Mini split sizing in Houston
I’m hoping on getting some thoughts on sizing Mitsubishi mini splits. I’m in the Houston area (zip 77536)
We’re gutting the entire second story and adding many big dormers to actually make the slanted ceiling bedrooms usable for our growing kids.
Walls will be 2×4 framing, fully filled with open cell spray foam and 1.5” of exterior rigid foam, then Hardie siding.
Roof will be 2×6 construction with open cell spray foam, 3” exterior foam and shingles. Bedroom 2 and west side of bedroom 3 actually have 2×10 roof rafters due to the size of the shed dormer.
The entire roof will be a fully closed, unvented design.
The bedrooms and bath will have high cathedral ceilings. So ceiling height will vary between 8’ at the walls and 11 feet at the ridge.
Window sizes will be about 30”x48”, low E with Argon.
Bedroom 1: 10’ dog house dormer with 2 windows on east side. 2 windows on north side. 11’ Shed dormer with 2 windows on west side.
Bedroom 2: French door on west side. 2 velux skylights 30×46” U factor 0.49 in roof
Bedroom 3: single glass door on west side. 2 velux skylights 30×46” U factor 0.49 in roof. 10’ dormer on east side with 2 windows.
Bedroom 4: two 6’ doghouse dormer on East side with one window each. one window on south side. 22’ shed dormer with 3 windows on west side. Half of that room is on top of my garage but I plan to put spray foam in the floor.
Bathroom has one window. I may take some of the bathroom square footage to make a closet for bedroom 1.
So my original HVAC plan was for 4 separate 6000 BTU Mitsubishi units for each of the bedrooms.
I’ve been playing around with coolcalc and coming in around 3500 and 4500 BTU cooling load for the different bedrooms. I’m really not sure if I’m doing the calculations correctly though.
My other option would be for a ducted Mitsubishi air handler in the tiny space that I have available over the hallway. From there I could reach all room, including the bathroom pretty easily. No idea about the necessary heat pump size though.
Is one option more efficient than the other? After reading through many posts, I was worried about short cycling with 4 individual wall units, but depending on how accurate my load calculations actually are, that might not even be a problem? Especially since the 6K Mitsubishi units modulate down pretty far?
I am somewhat concerned about not getting enough air flow into the bathroom, since the kids like to keep their doors closed.
With a ducted air handler option, would I still be able to regulate temperature for each room? What if I want to shut off one room completely? Like if one of the kids goes off to college…
I would appreciate any kind of guidance you could give me. My HVAC guy proved to be pretty useless. Trying to sell me a giant multi split pump with wall units between 9 and 12 K.
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Your gut feel that the one wall mount in each bed is a bad idea is correct. It gets even worse as you need a 4 zone unit which the smallest is 3 ton. You want the ducted unit feeding the rooms. You are in the land of ducts in the attic so this should be pretty familiar territory for your installer, the only difference is the ducts are inside conditioned space. The BOM cost a single zone ducted unit is also way less than the 4 zone multi split but most installers don't like to do ducting, so might have to ask around. They can make a simple plenum box with takeoffs for each room and home run them in flex:
https://www.kingersons.com/Images/ProductImage/FDMQ18RVJU-ducting-n.jpg
Coolcalc is not the most accurate especially when dealing with non-standard assemblies. I doubt your smaller bedroom cooling loads are above 2k although the ones with west facing doors and skylight could definitely be more.
This is a more accurate for sizing heat pumps for heating don't know how accurate it would be for your cooling load. Free and pretty simple to set up so it might be worth while to give it a try.
https://hvac.betterbuiltnw.com/Account/Login.aspx?ReturnUrl=%2f
As for turning off heat in the unused rooms, it is not an issue with a ducted unit if you oversize the ducting a bit. All that would happen is the airflow would now be sent to the rest of the rooms without any issues.
P.S. Once you add all that exterior rigid, you don't need any spray foam any more. As long as you tape the seams on the rigid or the sheathing underneath for a solid air barrier, you can insulate with regular batts. That is for both walls and roof. There is no benefit or need for the spray foam here which always has some risk of a bad install (plus cost more).
I have been playing around with the HVAC sizing tool. I found it somewhat limited as it wouldn’t let me enter a different ceiling size from the floor size, as needed for cathedral ceilings. Also, I could not figure out a way to specify wall construction and insulation for each room, only assuming the same construction and insulation level for the entire house.
Nevertheless, my results for each room actually came out pretty close to my coolcalc calculations (within 300 BTU for each room).
So what would actually happen if I put a Mitsubishi 6K wall unit in each bedroom and have each of them on a separate outside unit? Not one multi split unit but 4 separate heat pumps. I have the space and enough available breaker spots. According to the manual, the 6K unit can modulate down to 1700 BTU. My rooms come in between 3000 and 4500 for cooling load according to my calculations. So that’s still not big enough to utilize the 6K unit in an efficient way? What BTU load would be considered optimal for the Mitsubishi 6K?
My reason for being skeptical of the ducted unit is that bedroom 4 is actually a guest room/bonus room, and bedroom 3 belongs to my oldest who will be off to college in a few years. So then I’ll only have to cool half the area. Seems easier and maybe cheaper to just turn off two of the 6K heat pumps, than completely shutting off vents to two rooms and be left with a unit that might then be oversized for our needs?
People have posted that have done exactly that, a number of 1 to 1 wall mounts, with very positive results. If you don't mind the 4 outdoor units it is definitely an option.
I still think a single ducted unit is the simplest. If you want to zone off one or two rooms and don't want to adjust things too much here is what I would do. Size the ducting for the remaining rooms to handle the full flow of the ducted unit on low speed. For the rooms you are looking to turn off, install a motorized damper on the supply duct driven by a local thermostat. This thermostat doesn't control the heat pump, it simply opens/closes the damper. When you don't want heating in those rooms, turn off the local thermostat, the damper closes and you have no heat or cooling there. This local thermostat also gives you additional local temperature control when the rooms are in use. The cost of one or two zone dampers, simple thermostats and a 24V transformer to supply it all is a pretty small add on both in terms of labor and materials.
BEopt is another free tool that can give you a rough load calc and takes into account more complicated wall R-values.