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Community and Q&A

Minisplit retrofit in Climate Zone 2

AKShazam | Posted in GBA Pro Help on

I’m trying to figure out how to use a minisplit system on a retrofit project. I got a quote from an HVAC company (recommended by the equipment rep), but some of their opinions don’t make sense…which brings me here today.

The HVAC company proposed 4 zones with a 3 Ton compressor outside:
1 ducted 9k unit in attic, serving Guest Bath, Guest Bed, and Office
1 ducted 9k unit in attic serving Master Bed and Bath
1 9k ductless serving kitchen
1 9k ductless serving living

I don’t like the idea of putting units in the attic. That may be the only option for Zone 3, but not sure about Zone 1. I’m also concerned about sizing. Seems like there’s less room for error with mini splits.

My questions:
1. How difficult is it to mount a ducted minisplit on a ceiling and run ducts through interior walls?
2. Could 1 ductless unit condition the Master Bed and Bath?
3. There is a 40 sq ft opening between the Living Room and Kitchen. Could one 15k ductless distribute air widely enough to condition both rooms?
4. As you see from the load calcs (below), some rooms have a big difference between cooling and heating loads. If I size for the cooling load (due to climate zone), some rooms won’t keep up with heating demand in the winter. If I size to the heating load, some rooms will be significantly oversized in the cooling season. Thoughts?

Below are the load calcs, performed by a third party .
(Room – heating load, cooling load)

Zone 1
Ma Bath – 2788, 887
Ma Bed – 5200, 2928
Total – 7988, 3815

Zone 2 (if I can use one ductless for both rooms)
Kitchen – 4250, 7231
Living – 5214, 4764
Total – 9464, 11995

Zone 3
Guest Bed – 4034, 2639
Guest Bath – 1314, 997
Office – 4430, 2752
Total – 9778, 6388

Whole House – 27230, 22197
Latent Cooling – 4879

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "How difficult is it to mount a ducted minisplit on a ceiling and run ducts through interior walls?"

    A. It's not too difficult, especially if there is a high ceiling that allows the construction of soffits. The main problem with ceiling-mounted units is the difficulty of keeping the equipment and all ducts within the home's conditioned space.

    Q. "Could 1 ductless unit condition the Master Bed and Bath?"

    A. The answer depends on your expectations. If the ductless unit is in the bedroom, then the temperature of the bathroom is likely to differ by a few degrees, especially if the bathroom door is kept closed for long periods of time.

    Q. "There is a 40 sq ft opening between the Living Room and Kitchen. Could one 15k ductless distribute air widely enough to condition both rooms?"

    A. Probably.

    Q. "As you see from the load calcs (below), some rooms have a big difference between cooling and heating loads. If I size for the cooling load (due to climate zone), some rooms won't keep up with heating demand in the winter. If I size to the heating load, some rooms will be significantly oversized in the cooling season. Thoughts?"

    A. Assuming that the load calculations are accurate -- and that's a big assumption -- it's not unusual for heating loads and cooling loads to differ. The equipment is sized to meet the highest load.

  2. Chris_King | | #2

    It's actually quite easy to run everything on an interior wall. I just had that done and my HVAC contractor remarked that it was easier then trying to get the bend at the end of the attic where you are crawling around. Our unit is ceiling mounted in the living room and like you and we have a 6'x8' opening into the kitchen and there is no noticeable temperature difference. Granted it's only been a week but it's been a week of 95-99 out here in NJ.

  3. Expert Member
    Dana Dorsett | | #3

    Chris: In NJ you'll notice the temperature differences more when it's in single digits F outside (on the order of 60F indoors to outdoors delta) than on 95-99F summer days (less than a 30F delta). But with that big an archway it won't be much even in winter unless the heat loss in the kitchen is excessive (say, a big single pane glass slider from the kitchen out to the deck plus a bunch of other window area.)

    Green Heron: There are 2 ton and 2.5 ton 3 zone multi splits out there that might be more appropriate for your loads than the 3 tonner. Most load calculations have a bit of built-in slop, overestimating the actual loads, and 3 tons of compressor for a calculated whole house cooling load of ~27K is edging into the overkill zone (even if that were the actual load rather than a slightly padded number.)

