Designing a mini split w/ this floor plan

[Jmichaelp]

Hi, attached is the current layout of my 1964 ranch house in Jacksonville, FL. It's currently serviced by a 15- year old, 3.5 ton, 11 SEER heat pump. The Florida room has an in-wall unit. The original ducts in the attic are sheet metal with original insulation. The attic insulation is supposed to be R-9 (according to the inspection) but I doubt it. The walls are stick frame, not insulated with 50/50 brick/siding. The windows are 2-pane, low e. The roof pitch is 2.1 so limited attic space, which would make new ducts expensive to install (about $5k).

I intend to change the bathroom layout to make them both larger, move the front door North to create a better entry, get a new roof with radiant barrier sheathing & re-insulate the attic (air sealing if I can).

I was going to get a conventional unit, but a couple of neighbors said how much they love their mini-splits so I started looking into that. I thought I'd try to understand the design before I get any bids. I was thinking about a ducted unit to supply the three bedrooms (with a tucked away entrance would I need separate duct for the master bath?), a ceiling cassette between the dining room/den & a wall unit in the Florida room. I did room-by-room calculations using an online tool & came up with 22k cooling & 29k heating loads for the entire house.

Does this look reasonable? I'd appreciate any input. Thanks.

 

[WorthFlorida]

Mini splits are nice and with multiple cassettes around the home you can keep any unit on or off depending on the room occupancy. For one, radiant barriers really work by cutting down the heat load. My Orlando home has a heat barrier stapled to the roof rafters and the attic stays fairly cool on hot days.

New standard heat pump/AC units really are efficient as compared to just a 15 years ago. The federal law states a minimum of SEER 14 is required but to upgrade to SEER 16 is well worth it. Over that and the cost of the unit goes up quite considerably. The one problem you'll have is the old duct work. A large part of the efficiency gain over your old unit is increase the volume of air flow with larger ducts but at a slower rate. Most Florida AC guys can tell if your duct work is undersized. Age of the home is the first clue and since yours was built back in the 60's before SEER ratings were created, it definitely will be undersized.

Therefore, a mini split may be an advantage here. Just be aware that each cassette needs a set of lines and power with each cassette needing a condensate line. Generally, that means mounting on outside walls. Any inside wall is doable if you have a crawl space or the use of condensate pumps. Now the bathrooms, and walk in closets and as you know, AC is needed in these rooms. There are cassettes that can be mount in the attic and exhaust the cool air into the room from the ceiling. Trying to save $$ from redoing your duct work, adding cassettes around will increase the cost to maybe the same price as new duct work. Ask your neighbors how they handled it and how many cassettes.

Either way you'll see your electric bill drop and comfort increase.

Mini unit are now available with an inverter compressor motor. Here is a good description.
https://blog.totalhomesupply.com/what-is-a-mini-split-inverter/

 

[Sponsor]

 

[Jmichaelp]

Mini splits are nice and with multiple cassettes around the home you can keep any unit on or off depending on the room occupancy. For one, radiant barriers really work by cutting down the heat load. My Orlando home has a heat barrier stapled to the roof rafters and the attic stays fairly cool on hot days.

New standard heat pump/AC units really are efficient as compared to just a 15 years ago. The federal law states a minimum of SEER 14 is required but to upgrade to SEER 16 is well worth it. Over that and the cost of the unit goes up quite considerably. The one problem you'll have is the old duct work. A large part of the efficiency gain over your old unit is increase the volume of air flow with larger ducts but at a slower rate. Most Florida AC guys can tell if your duct work is undersized. Age of the home is the first clue and since yours was built back in the 60's before SEER ratings were created, it definitely will be undersized.

Therefore, a mini split may be an advantage here. Just be aware that each cassette needs a set of lines and power with each cassette needing a condensate line. Generally, that means mounting on outside walls. Any inside wall is doable if you have a crawl space or the use of condensate pumps. Now the bathrooms, and walk in closets and as you know, AC is needed in these rooms. There are cassettes that can be mount in the attic and exhaust the cool air into the room from the ceiling. Trying to save $$ from redoing your duct work, adding cassettes around will increase the cost to maybe the same price as new duct work. Ask your neighbors how they handled it and how many cassettes.

