The IBR Method is old. I have a copy of the early method from an Audel's book from the 1950's. It was a good overall method of heat estimation made to make it easy for installers to estimate homes. To eliminate errors by pencil and paper, the IBR method used Heat Transmittance factors ( 1 divided by R ) so multiplication could be done instead of division. With HeatLoss and HeatPro, the estimator doesn't have to do math AT ALL, other than measure the house and use the numeric keypad.
A large problem with the IBR Method was the air changes (infiltration method) which is concerned with whether a room had 1, 2, or 3 sides exposed to the outdoors. As today's Tyvek-wrapped and tightened houses sometimes have difficulty getting 1/3rd air change and might have to require air exchange devices to make up air, the IBR method is usually 'too generous' in its 'just-in-case' estimate. Some estimators add a few thousand extra btuh 'just in case' not realizing the long 'off-cycle' run-time that reduces boiler efficiencies.
It is not too excessive to say that the air coming into a 10 x 12 room is due to the 'number of exposed walls.' But, doing the whole 32 x 52 feet house that way gets TOO MUCH infiltration. As you can read in the comparison below, a commercial building can have a boiler installed that is twice as large as needed by using the method. The IBR Guide H21 warned in the front cover sheet that the method is to be used for smaller homes due to this overage.
However, some people just like to use the IBR Method, so I allow people to download it for FREE with the knowledge that it is not encouraged for buildings larger than 2000 sq.ft. It was written in Visual Basic 1 in 1993 and can recall older files from the black and white DOS era.
Some estimation methods were made to make it easier to do on paper. Converting them to computer methods doesn't make them more accurate, in fact it can 'institutionalize' inaccuracies.
Estimates of a 6000 square foot building with 400 square feet of openings using three methods:
Manual J Line 14 - Added 10% for duct loss 75,706 btuh
HeatPro Quick Method Total 75,674 btuh
IBR Method Total 134,460 btuh
As I made the first distributed IBR method computer program in the early 1980's, I had complaints that the IBR method made over-estimates in comparison to the Manual-J method, so I corrected the program to use R-factors and crack methods of air infiltration methods around doors and windows, As it was no longer the IBR method, the program became HeatPro. It is instead an accurate ASHRAE-formula method that I taught in the Advanced Guide 200 I=B=R course. Today's computers don't have a problem with division, so instead, the R factors can be read right off the blueprints directly and placed directly into the HeatPro program.
