Replies

1. 
   paul_wiedefeld | Aug 08, 2023 08:11pm | #1

   This isn’t directly addressing your question, but would change things: that’s a large heating and cooling load for a new house. Also, a cooling load exceeding your heating load is uncommon in a climate like that. Is there a wall of windows ?

2. 
   moe_wilensky | Aug 08, 2023 08:29pm | #2

   Do you have (or could you request) access to the energy model input assumptions. These might give you a sense of which (if any) sizing factors were used, modeling tools (and sizing approaches in general) apply factors to size equipment above the calculated peak loads, and if this is the case, it might give you more confidence moving a size down. You could also review any assumptions that are driving higher cooling load and which you might have control over, either though design changes or (the cheapest option) how you expect to operate the building (i.e., are the plug and lighting load assumptions in line with how you plan to use your spaces).

3. Expert Member
   Akos | Aug 08, 2023 09:29pm | #3

   In CZ5 that also jumps out at me. The heating load generally is above cooling unless you have way too much unshaded glazing.

   If that is the case, the better solution is to design in exterior shading now, you'll get a much more comfortable place with significantly lower energy use plus lot less glare. Dealing with solar gain with interior shades is a loosing battle once you have decent windows, they only block natural light but not much of the heat.

   Generally cooling load will be much higher for the 2nd floor unit due to typical open concept designs and stratification. The 2nd floor unit tends to do the bulk of the cooling for both floors. Your main floor unit should not be the limiting factor there (again unless you have something silly like a fully glazed west facing sunroom).

1. 
   maxwell_mcgee | Aug 09, 2023 10:44am | #5

   Thanks to all of you -- you're all pointing out something I should have caught myself immediately. (feel dumb that I missed this)

   Yes, there are in fact a lot of South and West facing windows that open into a great room with a tall cathedral ceiling. So I can see why the model is calculating a large cooling load.

   However, the model does not account for the fact that there are a series of tall, mature Maple trees that provide shading over the entire South and West facades of the home. The cooling load is almost certainly overstated. The only risk comes during shoulder seasons like April and October/November when the leaves have started to fall off, but on a bright and warm Spring/Fall day, I'll verily happily open the windows to deal with any thermal issues.

   So I think I will indeed step down to the 18 unit for the ground floor.

1. 
   Tim_O | Aug 09, 2023 12:42pm | #7

   Your energy modeler should be able to adjust the solar gain coefficient based on these trees. It's a little bit of guess work, but if it's a fairly complete shading, a SHGC of 0.1 or less would be fitting, which probably would significantly change the model. My home is also completely shaded by some large mature maples and oaks. The AC is way oversized because of this and hardly ever runs.

4. 
   Patrick_OSullivan | Aug 08, 2023 10:11pm | #4

   Definitely need more details. Overall square footage, "shape", and is any of that basement a walk out?

   Various things jump out, but an 18,000 BTU/h heating load for a basement *really* jumps out. Just to ballpark things, if I take a basement with the following characteristics, I get a 7,200 BTU/h heating load. So it's either a really big basement, or something else is different.

   - 40'x40' area
   - 8' walls
   - R-10 subslab, R-10 walls
   - 70 F interior design temp
   - 10 F exterior design temp, 55 F soil temp, midpoint temp averaged to 32.5 F

1. 
   maxwell_mcgee | Aug 09, 2023 12:20pm | #6

   Thanks -- super helpful data point!!

   The house is designed for multi-generational living and is quite large overall -- about 7500 sq ft conditioned area (5000 sq ft above grade, approx. 2500 in the basement).

   So the basement is about 2500 square feet and is sort of L-shaped. R10 floors, 10' ceiling height. I don't know what the soil temp assumptions are, but the rest of what you're describing is approx right.

   The actual heating load is about 13000 BTU/h for the basement itself, plus an additional ~4000 BTU/h to offset a HPWH that will be located in the basement mechanical room. Most of it appears to be driven by air infiltration (we assumed ACH of 1.0). Mostly driven by infiltration losses. If we drop ACH assumption to 0.6, the heating load drops to about 10,000 BTU/h (plus 4000 for the heat pump water heater).

   I think once you scale for size and for the less than ideal shape (L-shaped rather than square), this isn't too far off your assumptions above? What do you think?

5. 
   big__o | Aug 09, 2023 02:19pm | #8

   I wouldn't recommend undersizing. The mini split will happily cool at 500w or maybe even less if needed. if thats too much it might turn off but when it starts back, it doesnt have a huge startup surge like traditional HVAC units. its very gentle.

6. 
   Andy2022 | Aug 09, 2023 05:23pm | #9

   I'd suggest stop thinking in tonnage and look at the actual heat load charts. compare the output at the design temperature and see which one fits best.

   I went through a similar exercise recently, looking at the SVZ-KP18 and SVZ-KP24 units. I decided to go with the 18k unit as it's output was almost identical to the 24k, but cheaper and energy star certified (the 24k is not). That said I also added a backup heat source for days below the HP's operating temperature (CZ-7, -26C 99% design temp).

   Also, be aware the SVZ-KP18 units specs were updated in 2022 from ~9000BTU at -25C to 17200 BTU at -25C, so older dated documents may not be accurate at your design temperature.