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Mini-Split: Ducted/Ductless? Sizing? Location? (Manual J completed)

kashmi | Posted in General Questions on

Seeking advice about mii-split sizing, type and placement. I’ve been reading on GBA about ducted mini-splits, but wonder if that’s the right solution for the new/remodeled house.

Here are the particulars …
* A gut remodel of a 1940s 1,200 sq ft bungalow with 7’ 7” ceilings in the PNW.
* As per attached floor plan, mostly open space but with a 14 x 14 bedroom extension. Just one person living there; thus doors will be open almost all of the time.
* A finished basement, but a 6k BTU Mitsubishi hyper-heat unit should work for that space (840 sq ft) + a floor heat mat in the bathroom.
* Manual J calculation gives a heating load of 8,850 BTU/Hr and a cooling load of 7,830 BTU/Hr for the main floor.
* Heat mats will be installed under the tile in the bathroom, in hallway between bed and bath, and in the kitchen floor space between the main cabinet wall and the island.
* Propane fireplace insert in the living room.
* Tight building envelope.

All this suggests that a 9kBTU mini-split should be sufficient for the main floor (Yes?), but would a single wall-mounted unit do the job? If so, where to put it? One option could be Point A on the plan, but the closet is in the way. It (the closet) supports a beam that replaces a wall we are removing so it cannot be moved. The next best spot might be Point B in the Dining/Sunroom.

Alternatively, would a ducted mini-split be better? That approach solves the code issue of not having a heating source in the bedroom, but those systems seem to start at 12k BTUs. Seems like overkill. Also, the only place to run the ducts would be in the unconditioned attic – though we could cover the unit and ducts with batts.

The HVAC contractor’s first bid was for a 24k BTU ducted system with 7 ceiling registers. The Manual J (that I paid for) has convinced him that we do not need that much. The HVAC company is a Diamond Mitsubishi contractor, so we’ll be using Mitsubishi equipment.

Thank you in advance for your advice and thoughts.

Happy Holidays!

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  1. irin | | #1

    Did you consider a ductless multi-split system? Your situation is perfect for this scenario: one compressor outside that serves two heads on the main level - one in position B and another in the bedroom. You will not need any ducts then and have good enough airflow. We have a similar 1950’s raised ranch. We tore out all ducts (ducts in the uninsulated attic - disaster and in the basement ceiling - a different kind of disaster) and put Mitsubishi hyper heat minisplit upstairs - just one head in the living/kitchen area and $750 Pioneer minisplit in the basement. We have Amba electric towel warmers installed in all bathrooms - they keep the towels dry and pleasant and help keep the bathrooms warmer and we also have two e-heat envi wall mounted heaters - one in each of our bedrooms - as a back up heat for the coldest time. Washington DC area :) So far everything works :)

    1. kashmi | | #3

      @Irin -- Thank you for that suggestion. I had considered that initially, but opinions here on GBA seemed to be very anti multi-split systems. But I agree with you that that seems like an ideal solution and addresses the code requirement of a heat source in each bedroom (though a wall or ceiling mounted radiant heater could also work in the bedroom). It is good to read how well the set-up is working for you.

      1. irin | | #5

        When we bought our house it was a former rental infested with mice. The ducts were full of mice droppings and other nasty stuff. It made us very anti duct :)
        I agree that multi split might be less efficient and too complicated in the context of a larger house , but when it is only two heads - what are the arguments against it?

        1. Expert Member
          Akos | | #6

          The argument against the multi split is that the smallest unit is 2x to 3x oversized for the loads of the house. The multi splits also have a limited modulation range and much higher limits on the minimum output indoor heads. Unless the multi split is sized correctly for the load, chances are you'll end up with comfort issues and pretty low efficiency.

          Also a two zone multi split is about 2x the cost of a single zone system. So the only thing you are saving is a bit of extra install labor. Since most installers already charge by the number of zones, this is not all that much. So you are paying almost the same price for a unit that doesn't work as well and costs more to operate.

