Replies

1. 
   Trevor_Lambert | Aug 28, 2019 03:31pm | #1

   I obviously over-simplified this, having read Dana's detailed response.

2. Expert Member
   Dana Dorsett | Aug 28, 2019 04:09pm | #2

   The turn down ratios and minimum & maximum output temps vary with indoor and outdoor temperature. The ARHI test at +47F outside 70F indoors is the typical temperature point at which it's discussed. The max output at +47 is usually a multiple of the design BTU/hr requirements of some lower temperature in cooler climates, but comparing the minimum output at +47F to the heat loss/load at +47F of the space being heated is one way to get a handle on the outdoor temperature at which the compressor begins to cycle on/off rather than modulate.

   As a general rule, a bigger turn-down ratio yields higher efficiency and comfort when right-sized for the loads, since the compressor & blower don't have to spin up from zero as often. But depending on compressor type and system design it's sometimes more efficient to cycle rather than modulate with load at low loads.

   In the ARHI testing "rated' output is the modulation level at which it's efficiency is tested, not it's max output @ +47F. The unit has to be able to deliver at least that much at +17F at it's maximum modulation level at +17F, but isn't necessarily it's max output at +17F either.

   For instance according to the AHRI submittal sheet LG's 3/4 ton LSN/LSU090HSV5 has a max output of ~17,061 BTU/hr @ +47F, and a minimum of 1023 BTU/hr, nearly a 17:1 turn-down ratio.

   But it was tested for efficiency at a nominal or "rated" 10,900 BTU/hr. At +17F it can still deliver 11,080 BTU/hr, a bit higher than it's "rated" output at which efficiency was tested:

   https://refripartes.com.do/uploads/LSN090HSV5,%20LSU090HSV5.pdf

   But note that it's COP efficiency at only 1023 BTU/hr is a paltry 1.5, according to data they supplied to NEEP:

   https://ashp.neep.org/#!/product/25836

   But with a different compressor design their LAU/LAN090HYV1 3/4 tonner with nearly identical Rated capacity and identical minimum capacity (and a ~20:1 turn down) delivers a COP of 4.28 when dribbling out 1023 BTU/hr @ +47F:

   https://ashp.neep.org/#!/product/25817

   So it's not as simple as focusing on the unit with the lowest modulated output rate or biggest turn-down ratio. This is the same vendor, different model, where one has nearly 3x the efficiency under light load at temperate outdoor temps.

1. 
   charles3 | Aug 28, 2019 07:27pm | #3

   Wow, you really have to read the specs carefully.

3. 
   Jon_R | Aug 28, 2019 08:28pm | #4

   > it's sometimes more efficient to cycle rather than modulate with load at low loads.

   And quite often cycling provides better comfort in terms of humidity (because the fan doesn't modulate to match the compressor).