Minisplit head location?
I’m trying to decide where to locate my minisplit head in my new “pretty good” house. I would like to install it on my upstairs kitchen wall. Possible locations (that aren’t going to be covered by wall cabinets) include above the patio door, above the refrigerator, and above the kitchen sink. From what I understand about installation details, I should have enough room in all three locations, figuring trim and also keeping at least 6″ from ceiling, but I will have the most “wiggle room” at the refrigerator location.
Details:
Climate zone 6/7 (central ND)
8′ ceilings
1200′ upper level
1200′ lower level (4′ below grade)
8’x12′ open stairwell
Lower level will be use radiant slab heating and dehumidifier in the summer
Upper level will receive heat transfer from the lower level and will be supplemented with the minisplit
Leaning toward a Mitsubishi Hyper Heat unit, either 9K or 12K in the upstairs kitchen
I am going to pre-wire for a remote thermostat to be placed in the center of the upper level to control the minisplit, so temperature from refrigerator/window/door would be negated.
Manual J calculations, done by myself, are 10K cooling and 28K heating, for both upper and lower level
I would appreciate any recommendations.
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Replies
Adam,
It's hard to advise you unless you include a floor plan.
In most cases, a two-story house needs at least two indoor units: one upstairs unit to provide cooling during the summer, and one downstairs unit to provide heating during the winter.
Here is a link to an article with more information on this issue: Rules of Thumb for Ductless Minisplits.
Martin, here's the floorplan for the upper floor. Basement in very similar. The reason for only choosing one minisplit unit is that the basement slab will have radiant floor heating in the winter and in the summer I'm hoping to simply use a standalone dehumidifier to control indoor humidity. Upstairs cooling/dehumidification will be provided by the minisplit. And for what it's worth, the bedroom and bathroom doors will be open almost all of the time.
Adam,
I would locate the minisplit head where I have placed the red X.
.
Adam: Some potential issues if you locate it above the fridge:
My mini-split (Fujitsu) is about 12" tall. I assume the Mitsubishi units are similar. My fridge is 69 inches tall. You can get taller ones. If you set your unit so the top is 6" below the ceiling, the bottom of the unit will be 9" (maybe less) above the fridge. That may hinder airflow significantly. My unit requires placement at least 70" high. If the fridge is between cabinets, that may also hinder airflow.
In addition, you need access to clean the filters. It could be quite awkward if it's located right above the fridge.
I don't know if placing it above a heat source, like the fridge, will impact performance. Some people think 6" is too close to the ceiling. Mine is about 18-20" below the ceiling and I'm happy with the performance, but an 8' ceiling height limits placement.
I'd place the unit in the living area, on the west wall or the south wall, assuming the north wall has kitchen cabinets on it.
Using a second mini-split in the lower level might be cheaper to install and operate than whatever radiant heat system you are planning, if you also need a dehumidifier.
Thank you for the responses. Martin, if I install the head in that location (the shared wall between garage and house), could the lineset be run in the attic to that location before I blow in cellulose on top of the drywall? The outdoor unit will be installed on the north side of the house, under a deck. That location seems to make alot of sense. Do you believe it will also provide additional cooling to the lower level since it is right above the open staircase?
Stephen, thank you for the insight into the location. I have also heard of others that mention problems you cite when the unit is located too close to the ceiling. I hope that proper location choice combined with a remote thermostat will alleviate some of the potential problems.
Adam,
As far as I know, the lineset can be run either above the insulation or below the insulation. Just make sure that it is protected from physical abuse, and that the length of the lineset meets the requirements of the minisplit manufacturer.
Adam,
Q. "Do you believe it will also provide additional cooling to the lower level since it is right above the open staircase?"
A. If your house has a good thermal envelope (with enough insulation and low levels of air leakage), a ductless minisplit head on the upper floor should be sufficient to provide cooling to both floors.
Installing it at minimum clearance from the ceiling means it won't hit it's heating performance specs for either efficiency or capacity, since the specification is tested at 68-70F intake air, and the air at the ceiling can easily exceed that. A bit lower would be better, and not just for short-cylcle prevention. A few degrees difference in intake air temp is a real fraction of the total temperature delta that the heat pump has to overcome.
Putting it at the top of the stairs works, and would make it effective for whole-house cooling in the summer as well.
Without ANY common partition wall to speak of between the master bedroom and the fully conditioned space it's likely that you'll have to leave the door open to keep from freezing your tuchus off when it's -10F outside. With the smaller bedroom it won't be as big an issue due to the substantial surface area of the partition wall. There'll be a temperature difference with the smaller bedroom, but there won't be a 15-20F temperature difference. On the master bedroom it could be pretty high with the door closed.
What's the total calculated heat load of that 1232' of upstairs space, and the heat loads of the individual bedrooms, at an ~80F temperature difference? I'm not convinced that the 1-ton Mitsubishi really has it covered at -10F, but maybe.
Martin and Dana, thank you for the insight. As for building envelope, I have 6" of EPS covering all of the walls, with fiberglass between the studs (2x4 up and 2x6 down). Ceiling will have about 20-24" of cellulose. Triple glazed, tilt-turn windows. Very low air infiltration.
