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Fan coil units for mini split heat pumps

william_23 | Posted in General Questions on

We recently purchased a 20+ year-old town house near Birmingham, AL.  The HVAC is original, still working but the duct system is very leaky.  Most of the ducting is in attic space, but some runs between floors.  The A/C is 10 SEER and the furnace is an 80 percenter.  We are thinking of replacing it all with a mini split heat pump system with as high efficiency as we can get/afford.  The town house is two story, but has a bedroom and bath which are single story.  About 1650 sq. ft. total.  I have been thinking of using a combination of inside units, perhaps utilizing the portion of the duct system which is between floors.  However, I seem to be finding that the efficiency of the fan coil units which work with ducting have much lower SEER values than do the wall units, or even the ceiling cassettes.  Should I just forget about using the ducts and get used to thinking of having these wall units in every room?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"Should I just forget about using the ducts and get used to thinking of having these wall units in every room?"

    NO!!!!

    Unless it had a lot of glass facing an unobstructed "sunset view" western exposure, the real cooling load of a 1650' townhouse in Birmingham is going to be under 2 tons, probably under 1.5 tons. If you go with a wall-bl0b-in every room solution it's going to be dramatically and sub-optimally oversized, to the point that the efficiency numbers are meaningless.

    A 1.5 ton Fujitsu mini-ducted solution runs a hair under 20 SEER, but may or may not be suitable for your existing ducts.

    https://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/18RLFCD.pdf

    If it needs a beefier air handler the mid-static version tests at 19 SEER:

    https://ashp.neep.org/#!/product/25348

    But you'll probably want to zone it by floor possibly with two separate units, each sized correctly for it's zone load, or a 2 zone compressor with a pair of small ducted air handlers. There are 1.5 & 2 ton 2 zone multi-splits that can probably be made to work. The Fujitsu mini-duct units are a bit more flexible since they can be installed vertically if need be, and can eat up less than 10 square feet of floor area in a compact "utility closet". They even make a 7,000 BTU/hr unit, if that's more appropriate. But they can sometimes also be installed horizontally below the ceiling in the top of a closet, taking up no floor area. This is a 1.5 tonner mounted vertically heating & cooling an entire house in Berkeley CA:

    https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2019/08/02092349/Ducted-minispli-1-Dana-Dorsett-700x933.jpg

    https://www.greenbuildingadvisor.com/article/getting-the-right-minisplit

    First order of business is to run a room-by-room Manual-J type cooling and heating load calculation.

    If you want to micro-zone it room by room, a 2 ton Chiltrix with a buffer tank for the chilled/ heated water, and individual hydronic fan coils could do it, but I suspect that would come in substantially more expensive than a mini-split solution:

    https://www.chiltrix.com/

    Since you have a heating history on the place, running a fuel-use load calculation on last winter's coldest month gas bill would put a firm upper bound stake in the ground on the heat load, since it would also include the duct losses of the allegedly leaky ducts.

    https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

  2. william_23 | | #2

    Thanks for the reply, Dana. I have done a room-by-room-heat-load calculation using the online form by SlantFin. Since they are only involved in heating, they don't provide the cooling load calculator. I have a copy of Manual J on its way from an Ebay purchase.

    We have had two contractors give us bids on new systems. One of these guys actually lived in the house for several years some years back. We have bids on six systems, but four of them are for hybrid systems. With a climate as mild as Birmingham, is it necessary to back up a heat pump system with a gas furnace? We have used a ground-source heat pump as our sole heating and cooling source in Illinois, not too far from St. Louis. One of my goals is to eliminate the natural gas service.

    The current furnace and gas water heater are located in the attic, above the two-story portion of the house. I want to eliminate all of that duct work up there. There are two bedrooms, a bathroom, and three closets on the second floor. This level will be used infrequently, and I'm wondering if a separate condenser unit for this level would make sense. Below these rooms is a large, open living room/dining room, kitchen and hallway/stairs to the second floor. Adjacent to this is the first-floor bedroom and bathroom. My thinking is to place an appropriately-sized ceiling cassette to heat and cool the LR/R/K, which are now served via a duct system between the floors. I am thinking of then converting that duct to feed floor registers in the upstairs bedrooms. The only surface in the upstairs bathroom exposed to unconditioned space is the ceiling, to which we plan to add R30 to the R11 or so that is there now. The first-floor bedroom and bath are conditioned by a branch from that between the floors duct. I'm thinking of placing a ducted fan coil unit in the attic space above those two two rooms. I would plan on short runs of galvanized duct and then insulate them well. I'm mot thinking of this as a DIY. I just want to have a plan in mind when we again talk to contractors.

