the guy swears by it and says it is always pretty close.i know what you are saying about taking everything into account insulation etc,i even called slantfinn and they said they stopped making that programme a few years ago.anyway i am now looking at a hb smith boiler with an indirect tank,boiler co.says the boiler holds a lot of water preventing short cycling which will eliminate an ergomax been used as a buffer tank.what do you think.i know the right way to do it but am tight at the moment.thanks again for your help
How does the guy know it's pretty close- is he providing the post-installation energy use against heating degree days numbers, and demonstrating a very tight standard deviation over a few hundred sample cases as his validation of the methodology?
That kind of calc is pure crap. Yes, those methods are sometimes/often close when dialed for a particular location, if "close" means it's only 2x oversized on average, but never less than 1.5x oversized for the real 99th percentile design condition load.
The Slantfin freebie tool tended to shoot ~35% oversized on average (even more than that for very tight homes), but wasn't terrible. There are better heat loss tools out there. But if you have heating fuel use history on a place with known equipment it's pretty easy to put an upper bound on the whole-house heat loss at a given outside design temperature using simple math on fuel use against heating degree-day weather history, and the steady-state efficiency of the heating equipment.