Manual J and equipment sizing
Approaching the final stages of a 60’s ranch major renovation in climate zone 5. We added an approximately 1000 sf great room that will not be served by the existing ducted system, so I would like to use a ductless system to condition it. I have performed a room-by-room Manual J using CoolCalc which I feel should be fairly accurate given my inputs and I’ve come up with 16,725 heating BTU and 13,964 cooling BTU (13,955 sensible and 9 latent). Heating design temp is 14F and cooling is 95F. For infiltration I specified “semi-tight” in CoolCalc (whatever that means), but I am shooting for under 1.5 ACH50.
My first question is, this being my first Manual J, do these loads seem reasonable for a 1000 sf room with cathedral ceilings and a fair amount of glazing (about 100 sf on the South wall and 180 sf on the East wall)? The South glazing is solar heat gain and East is low-e. As for insulation, CoolCalc doesn’t give a lot of options for R-value so what I entered might be a little under, but ultimately we should have approximately R-50 unvented cathedral ceiling and R-30 walls.
My second question relates to equipment selection. I have read the discussions about single vs multi-zone units and would prefer a single wall unit for it’s increased efficiency and range of output. Is it reasonable though to expect a single wall unit to be able to evenly heat and cool the entire space, especially at the extremes of the design temps? The Manual J also calls for 704 cfm which seems high for a single head. All of the estimates that I received to date call for 2 to 3 heads, but they are all grossly oversized too, so I don’t know if the multiple heads are for coverage or just plain oversizing. If it does make more sense to go with two heads, is it still advisable to go with single zone units, or would that fact that both heads are in the same space allow a two zone unit to operate at maximum efficiency?
Any advice is greatly appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part