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Guidance calculating room CFM requirements based on heat load

kermit49 | Posted in Mechanicals on
Hello people.
 
Background
New home construction. Zone 3 southern Utah, 2100 sqft, one level. CDD 4293, HDD 1545 (1/31/2022 Las Vegas station). Manual J load calcs 14,850 heating, 16,730 cooling. House, tight and well insulated (R27 double stud/60 attic), all electric, E/HRV, all in conditioned space.
 
Manual D suggests proportionally dividing all rooms cfm by percentage of maximum system blower cfm using HF and CF factors and applying it to each room’s heating/cooling load so as to divvy up the blower cfm. 
 
This technique works fine, but makes each room’s cfm totally dependent on maximum blower output. Selecting a big blower yields higher room cfm than a small blower. I can see using this is you are stuck with a particular blower size and need to proportionally size up all rooms.

My ask.

Seeking guidance calculating room by room heating and cooling air supply CFM based on room load requirements.
 
I would like each room’s heating and cooling cfm calculations to be based on actual load. After summing all rooms, I can select a blower size that matches the sum of the rooms.
 
I know room (cfm) = room load (Btu/hr) / dT x 1.08
 
Cooling dT. Have read that for mini splits dt = 20-24 F.  If true am good with cooling.

Heating dT. Once running in steady state, can I assume dT = return air F at intake plenum – supply air F in supply plenum?
 
Ducted mini splits on my short list are the Fujitsu ducted 9k and 12k. 
 
For reference I see that the Fujitsu ducted 9k, rated specs (353 cfm, 12,000 Btu/hr)  dT = 31 F.  On the 12k dT = 39 F
 
Thank you 

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Replies

  1. kermit49 | | #1

    Just saw a post by Dana where he described how, in heating mode, output temperature varies quite a bit due to outdoor temperature and fan speed. Makes a lot of sense. http://www.nrel.gov/docs/fy11osti/52175.pdf

    So it looks like the using the Fujitsi 12k rated specs, see my question above, yielding a 39 F dT is real ! In reality it will vary, as the NREL.gov charts point out, but this is good enough for design purposes.

    If am wrong, someone please let me know. Feel free to enlighten me further.

    Thank you Dana for your clear explanation and link. Dana's comment was posted at https://www.greenbuildingadvisor.com/question/output-air-temperature-of-mini-splits-on-heat

  2. DC_Contrarian | | #2

    Do the ducted minisplits you're looking at even give you a choice of blower?

    The usual process is to select a blower, it will have a maximum allowed static pressure, you design the duct system to not exceed that static pressure, and to deliver to each room it's correct portion of the air flow. So long as you have sufficient air flow what's important is the relative share of airflow to the rooms. If you don't get the relationship right some rooms will be overcooled and undercooled.

  3. kermit49 | | #3

    Contrarian, yes the Fujitsu 12k has settings for 4 fan speeds, 283 to 383 cfm.
    Static pressure at maximum setting (383 cfm) is 0.36 iwc. My layout's ducting system (supply & return) calculates way under that static.

    The actual room by room heating and cooling supply air flow (cfm) calculated by room heat load only adds to to 282 cfm. That tells me the blower pressure setting would be set at 283 cfm, Fujitsu calls that the QUIET mode. By comparison, the HIGH mode puts out 383 cfm.

    The individual room flows based on room load directly correlates to room percentages generated using Manual D HF and CF. Both Manual D percentages and my calculations, based on room load, are side by side in my design spreadsheet. Easy to compare.

    Because the Fujitsu 12k can ramp up another 100 cfm, I wondered if the ducting should then be sized to the higher flow? That would either increase my design velocity of 400 fpm or indicate the duct diameters needs to slightly increase. Will compare both scenarios. Not written in stone that velocity needs to be 400 fpm throughout the system. Our target is a reasonably quiet system that feels like a slight breeze. High velocities are not the ticket.

    Appreciate your input and concern Contrarian. This is all a learning experience and member input and opinions are what zero experience people like me need.

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