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Comment on HVAC plan?

jameshowison | Posted in General Questions on

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:

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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  1. user-2310254 | | #1


    I'll give your post a bump.

  2. b_coplin | | #2

    Another bump.

    Not to muddy the water, but I'm skeptical you have a 29k btu/hr heat load @ 31° F design temp.

    Prior to some retrofit work, I had a heat load of ~33.5k @ 8° F design temp, based on fuel use. Slightly larger house w/a rubble stone basement, 1920s triple wythe brick construction. Very thin insulation in the (leaky) ceiling.

  3. jameshowison | | #3

    Thanks for the bumps. Who really knows on the design loads? I'm pretty confident on the Manual J inputs (although without a blower door test infiltration is a guess), so I'm not sure if there's anything more I can do there. You'd think that the units already installed could just tell me how many BTUs they had put out, but apparently that trick is beyond heat pumps (the fuel use method not being available).

    I think we'll be way oversized on heating, leading to cycling and short bursts of on and off heat, exactly what variable refridgerent is meant to avoid ... the more I read about multi-splits the less I'm convinced that anyone really understand the modulating abilities of different combinations and the amount of "drift" from unit to unit. For example, with our ceiling mounted minisplits if I set two of them to 70° and leave one room at 65° overnight then I end up with two rooms consistently at 72° and the other room at, well, 70° ish.

  4. pablov | | #4


    Not a HVAC designer but I share your pain. In March i had installed 2 ceiling cassettes and 2 wall units (MSZ-FH09NA).

    Long story short, I have hated every single units since the day they were installed. While they heat and cool each area very well (no sizing issues) I have been dealing with multiple service calls regarding leaks, rattling, frequent and random noises specially with the ceiling cassettes.

    Just like you, I have had with the wall mounted units and ceiling cassette and I will contacting my HVAC company to remove all the units except the outdoor unit. I am also going ducted with Mitsubishi including some ductwork in the attic because that's my only option at the moment. 100% efficiency has gone out the window and all I want is to be able to sleep throughout the night or enjoy a book in the living room without being bothered by my HVAC system.

    Hope we can get an update on how your new ducted system goes!

  5. Expert Member
    Akos | | #5

    600sqft per ton cooling sounds still a bit on the high side, the heating load is definitely way off even with an uninsulated house. Either way, your units are there and you have to make do with what you have.

    I think overall your 12k+24k is in the ballpark.

    I would downsize your 12k to a 9k unit and run a duct from the 24k to the inlet of 9k unit with a manual damper (over even better one controlled by a thermostat). This would give you the option to heat the whole house with just the 24k unit and use the 9k just as boost blower. This does have the feel of a hack though.

    No matter what you do, the unit will be way oversized for your heating so the units will cycle. Getting a handle on the efficiency of these under your condition is next to impossible (Mitsubishi just doesn't provide enough data), chances are it won't be great but not bad enough to be worth while buying a new outdoor unit. How this will effect comfort is hard to say.

    My home is heated by hot water but I have occasionally tried to heat it with my oversized multi split (LG) with wall mounts. I never found it to cause any comfort issues. COP was around 2.

  6. Jon_R | | #6

    Here's a good "big picture" mental exercise. Start with an assumption of ultimate HVAC - temperature/ventilation/humidity being zoned and controlled for each room. This probably isn't practical, so for each compromise short of this, review cost savings vs the negative effects.

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