Comment on HVAC plan?
I’d appreciate feedback on this HVAC plan.
Manual J via CoolCalc, was fairly accurate in orientation and entry (including internal shades on south and west facing windows). No futzing with design temps (although I question whether 98° F is the 95% percentile anymore for Austin TX, hard to see how that could be when we have months with the high above 100°F). Walls are concrete block and uninsulated, slab on grade, blown in cellulose to R-38 going in the attic.
One key element is that the master bedroom and the front bedroom are used as home offices (COVID, right?), so quite concerned to ensure that when sun loads hit western side of the house in the afternoon, the master suite isn’t over-cooled.
Cool Calc says 33k cooling, 29k heating. House is 1,650 sq feet. Room loads, and layout pic here: https://miro.com/app/board/o9J_lf8BTWg=/
Ducts in attic, unfortunately, hope to mostly buried underneath R-38 blown cellulose. I’m promised commissioning leakage and CFM delivery tests. I did enter ducts in attic in CoolCalc, but all equipment also in the hot attic, didn’t see a spot for that.
In addition a UltraAire 98H for managing humidity. To be largely delivered into bedrooms, as this is also fresh air ventilation. Treating this as adding a small cooling load (but not sure how to account for it).
Outside: MXZ-4C36NAHZ2 with 3 port junction box, but only two ports used. Cool: 15.5 min/36 max Heat: 22.5/45. (About right overall for cooling, but quite oversized for heating, see below this outside unit is already installed).
Inside: two zones, East (master suite plus child’s bedroom) and West (rest of house).
East: PEAD-A12AA7 (with short duct runs, like 6 feet). Design loads are 9,530. Equipment is 12k cooling, so mildly over sized? Quite a bit oversized for heating. Next down would be only 9,000, so would ~500 undersized for cooling and contractor doesn’t want to do that.
West: SVZ-KP24NA or MVZ-A24AA7-1 (depending on what’s available, which is apparently almost nothing to do COVID19 shutdowns and slow restarts). Those basically match the cooling needs pretty closely (24 830k). Again a little oversized for heating, (25k vs 21k).
Everything I read says try to avoid multi-splits but no options here. The outside compressor is already in place. Long story is that I re-did this place very recently with ceiling mounted Mitsubishi one-way cassettes MLZ-KP09NA-U1 in most rooms. Very Bad Call. They’ve been a disaster, with frequent noises, breaking, dust issues, service calls, as well as over-sizing set-point issues (a few posts on here about that). So they are coming out(!) and we are trying again, (with great pain) with ducts, hoping not to repeat the errors!
Adds the additional wrinkle that we have a second MXZ-4C36NAHZ2 outside, to be connected to three remaining ceiling mounted units, all part-time loads. A 9k mounted in the garage, a 9k mounted in a “garden room”, and planning to leave a 12k ceiling mounted unit in the large shared room as “party load” aka backup.
(Why were there two 36k units outside in the first place? Well, the properly sized outside units weren’t available from Mitsu at the last minute and they offered “an upgrade” and a story about modulating down … yeah, they modulate down to 15,500k but if you have two 9k units attached it’s a silly situation).
Ok, long post, but key question:
– Is this a reasonable design?
– Should we consider downgrading to a 9k unit for the East zone, and up to the “30” (27 cooling) for the West zone), somehow taking some of the load from the master/childs bedroom into that zone?
From a comfort perspective, should I worry about the lack of modulation down in winter (min capacity is 22.5k, but max design load is 29k …)? Could that cause issues sufficient to make adding a new 1-to-1 outside compressor worth it (I guess I’d donate the one here already?). That would never be cost effective, but really want to avoid comfort issues if we can.
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