Target CFM for a mini-split
ScottG
| Posted in General Questions on
We are doing a full renovation of a 3-story stone carriage house (built 1903) outside Philadelphia. 2000 ft2 per floor w/ 9.5′ ceilings on main floor. Due to ducting difficulties, I’m hoping to use a multi-zone mini-split (Mitsu h2i) to condition the bottom two floors. Based on my load calcs, the best match for my capacity and zones is a single 8 zone unit (I’m using mostly ceiling cassettes).
Following the advice of this forum, I made sure not to oversize the outdoor and indoor units w.r.t. to capacity. But, when reviewing the design w/ a potential HVAC contractor, he targets 4 air turnovers per hour for the blower. Though my design achieves this for six zones, due to the high ceilings and above-average envelope, I have 2 zones that come in below that: 2.5 and 3.0 ATH (max fan, heating — worse for cooling). Both zones are open floor plans.
To meet this requirement, I would need to upsize the IDUs. Not only would this lead to an over-sized IDU, but it cascades to the outdoor unit .. and then I’m oversized here as well.
Before I work too hard trying to achieve this, I wanted to ask: is there a “best practice” about how to size the blower based on the volume of the zone? If so, does it differ between heating and cooling? A related question is what classifies an indoor unit as “over-sized”. The Mitsu app note says 50% of max capacity.
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Replies
This sounds like he's using a rule of thumb.
My understanding of Manual D is you figure out the heating and cooling load for each room and size the ductwork to provide enough air to each room to meet its loads. You want the system to be roughly balanced. Providing every room with the same ATH defeats the balancing.
A rule of thumb for airflow is 400 cfm per ton. A rule of thumb for sizing AC is 600 square feet per ton. Put those two together and you get 40 cubic feet per hour square foot; with 10' ceilings that's 4 ATH.
This article talks about the evils of rules of thumb:
https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb
I remember a Corbett Lunsford video where he interviewed an IAQ expert and my notes from that episode say:
Shoot for 0.5 air changes per hour of “fresh” air which is any combination of circulating and filtering indoor air, diluting with outdoor supply air, and portable/room-based air purifiers.
So I think 4.0 ACH is more than you really need.
8 zones is over the top.
To my ear this sound a poorly designed system.
Will the top of ceiling cassettes and any ductwork be in the unconditioned attic space. That would be a bad idea.
8 cassettes sound like they stuck one in almost every room.
I think the multi splits are a bad idea they don’t modulate well. Sooner or later every system develops a leak and a system. The 20 or so field made connections are the most likely joints to leak. There is no easy way to isolate parts of the multi split system and narrow the search.
Consider making the space on each floor for a ducted miny split system. If you lower the hallway ceiling a foot, would it really be noticeable?
Note almost every unhappy post about mini splits turns out to be about an oversized multi split system.
Walta