One mini split for several rooms

[Yeager]

I am planning a small above garage (garage is heated/cooled) apartment, it is 780 sqft in South Carolina. Load calcs say I only need about 4k btu's. It will be for short term rentals, so no one living there, just staying for a night or two on vacation. I will be getting a 2 zone unit, probably 9k upstairs and 12k down. Upstairs will have two small bedrooms at 150 sqft each, small bathroom and a 400 sqft main room. FYI- It will be SIPS with R38 walls and R50 roof, and almost zero air leakage and the area is completely covered with shade.

So the question in, can I get away with one ductless unit to heat/cool such a small space? Or will the temp variations be too much room to room? My goal is to keep it simple. I can always get a ducted head unit with 4 outlets, which I know is the best solution, but significantly more costly and complex. One hybrid solution is to start with the ductless head and add a small powered fan register to move air into the bedrooms only if needed.

Has anyone tried something similar? Like maybe an office and restroom in a shop with a single mini? Thanks

 

[PoorUB]

Load calcs say I only need about 4k btu's. It will be for short term rentals, so no one living there, just staying for a night or two on vacation. I will be getting a 2 zone unit, probably 9k upstairs and 12k down.

Your math doesn't work out! Do you need only 4,000 BTU or 21,000? And if only 4,000 BTU, Really? I know SIP is great stuff, but 1/3 ton for 780 sqft?

If it is seriously that tight and well insulated one head upstairs should do it. You might notice a bit of temp difference from room to room but if the numbers are correct it shouldn't be too bad unless someone insists on leaving doors closed in the bedrooms.
Perhaps just a couple inline fans and ducts to transfer air from the living room to the bedrooms would do it. I see it done in motel mini suites all the time where the put in a PTAC and a small ducted fan to move air around.

 

[Yeager]

PoorUB, I was a little surprised also, but I ran the calculations with the zip code, exact dimensions, number and locations of all the doors and windows, insulation levels and sun exposure (which is zero, it's under a complete canopy of trees). The downstairs will be heated/cooled to the same level, so zero losses there. Since the smallest split unit I'm looking at would be 9000 BTU upstairs I have a large margin anyways. I'll run the calcs on a different program to confirm, but that's what it says. FYI- SIPS are usually 1/2 or 1/4 of the heating needs of standard construction and my climate is reasonably mild. It was 77 today, has only dropped below 32 once in the last 5 years.

 

[PoorUB]

See what 50% sun will do to the calcs.

 

[htmdude57]

I remember (around 1970s?) as a kid reading Popular Science, they said that insulating at R38, you need practically NO heating or cooling, because of such a small amount of heat loss through the walls. You are basically heating or cooling ONLY the air inside the house. They said that some research group built a super insulated house in Colorado, and it was basically heated by the people's body warmth.

 

[esanford]

I put a ducted mini split head in the attic between two bedrooms and split the intake and output sides between the two rooms. Just the plenums you see and 90 elbows, so not enough "duct" to matter to the unit. Despite the in/out both being in the ceiling 4ft from each other, it worked wonderfully. We controlled the proportion to each room with the ceiling vent louvers.

 

[Rc_Guy]

We live in a pole barn house in Minnesota, about 1600sqft, don't remember the btu's but I was told it would cool 1800sqft, it does great all summer long.

We use it for heat also but only until it is going to stay cold out for winter, then we turn on the in floor heat.

 

[Yeager]

See what 50% sun will do to the calcs.

PoorUB, I reran the calcs at full exposure and average insulation, it said 14,900 for cooling, so the calculator seems correct.

Htmdude57, with R38 & R50, plus the unit being inside the conditioned envelope with no air leakage, I agree that you could basically have very little heat if it weren't for air quality issues. You essentially just need a tiny bit of heating and cooling to condition the fresh air you are intentionally bringing in through the ERV and to remove moisture, which is important down here.

The calculations do look correct, but I am worried about over/undersizing. With mini splits, is being oversized a big problem? With conventional units- absolutely, but with inverters and variable speed fans, does being oversized matter? I can't order a 4000k ducted unit

 

[PoorUB]

You can oversize them, but you would need to be pretty far off to do it!

I would do like I mentioned erlier. A smaller minisplit head and two ducts with fans to circulate air to the other rooms. Run them off of a wall thermostat or a switch.

 

[Yeager]

You can oversize them, but you would need to be pretty far off to do it!

I would do like I mentioned erlier. A smaller minisplit head and two ducts with fans to circulate air to the other rooms. Run them off of a wall thermostat or a switch.

I was thinking the same, that you could oversize them, but with them being so variable, you would have to be way off. I know old units would modulate 40-100%, I think some newer ones are as much as 10-100%.

