The value that you need to know is the output rating - the amount of energy that is available for heating or cooling the home. Trane is publishing the BTU ratings of a number of different XR80 and XR80 models on their website - the tables are listed below. You will likely not find your particular model in this list. By ignoring any letters in your model number (which can be found on a label on the side of the furnace) and matching by number only, you may be able to find the correct rating for your model. If you do not have the installers manual anymore, which also contains this information, this is your best bet, short of calling the manufacturer.

For instance, the model TUX060C936D3 is not listed below. However, xxx060x936x matches TUX1B060A9361A, and both are 56,000 BTU models. TUD060C936K4, on the other hand, is a 47,000 BTU furnace, just like TUD1B060A9361A. For determining the total BTU rating of all furnaces in a home, it is necssary to collect all model numbers. Assuming that that the model in the attic has the same rating as the one in the basement does not work. Often times the attic furnace has a lower BTU rating, because the 1st floor has a bigger load (larger living area and higher ceilings, reserve for a finished basement, etc).