Either way you'll see your electric bill drop and comfort increase.

Mini unit are now available with an inverter compressor motor. Here is a good description.
https://blog.totalhomesupply.com/what-is-a-mini-split-inverter/

Thanks for the quick response!

I appreciate the comment about duct size. I had thought about reusing the duct work, but now realize that probably won't be possible. So a mini split might cost the same, as you noted.

I found this article http://www.fsec.ucf.edu/en/publications/pdf/fsec-rr-646-16.pdf which on first read seems to point me to using two systems, one for the bedroom/bathroom area & another for the remainder of the house. Apparently running it all off of one outside unit might not control humidity as well.

 

[Mage182]

I have been working on a mini split design for my house which has a similar layout just a bit smaller. A design I was given by a Mitsubishi engineer included multiple heads to cover the bedrooms and a larger one in the main living space. I'm concerned about microzones and the efficiency of multi head units. However, I also have doubts about the effectiveness (coverage) of one large central head. I'm interested to see what kind of solution comes out of this thread.

 

[Jmichaelp]

I have been working on a mini split design for my house which has a similar layout just a bit smaller. A design I was given by a Mitsubishi engineer included multiple heads to cover the bedrooms and a larger one in the main living space. I'm concerned about microzones and the efficiency of multi head units. However, I also have doubts about the effectiveness (coverage) of one large central head. I'm interested to see what kind of solution comes out of this thread.

Hi, was your quote for an actual unit in each bedroom? If so they would have to modulate down to pretty low BTU levels, right? I wonder how efficient they are at that level. Along with having to see a big box in each room, that's why I was thinking about a ducted system for that part of the house.

For the remainder of the house I'm thinking I could have a 4-way ceiling cassette that would blow down the hallway, into the dining area, into the kitchen & into the den. The hallway bathroom & laundry would get air indirectly.

Alternatively for this area, I could go with a wall unit above the window by the carport, so it's less noticeable. This would cover the den & blow towards the hallway bathroom. Then I could use a wall unit above the cabinets in the dining room, again less noticeable, blowing towards the kitchen/laundry.

 

[Mage182]

Attached are the plans showing each head location, and the diagram showing the hardware to be installed. Some changes would have to be made due to constraints of where where the heads can physically be installed, but it gives you a general idea.

My alternative plan is to install one larger output unit in the living room with uncertainty that coverage would be adequate in the south facing bedrooms downstairs (neither is used as a bedroom). And one 2 head setup with one head in each bedroom upstairs.

As Dana has told me, the multi heads units don't modulate. Which is what has turned me off to the Mitsu design. I've lived with an oversized boiler but the thought of that AC compressor turning on/off all the time as each room needs service is unnerving. Ducted options aren't really an option in my house. It's so small that there is no overhead or closet space to give up.

 

[Dana]

The heads of a multi-split can only modulate the total load is greater than the minimum modulated output of the compressor, which for the MSX-xCxxNAHZ cold climate multi-splits is pretty high. The Fujitsu xxxxRLXFZH cold climate multi-splits have a somewhat better turn down ratio, but still aren't great.

I've become a fan of Fujitsu's ducted mini-splits, which have a great turn down ratio can handle higher static pressures the competition, and can be mounted horizontally or vertically. While not a full-on cold climate mini-split (there is no pan heater for managing defrost ice build up), they use the same compressor technology, and have about the same capacity at -15F (though unspecified in their manuals) as their cold climate ductless minisplits. I'm aware of at least a couple of houses in northern Minnesota that are heated & cooled with xxRLFCD ducted minispits. The defrost ice issue isn't usually a problem, but checking it occasionally during extended cold snaps would be a good idea in a US zone 5 or higher climate, in case the pan drain develops an ice plug and needs to be manually thawed.

Oversizing a ductless head for the room load by more than 2x is often a comfort disaster- try to keep it under 1.5x, and try to keep the compressor oversize factor for the whole house load to 1.25x, 1.5x at the absolute max. Those points are nearly impossible to hit with the "ductless head in every room" approach.