Dana, the programs that I have used for load calculation (loadcalc.net and BEopt) don't seem to break down the load room-by-room. I just started using BEopt last week and I'm far from an expert, so maybe I'm missing something. I'm attaching the summary sheets from both programs. BEopt allowed me to customize the wall-type much more than the Load-Calc software. Load-Calc says 10K for cooling and 26K for heating. BEopt, as far as I can tell, has heating at 19K and cooling at 12K. Neither of these calculations take into account the fact that I will be using radiant floor heat in the basement.
I understand what you're concerned about regarding the master bedroom. In all honesty, that door will be open almost all the time. I was also planning to put my fresh-air HRV return in that room, but maybe I should put it in the other bedroom due to the lower-temperature incoming air. I'm planning to install fiberglass in the interior walls to help reduce sound transmission between rooms, so that will probably be another factor to consider.
Thanks again for the advice.
With R13 fiberglass in 2x4 framing with 6" of EPS on the exterior you're looking at a U-factor for those walls of about U0.03 BTU per square foot per degree-F. So at a temperature difference of 80F every square foot of wall will be losing 80F x U0.03= 2.4 BTU/hr.
You have about 28' of running exterior wall on the master bedroom and counting the joist depths about 9' of height for 250' of gross wall area, less maybe 20' of window area, for a net wall area of 230', and net wall losses of 2.4 x 230' = 552 BTU/hr. That's a tiny bit more than the heat input of two sleeping adult humans, so with an alarm clock or two let's just assume the humans & plug loads fully cover the wall losses with the lights off.
If the 20' of window is U0.2 windows, the window losses are 20' x U0.2 x 80F= 320 BTU/hr.
With ~22" of blown cellulose in the attic the ceiling U-factor is about U0.013. With about 185 square feet of ceiling area that's 185' x U0.013 x 80F= 192 BTU/hr
Add the ceiling & window losses together and it's about 500 BTU/hr that isn't covered, plus ventilation/infiltration losses. If there's a steady 20cfm (= 1200 cubic feet per hour) stream of air entering the room from fully conditioned space driven by a ventilation system, that's a heat loss of 500/1200= 0.42 BTU per cubic foot. Air has a heat capacity of roughly 0.018 BTU per degree per cubic foot, that's at temperature loss of 0.42/0.018= 23F. So if the kitchen is kept at 73F with the mini-split, you could see temps bottom out at 50F at design condition. It would be a bit less due to conduction through the floors and partition walls to less-cold rooms, but that's the order of magnitude. If you moved 200 cfm of fully conditioned air into the room the temps would only drop 2-3 degrees, but that's a bit of a breeze- with the wind-chill 200 cfm of 70F air feels like a cool draft- great in summer, not so much when it -10F outside and the air is dry.
This is just the rough handle on it, but it's the right order of magnitude. You can fine tune it on the real window & wall areas. An open door is probably good for 100cfm of convection at a room-to-room temperature difference of 10F, but probably not 200cfm, but it's enough to make a difference, perhaps bottoming out at only 60F instead of 50F.
Mind you, 500 BTU/hr of uncovered load is only about 150 watts. If you stayed up all night binge-watching Korean tele-novelas on NetFlix on nights when it's expected to go down to -10F, between the electronics and a bit of mood lighting and the fact that you're putting out a few more BTU/hr when conscious (even if you might wish you weren't :-) ) means it would probably stay in the mid-60s or higher. If that solution is too horrific to even contemplate, a 150-200 watt space heater would probably keep it cozy even with the door closed.
The HRV entry air is best delivered to the common area that's being fully conditioned, and not the bedrooms. That way the temperature effect of the ventilation air is always in the right direction, and not an additional heat load to the bedrooms & bathroom.
Dana, thank you for the thorough analysis. It's very enlightening. Will the same problem exist in the cooling season?
I think I will put the minisplit on that West wall, where Martin suggested. Running the lineset will be a bit more complicated, but I believe that is the smartest place for it.
That's not really a very thorough analysis at all, just the simplest crudest first-cut arithmetic to get the right order of magnitude. It takes simple algebra to make the corrections for the fact that when the room is colder the heat loss is lower, but it gets complicated when you try to model how much heat is coming through the door when it's close, or the partition walls, etc. (If the heat loss at a 70F room temp is 500 BTU/hr at a delta-T of 80F, it's only ~375BTU/hr if the room is 20F colder.) But it gives you the means to estimate roughly what to expect, and plan accordingly, and demonstrates that counting on the ventilation system to correct it would be a mistake.
As for the cooling season...
The master bedroom window faces north, with very minimal solar gain, and the indoor-to-outdoor temperature difference at the 1% outside design temperature is probably less than half what it is at the 99% outside design temperature. There won't be much of an issue for the master bedroom.
But the other bedroom's window is facing south, and if there isn't sufficient roof overhang or exterior operable shades/shutters to limit direct insolation on that window to reasonable levels it may have comfort issues by mid-afternoon. But it's not any worse than a bedroom in any other house with south facing windows and no air-conditioner. With the door open it'll be fine.
Dana, thanks again. The reason I mentioned moving the ventilation supply register out of the master bedroom was to remove the additional cold (winter) and hot (summer) air from outside. I think I will move it to the east side of the living room instead. I'm only planning one exhaust on that floor, in the bathroom. I know this is a bit off topic, but I was originally planning to install a Broan ERV. Since I have decided to use the ventilation system as bathroom exhaust, instead of installing a bath fan and wall penetration, would you recommend going with an HRV to keep the indoor humidity a bit lower? The price for the HRV is $900 and the ERV is $1000.