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Community and Q&A

Manual J – Calcs – Mini Splits

Peter L | Posted in Mechanicals on

Zone 4B – 3,300 sqft – 2 story – R10/R25/R60 with R8 windows – open floor plan design

The calcs came in at 3.25 tons heating/cooling. Two ductless minis per floor:
3 – 9,000 Btu Mitsubishi Hyper Heat Pumps – 26 SEER – 10.5 HSPF
1 – 12,000 Btu Mitsubishi Hyper Heat Pump – 26 SEER 10.5 HSPF

$12,000 installed (P&L) for all units – 10 year warranty

I know I can get by with probably 2 Minis (one per floor) but the calcs do NOT include the passive solar heat gain through the south glazing during winter. Plus the county would throw a fit if I don’t have a heating unit in each room as required per their code.

Overall I am relatively pleased and I know they are “over-sizing” the requirements but that’s the requirement by code. If not for the minis I would have to install resistant wall heating units in the rooms (600 watt) to pass code.

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Your post raises several questions.

    Heating load calculations do not consider heat gain through windows because the calculations are made for nighttime conditions, when the heating load is greatest. (The coldest temperatures usually occur right before sunrise.)

    Cooling load calculations should certainly consider heat gain through windows.

  2. User avatar
    Dana Dorsett | | #2

    At zone 4B type 99% outside design temps the output of a 3/4 ton mini-split is anywhere from 8000 BTU/hr to 15,000 BTU/hr depending on manufacturer & model, and your actual 99% design temp. They might be reasonably over sized, but it's also possible that they're INSANELY oversized.

    So rather than tonnage, what is the design loads for each of those zones (and the outside design temp, and our ZIP code to sanity-check that temp with), and which mini-split make/model are they proposing?

    Both the FH09 and FE09 Hyper Heating units have different HSPF numbers than that- the FH is much higher at 13.0 the FE is a bit lower at only 10.0, not 10.5. The FH09 runs an SEER of 30,5, to the FE's 26.0. Either one can deliver over 12,000 BTU/hr @ +5F, and I suspect your whole house heat load is less than 30K, maybe even less than 25K.

    From a cooling rule of thumb point of view you're at about a ton per 1000' would be typical or average for a code min house in climate zone 2 or 3, but the details matter. You may be undersized or oversized for the cooling loads- it just depends. A lot of west facing window area can drive the peak cooling loads sky-high even in high-R houses.

  3. Peter L | | #3

    Spoke to the mechanical engineer who did the Manual J and he stated that the home could easily get by with 2.0 - 2.5 tons total. He said the program always "puts the thumb on the scale" because it was added by Manual J software programmers to prevent complaints/law suits from people. He said people won't complain if the unit is too big but they will always complain if it is too small and they get cold in the winter or too hot in the summer.

    ZIP CODE = 86301 (warm side) 86323 (cold side)

    They designed for the EXTREMES (99F / 09F) but they also ran it for the AVERAGE (91F / 20F) and he said in the end it would save me a few hundred dollars by going from a 12,000 Btu to a 9,000 Btu but he doesn't see the value in it to save a few hundred dollars.

    He stated that most of the homes he is doing now (code min) require 1 ton per 500 sqft. With this home he said it came in at double or 1,000 sqft per ton. It was very energy efficient. He said had the home been code minimum it would have calculated out at around 6 tons for the heating/cooling.

    The units spec are the Mitsubishi MZ-FH09NA. He said currently it is at 26 SEER 10.5 HSPF but the newer model is out and going into circulation and the new model will be 30.5 SEER 13.5 HSPF

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