          1. irin | | #8

            Yes, two minisplits are most likely better than one multisplit with two heads. This is what we did in our house - one system on each level. But the question was why someone would choose ducted system over two headed multisplit? Ducts are noisier, they carry the noise of the air and they carry other noises from one area to another, they terminate in registers that do not move automatically, e.g can not follow the occupants as good air handlers do, can not change the angle of the airflow, etc. they accumulate dirt over the years, they need space to install, they are more expensive to install, etc etc the only reason we still cling to them is that they seem to be more “modern” , an improvement over the old ductless 1920s bungalow, for example. Fully functional air handler on the wall is so much more than a register over the hole in the ceiling. You can control the fan speed and position and movements of the fins, creating all kinds of micro environments. You can experience “breeze” as if you have opened a window or make it super quiet, let it throw air across the room or direct it into a certain part of the room. Some air handlers detect occupancy and adjust themselves accordingly. Meanwhile ducts just sit there and accumulate dust :)

  2. kyle_r | | #2

    You can get 9k ducted mini splits. If you are doing a gut remodel, why can’t you put ductwork for the main floor in the basement ceiling, between joists?

    From a heating perspective, most people like having their bedrooms a few degrees colder than living areas. Given this, I think a single ductless head would be fine. However, if you want a nice cool bedroom in the summer, it will be more difficult without direct cooling.

    A ducted mini split is going to provide superior comfort. In a gut remodel, this is what I would shoot for. I would typically only suggest ductless heads when you can’t find a way to make a ducted system work. My 2 cents.

    1. kashmi | | #4

      @Kyle R -- We could put the ducts in the floor joist area between the main floor and basemen. The question then would be where to put the air handler. Space is at such a premium; thus putting everything in the attic seemed to make more sense (EXCEPT for it being unconditioned space, so we'd have to deal with that).

      You are right about it being ok that the bedroom is a bit cooler in the winter. So far I am hearing that "we don't need AC in the summer in Seattle," BUT with warming trends, one never knows!

      1. Expert Member
        Akos | | #7

        Unfortunately Mitsubishi slim ducted units can't be mounted vertically. Even then, they take up very little space. You are looking about 3'x3'x1' box on the ceiling with ducts to each room. Since you are mostly using it for heating, and if your basement is open to upstairs, you can even skimp on the returns, just put a filter right on the unit. I would check local codes though as some require a return on every floor.

        A better option would a Fujitsu or Carrier/Midea slim ducted unit. These can be installed in vertical orientation, you'll need a mechanical room that is a whole of 3' wide and 1' deep. Even in the tightest space, you can find that.

        Unlike a ducted unit where you can swap out a filter every year, wall mounts are also not zero maintaince. Depending on how much you run the unit, the blower wheel will eventually need cleaning which is not a simple operation.

        From a cost perspecitve, a single wall mount with resistance backup in the bedrooms is still your best option. A well installed ducted unit would be much more comfortable and give you full AC with the doors closed if needed in the future.

        P.S Watch the static pressure of your ducting. The low cost option is the SEZ-KD09NA4 indoor head. These have 0.2" wg max pressure, which is doable but have to take some care. The PEAD-A09AA7 can do 0.6" wg for bit more cost.

        1. PBP1 | | #9

          Basement option with a ducted SEZ 9K air handler seems reasonable, though, as mentioned, can't be vertical. I have a SEZ 9K (in closet w/insulation blanket) that feeds a large bedroom, two baths and a laundry room (2.1K sq ft house w/high ceilings (upto 13.5') in MT has a 15K and a 12K too all on a single 30K ASHP (rated at 28KBTU, i.e., 15K+12K+9K (36K) all heating off a 28KBTU ASHP) - thus does seem like 24K is really too much for Seattle). If you get a filter box that mounts directly to the air handler, make sure it's big enough and that the filter size is common enough too. I have an anemometer and have checked air flow, I use web products washable filters on all the air handlers to get the most out of the 0.2" wg max pressure, studies show low pressure drop for those filters. And, as usual, use as large of ducts as possible with few elbows. My 15K and 12K each have two 14"x14" supplies with cross louvers that are adjustable (IMO better than a single wall mounted unit). If the unit is placed in the basement without insulation of the unit or ducts, it will heat your basement too. Why not try for a single air handler (12K?) for basement and first floor? HVAC had an option of two ASHPs but I went with a single ASHP, which has been fine even at -10 F. Attic is nearly never a good option - but Seattle seldom gets cold/snow.

          Just noticed this: "Mitsubishi - 12k BTU Cooling + Heating - M-Series Multi-Position Air Handler Air Conditioning System - 18.0 SEER". Also, SUZ-KA12NA2 ASHP is rated at 15KBTU 47F heating 10KBTU 17F heating and the SEZ 12K is rated at 13.6kBTU heating (specs need to always be checked against nameplate).

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