    The location of the ductless head and the direction it is blowing relative to larger archway/opening makes a difference in balancing temps. Blowing directly toward the opening is far superior to a head around the corner and directed elsewhere, counting on convection to drive the air exchange.

    If the combined cooling load of the kitchen + living room is only 11,995 BTU/hr there's no need to go for a 15K head, since most 12K wall coils are capable of delivering ~13,000 BTU/hr of cooling into a 78-80F room. The minimum modulated output of the 15K head needs to be compared to that of the 12K head if you're tempted to oversize. A 12K ceiling cassette mounted in the kitchen ceiling near the opening between the rooms can work pretty well too.

  4. AKShazam | | #4

    Thank you all for weighing in. I was hoping we could mount the unit to ceiling joists through the drywall. Plenty of room for soffits (9.5 ft. ceilings in master bath closet), but I can see how they don't want to deal with it since they've never done it before.

    The Master bathroom will be remodeled within the next few years, presenting an opportunity to create more air pathways between the bed and bath. In the meantime, the door is not closed very often, so I think we'll go with a 9k bedroom wall unit for Zone 1 and see if that works. While I run this experiment, I'll try to find someone who is willing to tackle the ceiling mount idea for future projects.

    Regarding load calc accuracy, this is definitely a concern. The current HVAC system is 10 SEER 2 Ton A/C with 80% gas furnace. It struggles to meet demand on some summer afternoons. Envelope air leakage is 17 ACH and insulation is R-11. Load calcs include envelope improvements (air sealing to 12 ACH & R-38), so it's logical that envelope improvements + half ton of capacity could meet cooling demand. However, if I size to the highest BTU in each zone, I'm over 2.5 Tons. Am I destined to oversize with mini splits?

    Dana: I get what you're saying about positioning of ductless units. For the living/kitchen zone, I'm thinking the kitchen is preferable due to higher loads. The best available location in the kitchen is at a 90 degree angle from the opening between rooms. I'm proposing a model that allows horizontal oscillation to blow air toward the opening. Thoughts?

  5. Expert Member
    Dana Dorsett | | #5

    The 17ACH/50 can probably be cut by more than half- the 12ACH/50 number would probably be exceeded just by sealing off the existing duct registers & boots.

    Most heads/cassettes can/will deliver more than their "rated" or "nominal" cooling capacity. The designated output number is the modulation level at which the efficiency was tested. It's sometimes tough to dig out the max output numbers when looking only at multi-split specs, but if the same head is used in a single zone configuration it often shows up in the submittal sheets for the 1-head minisplit, eg:

    Note, the above FH09 is nominally "rated" (= efficiency tested) when delivering 9000 BTU/hr of cooling, but it's fully capable of delivering up to 12,000 BTU/hr of cooling. When used in a multi-split it's max-capacity is still there.

    Similarly, the half-ton FH06 head can still deliver fully 9000 BTU/hr of cooling:

    When looking at which and how many heads/cassettes to hang on a 2-ton or 2.5 ton compressor, see if you can't get away with undersizing at the nominal rated output, by seeing if the maximum capacity still covers your 1% and 99% loads.

    In climate zone 2 the 99% outside temps are usually well above +17F (what is YOUR 99% outside design temp?) and the capacity will be somewhere between the 17F and 47F capacity numbers listed on the efficiency test submittal sheets.

  6. ktkcad | | #6

    For the master bath door check out the 'VANAIR' door from Lynden Door. It is designed with 65 si of ventilation and 'no' noise transfer or undercut. They are developing a residential version and with that much airflow and a continuous vent fan you will maintain temps in each room.

  7. Chris_King | | #7

    Sorry it took so long Green, couldn't get these to upload from my phone but here is a picture of the run down a interior wall chase and a view from the unit in the attic. Note the hole in the hole wasn't from having to run the lines, it was an old duct cover where the old AC unit used to be. Also included is a picture of the unit being put in the ceiling and a view from above. We had to cut one joist as you will for every unit I know of but it is not a big deal to box them out. I would suggest doing the box out before actually cutting the joist, just make sure you leave enough room on the lineset connection side for your HVAC guy to work. To seal the unit opening into the attic i used cut pieces of rigid foam sealed to the unit/sheetrock/joists with Siga tape then covered it in Roxul until i can go back and do blown in cellulose.

