2nd Floor Mini Split Placement in Short, Narrow Hallway
hi_res
| Posted in Energy Efficiency and Durability on
Hi all,
I’ve got a situation and question similar to that posed by Nathan Efrusy in the very helpful “Keeping Cool in Detroit” piece around best placement for a single ductless mini split to effectively cool and dehumidify a 2nd floor.
In our case, I’m wondering if 1) a single ductless mini split in the upstairs hallway will be effective given the short hallway distance (~9.5 ft) we have before the stairs start (see attached floorpan), and 2) what the optimal line run might be for said mini split given the layout of the upstairs?
Context: We are in Barrington, RI–winter design temp of 12, summer of 87. Humid summers. We have a 1.5 story 1880s house with 2 bedrooms and a bathroom on the second floor (~600 ft sq). We’ve had the exterior walls insulated with blown in cellulose, have an insulated attic with attic damming of 12″ R-38 unfaced fiberglass batts, 15″ of blown-in cellulose in the open attic, and 6″ of cellulose blown into the sloped ceiling; and new low e2 fiberglass windows just installed in the fall (new construction windows, not replacement). On the second floor, ceiling height (7’4″) and attic limitations make a ducting situation difficult, so we’ll likely need to go ductless.
I’ve run a couple of Manual J calculations using CoolCalc (block load and room-by-room), and on the high end of those for the 2nd floor, we’re at about 7900 BTU for heating, and 5000 BTU for cooling. The first floor will be heated/cooled separately with a different single-zone mini split.
My thought in terms of upstairs mini split hallway placement is reflected in the attached floor plan. I’d originally wanted to do a wall-mount in the noted location (south end of hallway), but there is only about 4 inches from the mounting location at the end of hallway and the doorway opening for the master bedroom, which poses a series head-knocker risk. So I’m thinking a floor mount might be needed here instead. In terms of sizing, the installer is a Mitsubishi shop, and the smallest Mitsubishi floor mount unit available is 9k BTU, which more than covers the heating and cooling loads (though by way too much??).
Is the fact that we have only about a 9.5-ft run before hitting the stairs going to result in all that cold air tumbling right down the stairs before it has a chance to try and cool/dehumidify the 2nd floor? A note that all the doors upstairs stay open during the day and night (the exception being the bathroom if someone is using it 😉 ).
My second question is around the line run for the mini split. With the hallway placement, the installer had suggested–for aesthetic purposes–running the lines through the closet directly behind the mini split, then going inside and along the south side knee wall to exit out the east side exterior of the house and down the back to the compressor. Even assuming the lines are insulated, is that just asking for trouble with moisture inside the wall? I like the idea of not seeing the lines run on the outside of the house, but I’m concerned about sweating condensate lines inside an exterior wall. My suspicion is that there is inherently less risk in running the lines initially through the closet (which I am fine with) and then straight out the south side wall to the exterior of the house, where they could run horizontally and then down to the compressor on the SE side of the house.
Thanks so much for reading this far, and for any thoughts and assistance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
You can do that but bedroom cooling would only happen if you keep doors open. The hallway mounted 2nd floor unit will also doing a good part of cooling the main floor, so you don't want to undersize it.
My $0.02. Install a slim ducted unit by the outside wall above the staircase. Even if you don't duct it, you'll at least have a proper filter and the option of ducting it down the road (ie bulkhead on either corner of the hallway will get you to both rooms and bathroom without making it feel too low).
Thanks, Akos! Appreciate that perspective. We'd thought about installing a unit in that location (outside wall above stairs), but were worried about people banging their heads while going up/down. Will look into the specs on the slim ducted unit.
Quick follow-up here. I'll be exploring the slim ducted unit by the outside wall over staircase, though with the sloped ceiling we have over that wall, it could be tough.
In the meantime, though, I'd appreciate a sanity check on some estimates I've received. I had the installer quote out pricing for a 9k Mitsubishi floor mount ductless unit in the upstairs hallway wall on an interior wall (lines to go straight through closet and out south side exterior wall), plus a 12k Mitsubishi wall mount ductless unit for the first floor (on an exterior wall). Both indoor units have their own hyper heat outdoor units (1:1 setup).
The quote that came back seemed outrageously expensive, despite the fact I had been preparing myself for the inevitable markup: ~$23k in total. ~$10k for first floor unit and ~$13k for the second floor unit, which includes ~$2,200 in electrical per unit (~$4400 total for electrical work). The electrical is just wiring up the indoor/outdoor units--our panel is already upgraded, so no extra work there.
Am I wrong in thinking the above pricing is absurd? I understand the equipment prices I see online are likely jacked up by at least a factor of 2, and of course labor is padded as well, but the estimate we got seems like quite the markup. Am I totally off-base here?
Consider putting a conceal ducted unit in a drop ceiling of the master closet with ductwork feeding both bedrooms and the hallway.
Walta
Thanks, Walta. This is exactly what we're considering now.
Is there such a thing as a duct run being too short? I ask because though that closet seems to be perfectly situated for this setup in that it shares wall space with the master BR, second BR, and hallway, for that same reason the runs to each room will be very short (esp as we attempt to keep the ducts inside our 2nd floor, instead of them going through the unconditioned (though insulated) attic.