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.
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.
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.
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.
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.
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