    I had looked into the Chiltrix units a couple of weeks ago. I contacted them to see if they had any installers in the Birmingham area. They don't and I'm quite old. I don't want to leave my wife with a unit which no one knows how to service.

  3. Expert Member
    Dana Dorsett | | #3

    >"I have done a room-by-room-heat-load calculation using the online form by SlantFin."

    The Slant-Fin tool usually overstates reality by around 35% , sometimes more. What number did it come up with?

    Run a room-by-room load calculation for the upper floor (even if using Slant Fin), and make a floor plan sketch with the heat load numbers on it, which will help determine if a ceiling cassette solution would cut it. It's often possible to do a ducted mini-split solution with in hard piped rectangular cross section ducts in soffits under the ceiling plane, if you're willing to build out the soffits.

    >"With a climate as mild as Birmingham, is it necessary to back up a heat pump system with a gas furnace? "

    A climate as mild as Birmingham?

    Even in a climate as mild as BUFFALO (NY) it's not necessary to back up a heat pump system with a fossil-burner. Even a 2-ton 2-stage Goodman GSZC18-0241C with heat-strip backup can do very well in a Birmingham climate, assuming it's not too oversized for your actual design heating load. A cool-climate ducted mini-split with a vapor injection compressor can put out a lot more heat at +5F or even -5F than the old-school types do at +17F (an AHRI test conditioned), and many do well even in cold climates without even the resistance heaters for backup.

    If I had to guess (and I have to until more information comes in) your whole house design heat load is going to be less than 20,000 BTU/hr, probably in the 15000 BTU/hr range. That would take less than 5kw of heat strip to cover 100% of the load even without the heat pump running. But even at 10F outside a right-sized standard heat pump would still be delivering the lion's share of the heat, so a smaller auxiliary heat strip would be more appropriate. According to the heating capacity charts, a 2 ton Goodman delivers about 16,000 BTU/hr @ +25F, but falls to about 13K @ +10F. If your load is 15K @ 23F the load at +10F is about 19K, so it would only need to make up for a 6K shortfall, which is about 2000 watts of aux heat. See the top of page 20:

    https://www.ecomfort.com/manuals/74f28fedf5e72cb07f4e96710ab9b0e4.pdf

    But the low-static 1.5 ton ducted Fujitsu is good for about 18,000 BTU/hr even at -5F, yet still modulates down to 3100 BTU/hr @ +47F- it's not terrible at all. See page 17 (PDF pagination):

    https://hvacdirect.com/hvac/pdf/(DT)ARU9-18RLF-AOU9-18RLFC2015061.pdf

    Almost ANY gas furnace (other than perhaps a Dettson https://www.greenbuildingadvisor.com/article/finally-a-right-sized-furnace ) is going to be oversized for the heat load of a 1650' town house at Birmingham's 99% outside design temp of +23F. If you're moving away from ducts in the attic it's probably simpler to stick with a right sized mini-split solution.

  4. william_23 | | #4

    When I looked at the results of the rooom-by-room calculation, it became apparent that there is a problem with the results. It shows a heat loss of nearly 10,000 BTU/H for the dining/living room area although it has only about 200 square feet of exterior wall space, and conditioned space above. There is a decent-sized window and an exterior door as well as a fireplace. We intend to plug the flue of the fireplace. At the same time, the calculations show a heat loss of about 8,000 BTU/H for the adjacent bedroom and bath. These rooms have a total of about 500 sq.ft. of exterior walls and unconditioned space above. So, there has to be an error in the calculation unless a fireplace can account for this discrepancy. I will be at that location in a few days. I intend to do a more detailed paper and pencil calculation as soon as I receive my manual J. I'll be back then.

  5. william_23 | | #5

    I did load calculations using CoolCalc and got a total cooling load of 32,275 BTU/H. The heating load which this gave me was ridiculously high, nearly double the cooling load. Since this did not generate much confidence for me, I paid someone to do it again. They came up with 32,311 BTU/H, so I figure that is good agreement. When they broke that into the three zones which I was thinking of, upstairs 13,560, downstairs immediately below the upstairs, 12,081, and single story part, 6671. When I played around with duct location on the CoolCalc, I found that moving the ducts into conditioned space cut heating and cooling loads by a great deal. So, the current duct system has a 60,000 BTU furnace in the attic with the A/C coils there. This feeds into a 24 X 24 plenum, off of which run two square ducts of 10" on a side. Each of these feeds two flex ducts. One pair feeds a bedroom and a closet which has only ceiling exposed to unconditioned space. The other pair feeds another bed room and a bathroom which has only ceiling exposed. There is a 10 X 16 duct which runs down to the space between the lower floor ceiling and the second floor. This feeds three ceiling vents in the kitchen, dining, and living rooms. There is a flex duct runs off of this to the attic space above the single story portion of the house. It then branches and feeds ceiling vents in a bed room and a bathroom. So, what I am wondering is the wisdom of conditioning the kitchen, dining, and living room areas with a single ceiling cassette. The duct above the ceiling would then be unused. I would think of putting an appropriately sized slim duct unit in the attic space above the downstairs bedroom and bath. This could easily be boxed in and heavily insulated. Then close off the downward vents in that ceiling duct and put another duct unit on that and use it to distribute to the upstairs bedrooms and bath. Is this feasible or just crazy thinking? Thanks.

