Manual J questions for a New England Cape
Hi guys – Long time lurker and fan of this website & Martin’s books, and I’m finally ready to ask a Manual J question to you guys (and hopefully contribute as I learn more!).
We live in a 1960’s 2 story dormer cape near Hartford, CT (1530 sq ft conditioned + unconditioned basement), and my wife has been asking to have real “A/C” for a few years. With our oil furnace getting long in the tooth, I figured a mini-split might be a good future proof option as we gradually go greener. Next step is Manual J! I modeled the house in a weekend with the iPhone, a tape measure, & MagicPlan (what an amazing & free app!), then entered the details into CoolCalc.com’s Manual J (MJ) calculator.
The heating results are close to what I expected when compared to my oil bill’s k-factor (38kBTU with 8.04 Oil K-factor vs 34kBTU CoolCalc ManualJ). Calculating cooling loads is new for me, since we have only used window units for spot cooling rooms in the past.
1. How should stairs get modeled for Manual J? Do I split the area for the first floor (Zone 1) and 2nd floor (Zone 2)?
2. Is warm air buoyancy accounted for if upstairs doors are left open? (less heat 2nd floor and more A/C). If not, should I adjust for it?
3. Do I model the original brick fireplace wall area as a window pane since the R-value of brick is poor, or is that overkill and should be ignored?
4. How should infiltration be modeled for both zones? Do I take our 1610 cfm @ Pa50 and divide by 2 for the upstairs and downstairs zone (since stack effect goes across both zones)? Or allocate by floor area %?
5. How should basement wall and sill plate insulation be modeled (since it’s not a conditioned area, but would affect heat load on the 1st floor). It’s currently not insulated at all, but is on my wish list. We’ve got 10″ thick concrete walls, 5′ below grade with 2′ above and five ancient single pane steel framed casement windows.
6. Dana often mentions a high A/C latent heat load in New England, but the CoolCalc.com calculations have my SHR at .904 downstairs and .84 upstairs (88°F design temp). Is that in the ballpark? I don’t know what to expect…
7. Should I bother doing a ManualJ for the basement, with a 55deg inside temp to see how that affects the whole-house load? Would that be included to the first floor because of conduction (hardwood floor over plywood subfloor).
Thanks for sharing the wealth of knowledge here! I’ve learned a ton on this journey, and am looking forward to getting our house more efficient.
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