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Sizing a Minisplit System for Wide-Ranging Heating-and-Cooling Loads

poissonn | Posted in Mechanicals on

Hi, I’ve spent the last few days reading pretty much all the different posts regarding the multi-zone vs multiple single mini splits trying to figure out what would be the best scenario for my project, without much success.

I actually had two contractors come at the house, but they both proposed only wall units using the rule-of-thumb technique…Being a mechanical engineer myself, that’s not enough for me and I’m having a hard time finding anyone willing to do proper calculation in my area.

My house is a 1250 sq ft single story (excluding basement), and I’m not looking to install a heatpump for the basement. I’m focusing on the ground floor for now and I  want to build a setup to heat the  during winter and cool during summer. I want to be able to close the doors of the bedrooms, so a wall mounted unit in a hallway is most likely not going to cut it.

As you can see on the attached house layout, the bedrooms are completely on the opposite site of the living/kitchen/dining, and are separated by a very closed off hallway so a single head for the whole house isn’t going to work either.

I decided to take some time to do a manual J calculation on coolcalc to figure out the different loads per room. Assumptions are the following for my house (in Quebec, Canada):

– Air infiltration = Tight
– Wall insulation = R21
– Ceiling insulation = R44
– All windows properly sized in coolcalc, double pane low-e (R4)
– Summer temp = 80F
– Winter temp = -10F

I’ve ended up with the following values:

Full house heating load = 24 000 btu, including basement room loads
Full house cooling load = 17000 btu, no cooling for the basement
Living/kitchen/dining: cooling = 11 800 btu, heating = 8171 btu
Bedroom 1: cooling = 1415 btu, heating = 1833 btu
Bedroom 2: cooling = 764 btu, heating = 1516 btu

The rooms not listed (entry, bathroom) will be heated through radiant floors and do not need cooling as much as the living area and bedrooms.

Option 1:
At first, I was looking at a 2-zone 18k unit with a 7-9k ducted cassette for the bedrooms and a 12k wall mount for the living room.

However, after reading on here, I’m questioning if I’m going to have issues with cycling on/off due to the very low loads in the bedrooms compared to the size of the compressor.

Option 2: 
Separate outdoor units:
– One 9k ducted unit for the bedrooms such as fujitsu 9RLFCD which can modulate down to 3100 btu (still oversized, but couldn’t find single ducted units below 9k…)
– One 12k wall mount unit for the living room, which can also modulate quite low for the in-between seasons.

I’m currently leaning toward option 2, but I’d really love some inputs from more experienced folks on here, as maybe I’m overthinking this and option 1 would do the trick just fine.

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Replies

  1. kyle_r | | #1

    How about one ducted unit for the whole house?

  2. poissonn | | #2

    You mean a "standard" furnace/air handler and heat pump? I was looking at mini splits mostly for efficiency and $$$ compared to the bigger central stuff and also for zoning.
    The install was much simpler as well with only a few duct lengths for the bedroom and I was okay with a wall mount in the living room.

    If you meant only one mini ducted cassette for the whole house, I was under the impression that the big difference in cooling/heating requirements between the living area and bedroom would be hard to adjust with only dampers.

    Am I missing something?

  3. kyle_r | | #3

    I doubt you will save money with mini splits vs a standard furnace and heat pump. However, if you want to go all electric or want greater modulation than a ducted mini split is the way to go.

    There is a spectrum now a days from low static pressure to mid static pressure to full out multiposition air handlers that are considered “ducted minisplits.” Being a ducted mini split usually implies the turndown and cold weather performance of a mini splits outdoor unit paired with one of the three types of air handlers I listed above. If you have an unfinished basement with enough space to run ductwork, this will likely be your best bet.

    The amount of BTU/ hr delivered to different room is just matter of duct design. You have larger or more ducts to the higher load rooms than the lower. With one unit for the whole house you will just have one thermostat with one set point.

  4. poissonn | | #4

    Yeah, I'm looking to go all electric since electricity cost here is all hydro-electric and pretty cheap. Based on your comment I went ahead and searched for other options such as a full house "ducted mini split". Seems like the fujitsu 18RLFCD could do very well with a proper ducting setup.

    The basement, and first floor actually, are down to the studs at the moment.

    I came up with a duct layout that can potentially make sense and keep static pressure under 0.36...My next step is to calculate the duct work...the attached layout shows the 18RLFCD unit mounted vertically in a closet next to the staircase wall in the basement. There would be 4 branches with minimal elbows/fittings running between joists. There would a central ductless return directly in the staircase wall. I'm not planning on doing the basement with the unit.

