Replies

1. Expert Member
   Akos | Apr 12, 2022 10:17pm | #1

   https://ashp.neep.org/#!/product/51515/7/25000///0

   Turndown doesn't look all that good on those. but this data could be wrong. Daikin support is your best bet for accurate answers

   Generally with multi split, best is to avoid installing small heads on them as they can be running a good part of the time bellow the min output of the outdoor unit. When this happens the outdoor unit would be bypassing refrigerant through all the zones even if off which can create overheating or cooling of zones and efficiency is pretty low.

   Even with larger heads, you might only get a 2:1 turndown on the indoor unit. The overall turndown depends on the exact configuration which is surprisingly hard to find.

2. 
   KKlouzal | Apr 12, 2022 10:58pm | #2

   Thank you for that website, it does give some insight into specific configurations. I was hoping to find the maximum supported turndown ratio of the particular heads regardless of whole system configuration, the 'theoretical turndown ratio' if you want to call it that.

   The RXTQ48TAVJUA for example supports a maximum of 8 indoor units. So if for instance I wanted to put 8 of the 7500btu heads on and 7 of them were running full blast but the 8th only needed to be on the lowest possible setting just shy of shutting itself off, what would that btu output be?

   I do understand that refrigerant will still run through the heads of a small system if the heads that are running don't equal at least the minimum turndown ratio of the outdoor unit, which for the RXTQ48TAVJUA is 14% or 6720btu.

   That being said, in my theoretical configuration above, with only a single 7500btu head running, the outdoor unit could only turn itself down to 6720btu.
   How about 2 heads, would they be running at 3360?
   Could only 3 running heads turn themselves down to 2240?
   Can 4 heads get down to 1680?
   5 heads down to 1344?
   6 down to 1120?
   7 to 960?
   Could all 8 heads running at their lowest capacity get all the way down to 840btu?

   I want to be able to plan for the edge cases based on specific room placement and cooling needs and the turndown ratios of indoor units is necessary to accomplish that.

1. Expert Member
   Akos | Apr 13, 2022 09:34am | #4

   Generally that is not how it works. When each head comes online the min modulation also increases a bit.

   There is also limited times when you get overlap as you suggest. The more likely scenario is one or two heads calling for heat at the same time. These will be running at near full capacity to meet the load so they'll hit the setpoint right away. By the time the other zones call for heat, they will be off thus this full on/full off cycle repeats.

   To get a multi split setup working properly, each head must be matched to the zone it supplies. So a 9k head, needs a near 9k zone, you can't rely on modulation for matching.

   If you want some more zoning, you can look at a larger ducted unit with an Airzone setup to feed the smaller rooms.

1. 
   KKlouzal | Apr 13, 2022 10:44pm | #6

   I agree with you 100% you need to match the head to the room as close as possible 9/10 times.

   Say you have two rooms, one requires 5000btu and the other requires 4000btu peak cooling capacity. Now in this scenario I believe you have two choices, place a 9000btu head and duct it to both rooms OR place a 5800btu head in each room that has the capability to modulate down below peak requirement.

   Now you have to worry about short-cycling. Having the modulation rate of the individual heads on hand can help you make a more informed decision whether or not to go with the one larger head or two smaller heads.

   Maybe this system was also designed so that the connecting hallway, which requires 1500btu peak cooling capacity, be fed from these two rooms as well. This could further work to help offset short-cycling, or during the hottest days could even work against you and result in the two bedroom heads being slightly undersized.

   Maybe too the building was designed to have a constant 50cfm of air circulation from ALL its rooms via a separate hrv/erv system meaning there could be some rooms slightly undersized and some rooms slightly oversized, basically giving you a small BTU buffer in sizing the zones. In this scenario, each room is its own individual zone, however the entire building is connected via the ventilation system and constantly circulating effectively allowing all the heads to work in tandem to some degree.

   There are just too many variables to calculate, and not having an important variable like turndown ratios on each individual head clouds your vision so to speak. Why does Daikin withhold this data? Is it their 'secret sauce' or maybe their ratios are horrendous? I'm afraid we may never know.

   Sorry, I've been over analyzing, crunching numbers, peering through engineering datasheets, and chasing down what-ifs for what feels like an eternity now.

2. 
   jameshowison | Apr 14, 2022 09:09am | #7

   Any links to an AirZone case study? Anyone know what the installed cost of these is? Don’t seem to be prices online.

   An article on damper zoning with vrf including AirZone and a comparison to multi-split woes would be a great article for GBA.

3. 
   KKlouzal | Apr 12, 2022 11:19pm | #3

   Maybe the information I am looking for is actually with the branch provider box, each branch has it's own electronic expansion valve (I thought the indoor heads had EEVs too) so potentially the refrigerant is limited at the branch box and not the head.
   I however am unable to find an engineering manual for the branch boxes...how convenient..
   BPMKS049A3U

1. Expert Member
   Akos | Apr 13, 2022 09:35am | #5

   If you do find the proper info, please post here. This type of information is near impossible to find.