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Mini-split design possibilities… suggestions appreciated

user-6179159 | Posted in Mechanicals on

Update to my prior thread where I was hemming and hawing about the right strategy for HVAC.

House summary:
North end of Climate Zone 5
All-Electric. No gas. No propane.
Main Floor: 1700 sq ft + 1000 sq ft garage. Main living/dining/kitchen are is wide-open with vaulted ceilings, and a woodburning soapstone fireplace in the middle. Master bed/bath on one side of main area (heated tile floor in master bath). Pantry/Laundry/Bath/Office/Mudroom off to the other side of the main area.

2nd Floor: 1100 sq ft (2 bedrooms, 2 baths, living room) primarily over the conditioned garage
Basement: 1700 sq ft, partial walkout + 1000 sq ft shop under the garage (separated from the rest of the basement)

Walls: R30 (2×6, OSB plus 2.5″ R12.5 foamboard outside, sealed joints, dense pack cellulise cavity fill)
Roof: Well-sealed to conditioned space at ceiling with R50+ of blown cellulose (decided against the conditioned attic I’d been considering)

Windows: Triple-pane, U values in the 0.17-0.19 range

Garage is insulated to the same R30 walls, plus a deceptively-rated “insulated” garage door with typically mediocre air sealing.

Basement Walls: R10 for the time being. Will be finished eventually to R20-30. Really won’t be used much until it’s finished. Shop under garage will be insulated to R10-20.
Paid for a “real” Manual J. For heating, it says (numbers are approx)
Main floor: 18000
Basement under main floor: 11000
2nd Floor rooms: 11000
Garage: 8000
Basement Under Garage: 8000
Total Heating Load: 62000 btu

My thought is a pair of mini-splits.
Looks like it takes a pair of ~48k units
One handles the main floor and basement underneath
The other handles the upstairs, garage, and under-garage.

MAIN FLOOR: I understand that airflow and open doors are the way to make temps even out, and not doing that can result in the “cold bedroom” effect. We prefer a very cold bedroom, so no problem in the master. Plus the m-bath has a heated floor. But what I don’t want is a HOT bedroom in the summer. I like a cold bedroom year round. Does that force a unit into the master bedroom that’s independently controlled?

One unit in the main living space/great room makes sense. Will one unit in the “bath/laundry” wing satisfy that large number of rooms if the doors are usually open? Or will the unit in the main room flow well-enough down a hall? The stairway is off the mudroom at the end of the hall opposite the great room area. Plus a unit for the basement, I think I see 2 minimum, 4 maximum in the main floor.

UPSTAIRS over the garage is essentially a 2 bed/2 bath… ok, call it a 1200 sq ft apartment complete with living area, minus-kitchen, for the kids. Kids = doors closed = complaints of too hot/cold. Their bath floors will also be heated. Do you go ducted here with the supply in the garage (understanding that the garage can’t be ducted on the same air supply)? Unit in each bedroom plus living area? Separate non-ducted means 3 units up there. 5 with the garage and under-garage workshop. Has to be a better option than 5 heads. Can systems even handle that many?

Many thanks in advance for the opinions!

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  1. user-6179159 | | #1

    btw, that soapstone fireplace will definitely get a lot of use all winter long. It's rated at about 10,000 btu.

  2. Reid Baldwin | | #2

    Most of what I know about this I have learned from reading Dana's responses to others. If he chimes in, ignore anything inconsistent that I say.

    I suspect that your AC loads are way lower than your heating loads. Is there a way to divide your system so that at least one outdoor unit can be turned completely off in the summer? For example, if you put the basement rooms (which probably have negligible cooling load) on their own compressor, you could probably turn that off in the summer. (and your garage if that will not have AC)

    Is the upstairs laid out such that a head in each bedroom would do a good job heating the common area when the doors are open? The common area may also get heat from the downstairs via the stairway.

    How about one unit in the great room, with its own compressor, sized for 90% of the heating load of the master and great room. Run this unit in the winter but not the summer. Then, have another unit, with its own compressor, in the master sized for the cooling load of great room and master. Run that one in the summer and for really cold weather in the winter. During the day, when cooling loads are high and you care about the temperature in your great room, keep your master door open.

  3. user-6179159 | | #3

    Hi Reid. Still need to get up to see your place one of these days.

    Total cooling load is only 42,000 btu, so any mini-split that is sized to heat on the cold days will be oversized on the cooling side.

    The upstairs is really where I'm struggling. Total load of 11,000, so does it make any sense to put in three 9k heads, one in each bedroom and one in the common area? That's what has me thinking that a ducted system up there might make sense.

    The master bedroom will probably be open most of the time, except to keep it closed off and cool in the winter.

    My thought on splitting up the two systems was based on a right-side/left-side approach. I don't see why an up/down approach for combining them wouldn't work also. Line lengths may be longer. And I do plan on heating and cooling the garage. It'll be heated to about 50, and cooled to around 80. The Manual J reflects those as design temps.