  8. AKShazam | | #8

    Dana: That is great news about ACH reduction and makes sense. Design temps are 31F/92F (outside db), 68F/75F (inside db), 37F/17F (TD). Actual setpoints vary a bit from the design temp, but not across all zones at once (70F during morning in winter in living/kitchen and 70F at night in summer in bedrooms).

    I'll check into the 2.5 Ton compressor. I was told that's not an option, but maybe because they wanted 4 zones. Sounds like it's better to risk undersizing for the rare loads, then add another compressor/ductless combo later if comfort goals are not met. How do you account for latent loads, since that's a whole-house number in the load calcs? Just make sure the total maximum capacity across all zones will cover it?

    Karl: Great suggestion! I will look into the Vanair for the Master Bath.

    Chris: Thanks for the photos. Is the wall chase located in the wall shown at the bottom of the 2nd photo?

    I really appreciate everyone's help on this.

  9. Expert Member
    Dana Dorsett | | #9

    A total heat load of 27230 BTU/hr @ +31F implies it's a pretty big house, or a load calculation with some fairly conservative assumptions. At reasonably tight one story 2x4 framed slab on grade house with clear-glass double-panes with that much true heat load at 31F would be ~3000 square feet- sound about right?

  10. AKShazam | | #10

    Assumptions are definitely conservative. 1500 square ft with mix of slab and vented crawl with clear double paned windows.

  11. Expert Member
    Dana Dorsett | | #11

    For a 1500' house I'd expect the true 1% cooling load to come in under 2-tons, and the heating load under 20K after air sealing & insulation up grades (maybe even under 15K).

    A vented crawl space in a hot humid climate adds quite a lot of latent load, but is fine in dry climates, as long as the floor is reasonably air tight, with R13s or better between the floor joists. Are you in climate zone 2A (gulf coast ) or 2B (Arizona/California)?

    With enough room in an unvented conditioned crawlspace you might be able to squeak by as a single zone with a 1.5 ton Fujitsu mini-ducted unit in the crawl. The 18RLFCD puts out about 21,600 BTU/hr of heat @ +17F, and ~24-25K @ 31F, and 18,000 BTU/hr of cooling. Insulating at the foundation walls rather than under the floor also peels a bit off the cooling load, "earth coupling" the house to the thermal mass of the subsoil, and it's much easier to reliably air seal the foundation walls than the floor, which would further improve the ACH/50 numbers and the associated ACHnatural loads.

  12. AKShazam | | #12

    I'm in 2A (Gulf Coast). Currently, the return vent runs through the crawl space and into a unsealed cavity beneath the air handler.

    Thanks again to everyone who offered input. I'm going to risk undersizing with 2.5 Ton compressor. Wish me luck! I'll post an update once everything is installed.

  13. Expert Member
    Dana Dorsett | | #13

    In zone 2A there is any air leakage on the return runs located in a vented crawl space it's dragging in a significant latent load into the cooling system.

    Simply having the crawlspace vented is also a significant latent load for the house. Even if there is almost no "payback" for insulating the foundation walls in zone 2, at the very least putting down a ground vapor retarder and air-sealing the foundation, limiting outdoor air infiltration and ground moisture load is going to be "worth it". That brings the crawlspace into conditioned space- the sub-floor and joists will run drier, for low/no mold risk, and it should reduce the latent load measurably.

    In flood zones you may be required by code to have the plugged off vents open automatically under hydrostatic pressure to relieve the water pressure from the foundation. This isn't difficult or expensive, but be sure to check if you're on a flood plane or hurricane storm surge zone just to be sure. Outside of flood zones you can just permanently seal it off.

    I doubt that 2.5 tons would be undersizing, even if the crawlspace is left vented. With the air handler and ducts all inside the pressure and thermal boundary of the house your true total load would likely come in around 1.5 tons, give or take.

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