    Let me know what you guys think! I'll report back once I have run the numbers on the ducts. If the 18RLFCD is not cutting it, I can always jump to the 18RGLXD or 24RGLXD.

  5. kyle_r | | #5

    Looks like a good plan. A couple of additional thoughts.

    1) The 1:1 ducted Fujitsu units don’t have a base pan heater. If this is your only heat source with a -10 F design temp, you may want to go with a Mitsubishi or Carrier (rebadged Midea) that does.
    2) It may be easier to balance the air flow if you do all home runs with dampers, and with your layout wouldn't be a bunch of extra ducting.
    3) Do an over sized central return (24” x 24”) at least. Also look at a 2” + filter to reduce the pressure drop. This will help get by with 0.36” of static pressure.

    For what it’s worth, installed a 12 RLFCD for my 1200 sqft second story and am happy with it.

  6. poissonn | | #6

    Thanks for the feedback. I'll look into the carrier/mitsubishi, but I really liked the vertical position of the fujitsu to put in a closet, as well as the low modulation and higher static pressure. Maybe a pan heater is something that can be retrofitted?

    The home runs is definately a good idea, looks like I may only need an additional 15' or so.

    For the return, any reason to go with a 24" x 24"? I was thinking of using the "bottom" option of the unit (see attached) to pull air directly through the wall. I guess this method would prevent me from having an appropriate sized filter though...

  7. kyle_r | | #7

    The Carriers can be installed vertically, the 18k has similar static pressure if I remember correctly.

    Yeah the picture looks like a good idea, but good luck finding a furnace filter that size (a real one, not the rock catcher they include with the unit) that won’t have a very high pressure drop at 600 cfm.

    Check this out.

    http://www.phius.org/NAPHC2018/Think%20Little%20Slide%20Deck%20-%20NAPHC%202018.pdf

  8. poissonn | | #8

    Kyle,

    thanks for pointing me to the carrier units...they seem to be easier to find around here. The unit lower threshold for heating is 5500 btu/hr which is a little higher than the fujitsu unit, but not all that bad.

    Now that holidays have passed, I started to build a spreadsheet for my duct sizing/manual D calculation.

    Where I'm struggling right now is what duct velocity should I aim for? The slide deck shared in the previous post seems to aim for 450 fpm, but is that for each duct branch, regardless of the desired cfm ?

    Also, manual D fittings EL are for 900 fpm and 0.08 IWC/100ft friction rate...I understand how to adjust the EL of fittings for the 450 fpm target (basically 1/4 of the value for 900 fpm), but how do I adjust it for the friction rate? I was under the impression that the TEL was actually used to calculate the friction rate...so if the FR has an impact on the EL of fittings, and that EL of fittings has an impact on the FR, how do I resolve this?

    1. kyle_r | | #9

      It can be tedious to calculate by hand, but energy vanguard had a good series on it, https://www.energyvanguard.com/blog/basic-principles-duct-design/

      The way I would do it is take the maximum cfm the unit puts out and distribute that to each room based on heat load. Then look at this chart (https://www.energyvanguard.com/blog/best-velocity-moving-air-through-ducts/) and determine the duct size needed for each home run at a velocity of around 400 fpm. Keep the fittings to a couple elbows and a duct boot per home run and you should be fine. By keeping the velocity low you really minimize the duct losses.

  9. PBP1 | | #10

    Not sure why so many seem to have overestimated heating loads, my house is only around 50 HERS rating and has 28kBTU heating load (2,100 sq ft, high ceilings, tons of glass at R4+ and R3.5 and insulation on par with your specs) with elevated floor in Montana. A single ASHP rated at 28kBTU (47F) has been sufficient, with three ducted air handlers 15k, 12k and 9k. The ASHP and 3 air handlers (when powered) consume around 150 W baseline and have handled sub-zero temperatures (indoors 68F or above), with electrical usage for the ASHP hitting around 1500kWh for a very cold month (as low as 50 kWh in transition months). Maybe the 18kBTU ASHP is enough for your entire house. Many say undersize the ASHP.

  10. poissonn | | #11

    PBP1, what's your system make & model if I may ask for your 2100 sqft house? Where are you located?

    I'm posting here to update the status of my project. I've basically found a nice layout with max run lengths of around 30' with 3 elbows each. Each run will be 8" rigid ducting and will be balanced using dampers. total pressure loss including 2" pleated filter (24x24) but without the supply elbow at the register (which I couldn't find pressure loss for in my fitting database) is around .22. I figure that fitting can lose much more than .2-.3 so I'm good on the pressure side of things.

    I'm using ASHRAE method to calculate instead of manual J this time so it is not based on EL of fittings, but a database of fittings with straight up loss values based on velocity, diameter and cfm. I find it much easier and precise to use. The database is available on Ipad and iphone only for 10 bucks.

