We are about to replace our heat pump system.
I've had a bunch of contractors out, most of them sales reps but a couple with technical/repair backgrounds, and not a single one has brought up the subject of "Manual J". In one case, when I asked about getting one, the guy did not know what I was talking about. They all seem to assume that if a 2 ton system is in here now, that's what the replacement needs to be.
I've downloaded a bunch of spreadsheets to try to do calculations myself.
Some versions are easier to use than others, but they all seem to show a 22- 24KBtu heat loss. I'm getting different figures for the heat gain but the best estimate seems to be about a 12KBtu morning (AM) heat gain and ~24K Btu PM heat gain. That's with a fair amount of detail on interior appliances, window orientation, etc. My experience is that, yes, the heat gain in this house does increase greatly in sunny afternnoons.
Are there any particular "gotchas" that a homeowner should beware of in these do-it-yourself calculations? Or are there so many that I really need to try to find a contractor to do it? Some of the data points ("dry bulb" this and that) in some of the sheets require more technical knowledge than I have.