) or is the HVAC tech supposed to select a system 25% larger (I suppose not).  If the latter we would be blaming the techs for over sizing and not MJ.

    In addition, the 51,000 number certainly has been adjusted for internal gains, so before we work that number backwards we would need to add back those internal gains.  Between the plus 15% for internal gains and some potentially unknown over sizing adjustment I would have little confidence in working backwards as you outlined.

    @Dan:  Your question is a bit unclear as it sounds like you are asking for the fuel consumption to reach the set point while experiencing the outside design temperature.  In most cases we only look at the total fuel consumption per year.  Let us know what you are looking for.


    • Bud,

      The question was how to translate- from design heat loss calculated by using  ACCA manual J protocol to a yearly fuel usage.   I outlined a crude way to do it.  Is it perfect? Nope.  There are a lot of caveats to it.  Generally speaking when you do it and then also compare it to actual fuel usage, actual fuel usage is lower. 

      You are asking how can you trust the person who did the MJ8 calculations did a good job and used correct assumptions?  I CAN’T ANSWER THAT.  It is certainly easy to  do a bad job of it.



      • Hi again,

        Just to be clear, I wasn't questioning the person who did the MJ work, but I was questioning MJ itself as I do not know if or where those factors mentioned are included.

        There's another distinction between loss at design temp and total heat load.  If they oversize the appliance by overstating the size requirement at design temp that has only a small effect on the total fuel consumption.  Your rough number of 1175 gallons might be overstated by 20%.

        Do you know (I don't) what/how MJ calculates their outputs?

        I'm well aware of all of the equations involved, just relating to the often stated over sizing that results from MJ and trying to get his numbers closer to the real world.


        • Bud,

          I think these 2 articles will help illustrate the difficulties of modeling energy usage.



          The first speaks to the common  practice of  manipulating inputs to add a “Safety Factor” to the calculations. 

          The second shows real world data from different software approaches to modeling heat loss and gain.


          In my own very limited experience, I place the most trust in calculating the information from actual fuel and electrical usage if that information is available.  Of course this is highly dependant on thermostat settings, and occupant behavior, and the efficiency of existing equipment and ductwork  so there is quite a bit of variability in that also.   I use REM Design for determining Heat loss, yearly consumption,  DHW, and utility bill disaggregation  etc.  What worries me about it is that it is a bit of a black box.  I have convinced myself it is OK by comparing it to a spreadsheet  I used to use for calculating heat loss , solar gain and annual fuel usage.  Which I admit is  a dumb way to decide it is ok.

          Generally I find that with REM Design If I measure infiltration and duct leakage, mechanical ventilation, and carefully look at insulation levels, and can figure out what they are using for windows, I can get reasonable results.    I arbitrarily define reasonable as within 20% of values calculated from actual fuel usage. In all cases so far actual usage is lower than calculated usage by ~20% I think that there are honestly too many independent variables to do better than that. 

          I also think that for our purposes, this is probably good enough- on the heating and cooling end of things,  the important part is to quit over sizing things by 300%, when you get down to over sizing by 10-30% which manual J tends to do when applied correctly, there is little performance penalty. 

          In terms of modeling what is cost effective in terms of energy efficiency measures, I   model based on calculated values and then adjust based on actual usage so it is more relevant to the actual occupant. 

          Where I worry most about my numbers is when looking at the effectiveness of heat pumps.  There it seems to me that I should be using binned hourly temp info to determine how often any resistance heating elements will be used.   I do this separately and crudely  in an excel spreadsheet. 



          • "Actual fuel and electrical usage" is anecdotal at best. Bin is the thing and ASHRAE/Manual 'J' have the average weather data relevant to the building site. 

            The operate can cheat the numbers to add what ever he likes, but all the programs I have use in the past warn against it. 

            Burdick says the first step in HVAC design is the load calculation. In residential application this the step most often never taken. Thus the tradition of over-sizing HVAC appliances persists. 

            This is the "how-to" for Manual 'J' and if followed will result very accurate calculations in both new and old building HVAC work. In my own case I have on several occasions built to design and had customers confirm that the thermostats dropped below IDT when ODT was exceeded. 

            Zero over-sizing using Manual 'J' 8 as designed. 

            The only thing to improve a careful Manual 'J' is a blower door to more accurately predict the air changes. 

            • Morgan,
              From the perspective of the energy auditor, I don’t agree with that statement.
              “Actual fuel and electrical usage" is anecdotal at best”
              Unless you change something, the best predictor of future energy usage is past energy usage. Now if you are going to use fuel and electrical usage, you have to have a minimum of one years data, you need to know the heating degree days and cooling degree days over that exact period, (Or binned temp data) And you need to measure the combustion efficiency of the heat source.
              I will try to dig up a paper I have on the subject- the punch line was that identically built and situated houses had a 2 fold difference in energy usage based on occupant behavior. Both have identical manual J heat loss, but use very different amounts of energy. If you are trying to make rational choices based on payback, you have to know the actual usage- which is related to occupant behavior.
              An example- If you are going to recommend a solar pool heater to save money- you better make sure they are actually using their existing electric resistance pool heater. Likewise recommending a high efficiency AC replacement makes no sense if the homeowner hates AC. A customer who keeps his house at 80°F all winter, uses a lot more energy than someone who keeps the thermostat at 55. Some efficiency measures that have a reasonable payback in one situation will not in the other.
              The customer generally does not care about manual J- design temps, etc. They want to know what it is going to cost to heat and cool their house, and what is the best way to do it. Manual J is one tool in the box to help do this.

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