    Now...as I know a ducted unit will do the trick...I was calling around for a Fujitsu 18RLFCD and the best price I got without ducting is 7600$ (CAD). However, I've found one contractor who's offering the following Midea units at MUCH lower price. SIngle zone was 3000$ and multi-zone 5500$, but I have access to 2500$ government incentive on the multi:

    Single zone 18k btu:DLFSDAH18XAK (indoor) / DLCSRAH18AAK (outdoor)
    Multi-zone 27k btu (18k ducted for first floor and 9k wall unit for basement): DLFSDAH18XAK (indoor)/ 9k wall unit (indoor) / DLCMRAH27CAK (outdoor)

    The reason they suggested the multi zone instead of the single zone is that for some reason the turndown is actually better on the multi than the single zone. The multi 27k heating range is 6000-36000 btu, and the 18k single is 8900-21400 btu. Cost-wise, it appear to be much better BTU/$$$, especially since now I can also heat my basement with the heat pump.

    Any red flags not to go with the multi in this case? I mean...the minimum heating is better than the 18k and I've got plenty of room to spare...Clearly the upper range is way oversized for my needs, but if it can go low enough, why not?

    Other than the Fujitsu which throttles down to ±3000 btu, I couldn't find another make/model of ducted mini that could modulate lower than that multi unit, especially for the price.

    1. kyle_r | | #12

      Carrier sells rebadged Midea units. Their documentation might be more complete, Midea itself may not have great documentation in English. It doesn’t make sense that the 1:1 had a higher minimum than the multi. Maybe cross reference with the Carrier documentation to confirm. You can see the tech specs of the out door units here

      https://www.shareddocs.com/hvac/docs/1009/Public/0C/38MAR-08PD.pdf

      Given your design temperature and the fact that these have base pan heaters, I think they are a better fit than the Fujitsu.

    2. Expert Member
      Akos | | #13

      I'm using the DLCSRAH18AAK for a project and can confirm the turndown on it is something to be desired, the data sheet seems to be correct. A bit strange because the 12k version has much better turndown. It still works great and would have no issues with using it again.

      In your case, I think the multi is not a bad option, I would just watch how running the basement unit will effect your power use. Sometimes a smallish head on a multi split can create issues, but at least in this case the min of the unit is above the head capacity in all cases.

      If not possible to gravity drain (always the best) and if you are using their built in condensate pump watch your routing. The pump has a very low lift.

    3. PBP1 | | #18

      I’m in Montana with a design temperature of -1 F, and its a Mitsubishi hyperheat rated at 28.6 kBTU/h, today was 0 F and house (2100+sq ft) was at 68 F set point. I have three ducted 15k, 12k and 9k SEZ air handlers. All was OK’d by both HVAC and HERS rater/energy consultant. No complaints. And, I find Akos’ posts informative.

  11. poissonn | | #14

    Kyle, I just went through the Carrier documentation and it states the exact same thing as Midea's. This is odd...as Akos stated, the 12k unit has a min of 1800 btu, which is really great.

    I'm really debating going with the multi or not. I'm tempted to oversize on this one..haha. the 18K unit is for sure going to need some backup heat when it's -22F around here, but with the 27K, I'd most probably be fine and the turndown is ***theoretically*** better. It would also end up costing the same or less than the single zone due to the government pushing hard on multi-zone mini's for whole house heating. They actually do not give anything if you go single zone wall/ducted/whatever, only 2+ indoor units or central ducted.

    Akos, since you have a bit of experience with the 18k single zone...do you have any idea of how the not-so-great turn down is affecting the heating/cooling bill at the end of the day? I keep reading on here that modulating is important for efficiency...but without any real world usage data to back that statement it's hard to figure this one out. I searched hard for the Fujitsu, but prices are outrageous. I just got a quote from another contractor... 11 000$....

    Going back to the multi idea...The 9k unit would heat the whole 1200 sq ft basement, so I'm thinking it may not cycle too badly. What kind of issues can I expect from a small indoor on a larger outdoor?

    1. Expert Member
      Akos | | #19

      I don't have a power meter on the unit, so I can't comment on the efficiency. About the only thing I noticed is on warmer days I can hear the indoor air handler cycling (it ramps up from low whenever it is delivering heat on auto fan).

      Doing a quick look at the 3 zone unit, I really can't see why not to go for it in this case. The unit has much better modulation range than most multi splits and the price is right. Something with slightly better turndown that costs much more will have no real ROI.

      Most heat pump issues from here come from two things:
      -oversized units
      -sloppy install

      In your case, the 27k is a bit oversized, but not far off if you include the basement heating load.

