Locating Minisplits for Bedrooms
Hi all,
From reading these boards over the last year or so, I’m pretty sure I want to install Daikin Quaternity, because in my 4a climate, it’s not the heat, it’s the humidity that makes sleeping uncomfortable.
Background — I live in an old duplex with plaster walls and ceilings and radiator heat. We have lived without any air conditioning 97% of the time in the first two summers in the house, only resorting to sleeping in a room with a window unit (not our bedroom) when the forecasted low is in the high 70s or 80, so three to five days a summer.
My memory is that the temperature is often 84 degrees upstairs in the evening? It is frequently hotter in the bedrooms with the windows open and box fans going than it is outside at that time of night.
[Normal highs are 87 to 89 June 24-Aug. 18; normal lows around 70/71 same period; average dew point 67-68 in July & Aug.]
The smallest Quaternity is 9,000 for cooling, which I know from reading the boards here is considered too big even our largest 172-sq ft bedroom. (Second bedroom, which we also want cooled, is 145 sq ft.) Third bedroom is very small, under 90 sq ft, but is also attached to a former sleeping porch, now enclosed, but not heated, which is 54 sq ft)
Our bedroom is the front one on the second level diagram above, the largest; at times we will have foster kids in the second bedroom. Third bedroom is a home office, and last summer, we put a window a/c in the sleeping porch to cool that, but barely used it, because our desire not to add to global warming is stronger than our need not to sweat.
So, here’s the question: is it better to put a minisplit on the wall between the two major bedrooms and do some kind of short duct arrangement to have them both cooled by it? And then put a Midea modulating window unit in the sleeping porch to serve the home office? Or is the noise transfer between two rooms too disruptive to kids trying to sleep if we’re talking in bed?
According to earlier Q&A on Midea window unit, its 8,000 BTU window unit modulates down to 2,000 BTU, but usually runs a little above that and uses 147 watts.
I don’t know what size the GE window unit is that we currently have in the sleeping porch window, but I know 5,000 BTU is the smallest on the market. Is a too-big window a/c with an inverter better on power use than a smaller ‘normal’ window unit?
There is an extra deep closet in the second bedroom, whose back wall is shared with the third bedroom, which is 37 inches wide. Daikin unit is 35 inches wide.
So should the solution be a minisplit in that closet to serve both those bedrooms, since noise is not an issue between them, and then put another unit in the main bedroom, even though 9,000 BTUs is overkill?
Is it a lot more expensive to run the coolant lines from interior walls out to the exterior wall? We are in a duplex, and the exterior wall is the left (East) side of the drawing above. If we did one unit to serve the master bedroom, it would go on the exterior wall. If it was serving the two bedrooms, the unit and/or vent would be directly above beds in both rooms.
Our attic is insulated at the roof line, and only runs a few degrees hotter than the second floor in the summer, however, it is fully floored, so I’m guessing ducting up there isn’t that practical?
I don’t think putting one unit in the hall to serve the three bedrooms would be feasible, because the backside of the wall facing the door of the second bedroom is the staircase to the attic; moreover, I don’t think there’s even a way to run the coolant lines in that scenario, given that the exterior wall is on the opposite side of the duplex.
Not to mention that the fact that we won’t be cooling the downstairs at all seems like it doesn’t make sense to have the cooling in the hall, where it would mix more with the hot air rising from the first floor…
From writing this question, it seems the answer is there is no ideal solution, maybe just a least bad one!
For what it’s worth, we would probably find a 78 degree setting comfortable if the humidity was brought down to non-muggy levels.
Mara
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Replies
Is the attic part of the conditioned space? Have you considered hiring an engineer to complete a Manual J and make a system recommendation?
To Steve Knapp:
I *think* the attic would be considered 'semi-conditioned,' but I'm not 100% percent sure.
I've read here that when the attic is insulated at the roofline but there is no return register, that's considered semiconditioned.
But I also don't really understand if that applies to my attic, because there is no forced air heat in my house, it's hot water radiators. (There are no radiators in the attic or the basement; the attic, since we added insulation, tracks more closely the temperature of the second floor than the basement does the first floor. Basement ceiling is insulated, walls and slab are not).
Further adding to my confusion, there are smallish registers of some kind in the larger two bedrooms, and they do connect to the basement. Before we replaced the ancient dryer, we could hear it run in the bedroom through that register. (About a foot tall, not as wide)
Looks somewhat like this, is low on the wall in both rooms
https://cdn11.bigcommerce.com/s-qo2d3v/images/stencil/800x800/products/8537/40284/cast-iron-scroll-grilles__86992.1607675536.jpg?c=2
For what it's worth, current temperature at every level, as boiler is working to warm house up from setback:
basement: 55
first floor: 67
upstairs: 63
attic: 59
Thanks!
Mara
I have not paid for a Manual J. I did run a coolcalc estimation, and it said
15,178 heating BTUs and 2,367 cooling BTUs for the three bedrooms upstairs (I did not include hallway or bathroom)
A guy at heatinghelp.com said a ducted minisplit with the ducts in the attic is the way to go. Which is okay, except that the Daikin Quaternity is only available as a wall unit, and I was really sold on its dehumidifying feature. I've seen a lot of people on this board complain that their Mitsubishi, even in dry mode, doesn't work well for dehumidification.
A different guy at heating help said to put it in the hall opposite the second bedroom and the professionals would figure out a way to get the condensate line up and over the hallway, across the width of the bedrooms and out the wall. Obviously, that would be a lot more expensive than placing a wall unit either on an exterior wall or on a wall that borders an exterior wall, but maybe less expensive than a ducted minisplit?
I am less concerned about dehumidifying when there's not much call for sensible cooling, as we would not run the system then. I am concerned about being able to dehumidify without over-cooling, as I hate feeling cold from a/c in the summer.
Do your attic temps track that closely to second floor in summer? If so, it might be okay to put the air handler for a ducted mini-split in the attic. (I'd like to see input from one of the GBA experts, however.)
FWIW. I have a Daikin ducted unit (Daikin RZQ30PVJU9 Heat Pump and FTQ30PBVJU Air Handler and Coil), and it has a dry mode. One potential advantage of this type of system is that it might be able to serve the entire unit.
If you went with a single-heat unit, that might work fine as well as long as you keep the bedroom doors open.
My memory is that they are no more than 5 degrees hotter than second floor in summer. But it's all quite hot at that point -- about 82 on on first floor, 84 on second floor, maybe 88 in attic??
How low do you keep your temperature with your Daikin ducted unit? Do you think it could lower the humidity to a comfortable level if you had a high temperature setting, like 76 or 78?
I think you will be comfortable if the system is properly sized and commissioned. We are in Zone 3A and like it pretty warm real-round. Thermostats are set in the 75 to 78 degree range all the time. (Air sealing as much as practical also helps.)
You should read: https://www.greenbuildingadvisor.com/article/who-can-perform-my-load-calculations.
Thanks for all your advice!
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