Low CFM of Concealed Slim Ducted Minisplits

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How do people deal with low CFM of concealed/slim ducted minisplits?

Most slim duct or concealed ducted air handlers put out max 350cfm, whether its a low static or high static model. These are often recommended to use for multiple bedrooms so as to not over size with multisplits. But as an example, if I want to use a 9k or 12k air handler to supply 3 spaces, each needing 3kbtu/h according to manual J, each room will get max 100cfm, and only from 1 supply register, and likely not in the floor, usually high up in a wall. Is that CFM enough? and if not, what are the solutions?

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Replies

1. | | #1

CFM is calculated based on air temperature delivered and the load. If the unit is rated at 12k BTU and 350 CFM, it is likely delivering 12,000/3.5 =~ 3400 BTU/100CFM. You can back calculate air temp from there and see if it's reasonable. BTUs = CFM x ∆t x 1.08. So a temp rise of about 28*F yields 3k BTUs at 100CFM. 100*F air from a mini split sounds reasonable.

The thing to be careful about with the ducted units is the really low static pressure. Running it to only a few rooms it should be fairly easy to keep the duct losses down.

I guess one thing I have wondered about using units like this in a fairly low load per room situation, how important is the supply register location? With a furnace, you often try and put ducts on outside walls, washing the windows. In a high performance house with low air flows, I'd imagine this is not that important. Helps save some duct losses.

2. | | #2

100 CFM is fine for a room with that heat loss. Very typical, a furnace would often handle that load with a much lower CFM.

3. | | #3

Good to know its enough. The 3 spaces are all adjacent to each other so the ducts will be literally be an elbow worth from plenum to register - or, if I can figure out how to mount the register boot on the plenum box, skipping the ducting - and I don't know if code prohibits that. I do have a 4th space another 3k I'd like to duct to but thats 20' away so I don't think its worth the effort with such low divided CFM and heat loss too low for another minisplit, and will just have to use a baseboard heater there.

4. | | #4

I have three of these slim ducted minisplits. Energy Vanguard designed the ducting system and the contractor installed it per their design. Air distribution part works awesome - very comfortable.
Largest unit has 6 supplies
Smaller unit 1 has 4 supplies
Smaller unit 2 has 4 supplies
The mini split runs constantly so there is usually conditioned air coming through. They did design most of the supplies to be at the perimeter or close to it.
One supply per room, including walk in closet, corridors, alcove and a small loft area

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