I'm still thinking 9000k ducted with 4 ducts, two into the open kitchen/living room, one small into each bedroom with only 1 filtered return. That would be double the size needed, but the ducted units only go so small.

 

[PoorUB]

I was thinking the same, that you could oversize them, but with them being so variable, you would have to be way off. I know old units would modulate 40-100%, I think some newer ones are as much as 10-100%.

I'm still thinking 9000k ducted with 4 ducts, two into the open kitchen/living room, one small into each bedroom with only 1 filtered return. That would be double the size needed, but the ducted units only go so small.

Can you buy a 9K ducted? That is pretty small. The brands I dealt with were around 24K and up on ducted units.

 

[Yeager]

Can you buy a 9K ducted? That is pretty small. The brands I dealt with were around 24K and up on ducted units.

Several companies make 9k ducted heads, they are part of a typically 24k multi zone system. I will have a 750 sqft garage shop downstairs which would get a 12k ductless due to the uninsulated slat and overhead (R18) garage doors. That would have me using 21k of the 24k available from the outside unit.

 

[PoorUB]

I see Gree does have a 9k ducted. Probably the best bet for the situation. The smallest wall hung unit I am aware of is 7K so not much difference. The 9K heads, one ducted, one wall for the lower level and a 18K outdoor and he should be good, or as good as it will get.

 

[Yeager]

I see Gree does have a 9k ducted. Probably the best bet for the situation. The smallest wall hung unit I am aware of is 7K so not much difference. The 9K heads, one ducted, one wall for the lower level and a 18K outdoor and he should be good, or as good as it will get.

I'm not that familiar with mini split sizing on multi zone systems, but I was thinking 18k outdoor, 9k ducted upstairs and 12k downstairs due to the garage doors downstairs. When it's 100 degrees and 98% humidity out, as soon as I open the overhead door the cool dry air will flood out. I don't want to sweat for the next hour waiting for it to catch back up every time I do. Figured 12k would rebound faster. I doubt the two units would ever hit peak load at the same time, so I was thinking having 21k of potential load hooked up to 18k outside wouldn't be an issue, but I don't know. Outdoor units at 21k and 24k are about the same price, but I've heard of issues with too much outdoor capacity with too little load. Thoughts?

 

[PoorUB]

I can see where you might want to bump the garage up faster so a 12K would be fine, and then a 9K upstairs, 18K outside should be fine, but it might vary by manufacturer which heads you can use, but I suspect an 18K would be fine in any brand.

If you go with the ducted unit be aware that most of them are very low duct static pressure. Where a 6" duct woulld normally be fine you might need a 8" duct to get proper air flow.

 

[Syberia]

The calculations do look correct, but I am worried about over/undersizing. With mini splits, is being oversized a big problem? With conventional units- absolutely, but with inverters and variable speed fans, does being oversized matter? I can't order a 4000k ducted unit

If you can find the specs for the units in question, they should list the minimum and maximum outputs. For instance, the 9k mini split in my shed goes down to ~3,800 or so, and the 24k in my garage goes down to 6,000.

 

[Yeager]

Thanks for all the input.
One unit I found today, the Daikin (2MXS18NMVJU) Two Zone Mini Split, is 18k btu's. It says it can have 2 units connected, as low as 7k and as high 15k each, with 24k max allowable connected. So a 12k and 12k, or a 7k and 15k with no issues. They also have a low or mid static pressure ducted unit, the mid static would solve the duct length issues. I'm sure other brands do too, but it seems like that would work out very well.
With the outdoor unit variable and the indoor variable, any reason not to use all the 24k allowable (even though only 18k max at a time)? I mean any disadvantage to 12k and 12k even if I only need 9k and 12k?

 

[chipjumper]

Just my $0.02 but I’d get exactly what you need based on the mini-split calculators. Find one that is very specific (that consider cubic feet). You don’t want too many cooling BTU’s as you might run into humidity issues. The system may not run long enough to dehumidify as its cooling the room too quickly.

 

[PoorUB]

With the outdoor unit variable and the indoor variable, any reason not to use all the 24k allowable (even though only 18k max at a time)? I mean any disadvantage to 12k and 12k even if I only need 9k and 12k?

I wouldn't size the indoor head any larger than you have too. A smaller unit will run longer and dehumidify better. Sure they will modulate, but a 7,000 running at 1/4 speed is better than a 12,000 running 1/4 speed if all you need is 2,000BTU.

On the other hand often they will only modulate down to a certain point, so the 70,00 might only go down to 4,000 and the 12,000 might be the same 4,000. But still I would avoid over sizing if you can.

The only time I would recomend larger is if you need the unit for heating in a colder climate. In your part of the country AC is the major issue.