      For the install, I would make sure to read this excellent article from Jon on proper install:
      https://www.greenbuildingadvisor.com/article/reducing-refrigerant-leaks-from-heat-pumps

      You want to see a pressure test with soap for leaks and a vacuum decay test. I mean actually see it. I've had guys not even bother with vacuum gage until called out and of course followed by the usual "you don't trust me to do a good job".

      1. poissonn | | #21

        Akos,

        thanks for the feedback. I am too pretty much set on the multi. It just looks to be one of those "too good to be true" thing..lol. Especially since it's a Midea and not Mitsu/Fujitsu/Daikin.

        I've checked again the government website and they give 5000$ (!!!) if you go with 3 indoor units. I'm thinking of adding a 9k unit to my attached garage :). By doing this, I'm definately not going to be oversized anymore!

        1. Expert Member
          Akos | | #23

          I don't know about that specific multi, but most multi splits put some refrigerant through all the heads even if the head is off. For example, my LG multi puts pretty much rated BTU worth through it, if I run the bedroom head in fan only monde when the other head is set to heat, it will quickly overheat the room.

          Keep that in mind that adding a head to the garage might cost extra to run even if you are not using it. I never checked how much this is, it very well could be small enough to not worry about it. On the other hand if you are looking to put some minimal heat into the garage at all time, this is a non-issue.

  12. kyle_r | | #15

    So at the link I shared on page 11 you don’t see a turndown of 5,500 btu/hr? Or is that a different unit?

    I actually don’t think the turndown on the heating will make much of a comfort of efficiency difference in this case between the two. What are the cooling minimums?

  13. poissonn | | #16

    Kyle, yep I see 5500 btu. That 38MAQB18R--3 outdoor unit seems unavailable though around here. I was actually asking for this specific unit when calling around but all contractors only had only access to the 38MARBQ18AA3 model, which is the exact same thing as the DLCSRAH18AAK I was quoted and that Akos worked with.

    The only place I've seen the 38MAQB18R--3 for sale is online...Seems like a unicorn around here!

    Cooling minimum on the DLCSRAH18AAK is 6500 btu, whereas the 27k multi it is 7765 btu. For what it's worth, the ducted indoor with the 38MAQB18R--3 unicorn minimum is 4500 btu...

    So, will 1265 btu between the two more available unit make a big difference?

    Something tells me I should continue to look for the Carrier unicorn...

    1. kyle_r | | #17

      With the multi turning town to almost a half ton, I would go with multi considering the $2,500 from the government. Are you installing yourself? I would make sure you understand any nuances with the multisplit (min/max line set lengths, and branch box considerations if any).

  14. poissonn | | #20

    Thanks for your input, appreciate it.

    I'm only going to do the ducting myself for the slim duct and a contractor will handle the refrigerant lines. I did however take a look at the line set lengths, seems pretty straight forward: Max total length (added lengths of indoor units) and max total length for each branch/indoor unit. This specific multi doesn't need a branch box, it can take up to 3 indoor unit directly off the condenser.

    Last item so far to take my final decision is finding out if the multi condenser is fully modulating like a 1:1 unit, or only doing it in "steps". I've seen that comment a few times on this forum from Dana and others for other units such as Mitsubishis. Seems like this info is somewhat harder to find in the manufacturer specs.

  15. BenRoc1979 | | #22

    Oh we will need to talk. So many questions for you.

    In Pierrefonds here (Mtl island) and starting some renos soon. Panasonic ERV, plus I know my minisplit will die soon and looking into multi zoned unit, probably 2 ducted ones (3 bedroom for one, living room/kitchen and maybe a tiny bit basement for the other one. Wall unit for garage? Hmm

    Curious about the hvac company. I have prices done, no one is doing a manual J. Its like they don't care about that in QC. Even had someone told me they dont seal duct and air escaping will heat the floor anyways. Wtf.

    If you wanna hit me up at benoitrochon, on the gmail

    1. poissonn | | #24

      Sent you an email!

      You're right...no one seems to care around here. Manual J is mostly for united states though. In Canada, CSA has another method based on the ASHRAE documentation for load calculations. I stopped searching for one a while ago and just did mine through coolcalc by being very careful about the details I input, otherwise it messes up the numbers.

      Quick update on the project:

      It seems like the government credits for multi-zone heat pumps may apply only to non-ducted units...Now, hard to say if a "slim duct" qualifies as a non-ducted unit, as usually the manufacturers list them in their ductless catalog along with wall units and ceiling/floor cassettes. Way to be confusing...

      I guess if I commission a slim duct unit in my basement without any duct connected to it when the inspector comes for the credit inspection, perhaps that can work? haha

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