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Mini split: multiple single zone or multi-zone condensor?

krumly | Posted in Mechanicals on

Preface: I’m moving into ac installation on a full-gut renovation of a bungalow in St. Paul, MN. (CZ 6A). Topped the old attic and added a full second story. We now have a simple gabled ‘box’ with extensive insulation and glazing retrofits. Total design cooling load of 23,400 Btuh sensible and 5800 Btuh latent. House plan is very open with a big central stairwell from basement to 2’nd level. Cooling load splits pretty much 50/50 between 1’st and 2’nd level. Basement can be closed off and was considered as conditioned for heating but unconditioned for cooling. I may cool it in the future, as I suspect a right-sized mini split down there could keep it dry at same or less cost than running a dehumidifier.

My heating system uses Myson panel radiators, each home-runned to a 10:1 turndown, natural gas boiler, with discrete TRV’s. I really like the precise temp control across the house. It would be nice to achieve something similar with the cooling

I’d originally planned on installing a 2-stage compressor outdoors with an ECM air handler and ductwork indoors. Air handler and ductwork would all have been in conditioned space. I’m thinking of scrapping that and switching to mini-splits, mostly for efficiency reasons, but also because I’m sick of the build and don’t really want to run all the ductwork 🙂

Looking at multi-zone mini-split condensers, it appears they don’t have nearly the min-max cooling range of single zone condensers. For example, an LG LMU18CHV 2-zone condensor with 2 each 9kBtuh capacity wall units ranges from 10.200 – 19,000 Btuh cooling. A single-zone LG LS090HEV1 condenser ranges from 3,070 -10,500 Btuh cooling. Up-front costs notwithstanding, am I better off buying single zone condensers to equal the comparably sized multi-zone compressor?

Assuming I install a mini-split system, I still want to be sure that 2 smaller bedrooms that don’t justify their own head get adequate cooling. These bedrooms will have ceiling fans. Seems my options are to use standard wall units in a common area and then either leave a door open (not so good) or install a room-to room fan and return grill to move cool air into the bedroom. The other option might be to install low-static pressure ducted units with a custom diverter that feeds part of the supply air into the bedroom and the other part into a common room. I could do this, but it sounds like more of an install PITA. Any opinions on efficacy of either option?

Thanks much.

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