  6. Expert Member
    Dana Dorsett | | #6

    >"So, what I am wondering is the wisdom of conditioning the kitchen, dining, and living room areas with a single ceiling cassette. The duct above the ceiling would then be unused."

    The square ceiling cassettes require modifying the ceiling framing- they don't even fit between 24" o.c. framing. Mitsubishi makes a set of ceiling cassettes that fit between 16" o.c. joists, but they are uni-directional, whereas the square ones blow in 4 directions.

    http://meus1.mylinkdrive.com/M-Series/R410A+Systems/One-Way+Ceiling+Cassette/index.html#MLZ-KP+Models

    The smallest of the line MLZ-KP09 is good for 9000 BTU/hr cooling, 12,000 BTU/hr heating:

    http://meus1.mylinkdrive.com/files/MLZ-KP09NA_For_MXZ_MULTI-ZONE_SYSTEMS_Submittal-en.pdf

    The KP12 is good for 12K cooling, and 15K heating, which may be more appropriate for a 12,081 BTU/hr design load. (Was the 12,081 BTU/hr cooling, or was it heating?)

    http://meus1.mylinkdrive.com/files/MLZ-KP12NA_For_MXZ_MULTI-ZONE_SYSTEMS_Submittal-en.pdf

    With some floor plans kitchen, dining, and living room areas that might work, for others maybe not.

    >"I would think of putting an appropriately sized slim duct unit in the attic space above the downstairs bedroom and bath. This could easily be boxed in and heavily insulated. Then close off the downward vents in that ceiling duct and put another duct unit on that and use it to distribute to the upstairs bedrooms and bath. Is this feasible or just crazy thinking? "

    That can work. All of the joist bays running under your utility chase would need to be rigorously dammed off and air sealed, and you'd have to design an insulated weatherstripped service access for changing the filter media and dealing with clogged condensate drains, etc, but it's do-able.

  7. william_23 | | #7

    I am still working on this. I am currently considering two options. One would be to use a unit like the Mitsubishi MLZ-KP12NA in the large open area of the kitchen, DR, and LR. This measures approximately 17.5 X 35. However, there is a beam which projects a foot down across the ceiling about 14' from the one end. Would this prevent proper air movement from such a ceiling unit? My othe option would be to put an air handler such as a SVZ or PVA and tie it into the existing duct system. The duct system serves the upstairs also, but since the upstairs will only be used if there are overnight guests, I'm currently thinking of leaving the existing HVAC system there and use that on those rare occasions, So, it would be necessary to install a damper in the trunk line above where the air handler would tie in. Any thoughts?

    Added thought. Does anyone other than Mitsubishi produce a system which fits between the joists? Joist spacing in this unit is 19.2" Thanka.

  8. Expert Member
    Dana Dorsett | | #8

    The Mitsubishi MLZ-KP12NA is almost surely going to be 2-3x oversized for a space that size, but might be right-sized for the whole-house loads. The MLZ-KP09NA may be a better fit, though the minimum modulation levels are dramatically lower than the -KP12:

    http://meus1.mylinkdrive.com/files/M_MLZ-KP12NA_SUZ-KA12NA2_SUBMITTAL-en.pdf

    http://meus1.mylinkdrive.com/files/M_MLZ-KP09NA_SUZ-KA09NA2_SUBMITTAL-en.pdf

    The MLZ series is directional, and if oriented correctly should be able to project air far enough. The beam is only an issue if the MLZ blowing toward the beam and fairly close to it.

    Run the load numbers before picking it, but if it's just serving the 615 square foot fairly open space, that's probably a reasonable way to go.

    So far as I know Mitsubishi MLZ series is the only ceiling cassettes that fit between 16" on center framing. I don't know of any others that make it between 19.2" o.c. framing either.

    A floor plan showing the rooms, the location of the beam and the direction of the joists would be useful here.

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