  4. Expert Member
    Dana Dorsett | | #4

    Without re-reading the lengthy prior thread those load numbers seem way high to me, as does the prescribed pair of 8 tons of multi-split. A cooling load of "only 42,000" BTU/hr is way high for 4500' of conditioned space, some of which is below grade.

    Is the Manual-J even credible? What were the 1% & 99% outdoor design temps used, and your ZIP code (for outside design temp purposes.)

    A room that has less than 2000 BTU/hr of load at the 99% design condition isn't a good candidate for getting even a half-ton head (eg: Mitsubishi- FH06 or GE06, or Fujitsu 7RLS) let alone a 9K head. You may be able to get there with 9K -15K mini-duct cassettes supporting two or more rooms as a single zone rather than short cycling the oversized wall-blobs (and the compressor) into poor efficiency.

    A drawing of the floor plan with the load numbers labeled in each room might be useful starting point for this discussion.

  5. user-6179159 | | #5

    What seems off on that, Dana? Or how high is "way high"? I thought it was much better than the 110k btu figure I got from one HVAC contractor!

    Design temps were -3/72F (75deg dT). Wrightsoft was the software used.

    If I figure just the "normal" part of the house, ignoring the garage and basement under the garage, I come out with 41k btu for 2900 sq ft plus a 1700 sq ft basement. Figuring all of the above-ground area, about 4000 sq ft with the garage, I'm at about 38k.

    Why is a pair of 48k btu units over-sized for my figures? If I look at the performance at the coldest temps, the effective rated output is right around 30k btu. Or is that not how they're sized? The heads, yeah, I totally agree. Thus my thoughts on going ducted upstairs.

    I'll see if I can get a plan uploaded.

  6. user-6179159 | | #6

    Here's the layout with the btus per room noted (attached). North is to the left.

    I’ll note also that the house was not designed to be super-efficient, but it will be built to be efficient. By that, I mean that it’s not an over/under box with small windows and a simple roof. It’s a bit sprawling, and windows are where they look good and where they offer good views. They’re efficient triple-pane, but the glazing area isn’t small.

  7. Reid Baldwin | | #7

    The design temperature for my house, located about 5 miles away, was 8 degrees on my manual J. That is exactly what my outdoor thermometer measured this morning from 3-4 a.m. We are forecast to see temperatures below zero for a few hours over the next week. However, as is often mentioned here, you don't need to design to the minimum temperature. A well-insulated house cools down pretty slowly even when the heating system is producing only 80-90% of the heat load.

  8. user-6179159 | | #8

    Comfortable with the recent cold, I assume, Reid?

    That's where I'm struggling a bit is in the design temperature. (I'm surprised that we had different design temps, when the same person did our Manual Js. Interesting) Naturally, we don't have see 0F for days on end, it's usually a nighttime dip. In those cases, I'll more than likely be burning a fire in the fireplace (rated around 10k btu) and a house temp of 60F is more than enough at night.

    The more I look at it, the more I think a ducted system makes sense upstairs. One head could easily handle the rooms up there. Putting in more heads would just be a waste. The question really is how to apply the btus, or put differently, how to spit up the house between the two systems that will be required.

  9. Reid Baldwin | | #9

    I re-checked my manual J. The design temperature is 7 degrees (not 8). The predicted heat load is 34,794 btu/hr. My two-stage furnace has an output rating of 39,000 btu on high providing very little margin. On the recent nights that the temperature hovered around the design temperature all night, the furnace ran continuously from at least 3 a.m. until about 9 a.m. When I got up at 6:30, the temperature was about 1 degree below the setpoint, so it apparently wasn't quite keeping up. Even so, we were comfortable. One degree is barely noticeable. Soon after the sun came up, the house started to heat up. We get a lot of solar gain, especially in the morning hours. By early afternoon, the temperature was several degrees above the setpoint due to the solar heating.

    The less comfortable situation actually occurred one cloudy day last week when the outside temperature hovered around 20 all day. Apparently, our dog has learned to open the door between the house and the garage. The door was open for about 4 hours while no human was home. (We are going to change that door knob.) The garage was about 40. My daughter closed the door at 1:30 p.m. When I got home at 6:00, the house was still three degrees below setpoint. A couple hours of running the gas fireplace brought it up to temperature.

    With your level of insulation, don't expect much difference between daytime indoor temperatures and nighttime indoor temperatures. The house simply doesn't cool down very fast (unless someone leaves a door open).

  10. GBA Editor
    Martin Holladay | | #10

    Yes -- if you have lever doorknobs, dogs can learn to open them. (Clever little monkeys.)

    Old-fashioned round knobs are harder for dogs (and old people) to use. What to choose depends on the species mix in your home.

  11. user-6179159 | | #11

    Another justification for heating my garage...

    "Why do we need to heat the garage??"
    Because you like those doorknobs.

  12. Expert Member
    MALCOLM TAYLOR | | #12

    Interestingly, jurisdictions across Canada have started to mandate the use of lever-knobs as part of their disability provisions. Dogs always win!

  13. Reid Baldwin | | #13

    Yes, we have lever door knobs. I will be changing the one between the house and the garage to a traditional round knob. The front door also has a lever knob on the inside but we always use the deadbolt on that door.

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