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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.

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