I bought the HVAC-Calc residential version I saw mentioned here because I wanted to get some idea of what variability might be in the load calculations results depending on things like ceiling insulation and so forth.
But I was surprised at the program's BTUH range associated with fireplaces! The program provides 3 choices of fireplaces - Best (combustion air from outside), Average (glass doors, damper), and Poor (presumably a damper only - but not listed). The BTUH figures used by the program are 2543 (Best), 5086 (average), and 15,257(Poor). Since the difference is so large I have to wonder how much variability might there be in the Poor value? I have 2 "Poor" fireplaces (no glass doors) so a 5,000 BTU variance would be 10,000 BTU's out of my home's total 80,000 BTU.
The other big variable seems to be the Infiltration value (I used the "normal" default value), which is about 18,000 BTU out of my 80,000 BTU total. That seems to be something of a guess since there are places to simply plug-in a different value. How much variance might there be in that number?
I'm not knocking the program I'm just trying to get a handle on how much faith I should put in sizing the furnace accordingly when the two biggest contributors appear to have some considerable variability. BTW, the numbers I'm getting from contractors are lower than the program produces by about the difference in the Poor and Average fireplaces.
I'd be interested in anyone's thoughts or guidance. Thanks.