Replies

1. 
   user-2310254 | Jun 15, 2022 09:50am | #1

   Matt,

   Just curious why the basement load is higher than the main floor load. Basements usually require much less conditioning than above grade sections of a home.

2. 
   Pott0120 | Jun 15, 2022 06:06pm | #2

   I’m not really sure why the basement has a higher load. I don’t have a lot of experience with these, other than my consultant that did the manual j said it was normal to have a higher load in the basement. It is a big house at 2000 ft2 per level. The btu/hr from the manual j seem reasonable from the examples I have seen posted on here.

3. 
   user-2310254 | Jun 16, 2022 08:54am | #3

   Is your consultant an independent HVAC engineer? What he/she told you is the exact opposite of what is typical. Before you do anything, let see if one of the site's experts can review the documents you attached.

   FWIW. Daikin makes great equipment, but you could use Mitsubishi or Fujitsu as well.

4. 
   Pott0120 | Jun 16, 2022 11:37pm | #4

   Yes, the consultant is an independent HVAC engineer. My contractor will only install Daikin so I need to stick with their product line. My HVAC engineer is recommending option 1 but I believe option 2 looks like a better solution. Any advice would be helpful.

5. 
   user-2310254 | Jun 17, 2022 08:10am | #5

   Matt:

   I suspect the engineer is recommending option 1 because it will operate (mostly) without the need for supplemental heat. Heat strips increase operating costs.

   Let me give you a bump since I'd like to know what one of the experts thinks about the proposed systems.

6. 
   AlexD2022 | Jun 17, 2022 02:13pm | #6

   I am not an expert but I believe manual S is very clear that equipment should not be sized over 140% from the load determined from the manual J calculation.
   Keep in mind manual J already has some buffer built into it so an honest (and correct) manual J would most likely give you slightly oversized equipment without having to go over 100% of the expected demand.

7. Expert Member
   Akos | Jun 18, 2022 10:56am | #7

   Looking at the heat loss in the basement, I'm guessing it is not insulated. If this is the case, insulating that can easily cut your heating costs by ~20% so ROI is pretty quick. Even something as little as 1.5" of rated polyiso (ie Thermax) installed on the top 4' of the foundation will make a big difference especially if combined with sealing and insulating the rim joist.

   I don't see an issue with using the non hyper heat unit. With 14F design temperature you have a lot of headroom even during a polar vortex day plus you also have the other unit bellow. The important item is to have a base pan heater which looks like is included.

   For most heating dominated climates, going for a hyper heat tend to be a better option as it matches heating and cooling loads better. A non-hyper heat will generally have too much cooling capacity when sized to carry the heating load. In your case, I think the RXL12 or RXL15 would be better.

1. 
   Pott0120 | Jun 18, 2022 12:56pm | #8

   Thanks for the reply. I can't quite figure out why may basement is higher. It is a 2000 ft2 walk out basement. R10 under slab, R10 foam with insulated 2x4 against foam along the cement walls, R40 double stud on any exposed walls, and R40 closed cell on the rim joist. I tried to review the data by room and I just don't understand the input to determine if it is correct. If it would help, I could attach my plans or the Manual J/S. I asked the designer if it was a normal load and he said it was very common to have the bsmt require more heat. Is it the infinite thermal mass swaying the data? The more I look at the data I am more convinced the RX18 for the main floor is a good choice. You are correct the cooling loads are smaller relative the heating loads and it is difficult to get the sizing aligned.

   How well does the wright Manual J software account BTUs moving back and forth between two zones? Will I be able to run my house on one heat pump most of the year just through thermal transfer between my basement and main floor?

   I looked at the RXL models, they appear to not have the low-end range when cooling compared to the RX models. It appears matching heat pumps is a balance of compromises. My cooling on the main floor is 11747 btu/hr and lower is 8663 btu/hr.

   Thanks again for everyone's thoughts and input.

1. Expert Member
   Akos | Jun 18, 2022 03:44pm | #9

   Based on that level of insulation, there is something off on the Man J. Most likely the infiltration is set too high. If you have the detailed man J post it up and we can see if something seems out of place. Transient effects from structure heat capacity is not part of standard design so it won't effect the man J, more of something you need to look at for a passive house with a lot of solar gain.

   If you 2000 sqft of walkout with same above with R40 walls at a 14F design temperature, I would not be surprised if you can heat the whole place with a single RXL18.

1. 
   Pott0120 | Jun 18, 2022 09:56pm | #11

   I may have figured out the error in the calculations. Bedroom 2 is a walkout basement with a double stud R-40 wall which is the same as the wall above in the master bath. Bed 2 outside wall appears to have 1311(593+718)BTU loss and the exact same wall exposure above has 188(43+145)BTU loss. These value should match exactly since it is the same wall assembly. Half the basement is walkout so I should drop significantly on the BTU/hr.

   How can I tell if the basement walls that have cement behind them are calculated correctly? R10 foam + 2x4 with blown fiberglass. Looks like he is using a U=0.088 which is R11.4. Doesn't seem right but maybe I am missing something. Should it be closer to R17 and a U=0.059?

   Do you see anything else that sticks out?

   Thanks!

1. Expert Member
   Akos | Jun 19, 2022 12:07pm | #12

   12" dense pack fibergalss double stud wall is closer to U 0.023 but that won't change much.

   The basement does look too low, should be around U 0.042.

   If you are building a house with that level of insulation, I would assume you can safely hit less than 2ACH@50PA.

8. 
   Pott0120 | Jun 18, 2022 08:20pm | #10

   Attached are the reports. Let me know what you see. I tried to find errors and I am just not familiar enough with Wrightsoft to spot them. Thanks!