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Community and Q&A

Single Fujitsu 12K on AOU24RLXFZH Multi

kjmass1 | Posted in General Questions on

In a multi-head set up where only one wall unit is primarily used for heating (other unit is in 3rd floor attic), is the max output at 17F and 5F reduced even though the exterior condenser has a max 25K BTU available at 5F? What can I expect for an output at 5F?

I used 1150 therms in the ’16-17 winter months with an old steam boiler at best 82% efficient. With 5332HDD, I came up with an average of 790 BTU/degree hour. If I remove the 33% pickup factor maybe I’m down to 530 BTU/degree hour in my conditioned space of 1900sqft outside of Boston. 

Just curious how much I can expect from that head on the first floor to carry the load at lower temps in conjunction with the radiators. That master spreadsheet was very helpful. 


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  1. Expert Member
    Dana Dorsett | | #1

    The active head has to have the capacity to deliver the 25,000 BTU/hr for the system to deliver 25,000 BTU/hr. As long as the compressor still has the capacity, the heads will deliver their nominal output numbers at cooler outdoor temps.

    If you stick three 7RLF heads on an AOU24RLXFZH the most any ONE head will deliver would be the nominal 8100 BTU/hr (but three would deliver 24,400 BTU/hr).

    If there were a 15RLS head on the first floor with one (or two) 7RLF(s) on the other zone(s) the 15RLS would deliver about 18,000 BTU/hr, as long as they all weren't calling for heat at the same time. The combined capacity of a 7RLF + 15RLS is slightly more than the capacity of the compressor, but 2x 7RLF + 15RLS would be quite a bit more capacity than the compressor.

    The most you'll get out of a 12K head is 16K:

    To get the most BTUs out of it, size the heads proportionally to their actual loads. A 7RLF head in a bedroom with a 2000BTU/hr load is going to just cycle on/off all season, even if the other zones aren't keeping up due to being undersized for their design loads.

  2. kjmass1 | | #2

    So an ASU12RLF1 running on the non hyper heat multi 24k would output 13k at 5F no problem? Even with defrost cycles?

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