Multi-zone mini-split vs. ducts

[JackOfDiamonds]

What do you do when your total square footage is too small for a normal central system, but you have 3 spaces that are each too small for even a 9,000 BTU mini split?

I'm a big fan of mini-splits and already have one in my shop. I'm now planning to partition out a bedroom, bathroom, and office out of existing space. Total square footage will be maybe 600 sq ft, so 12,000 BTU or 1 ton should be enough, but I need to cover all 3 rooms.

Since I'll have 3 separate rooms, a single mini-split won't work. I either need 3 separate mini-splits, and the smallest ones you can get are 9000 BTU, so overkill. Or one of those 3-head mini-split units. That adds up to something like $3000 either way, not to mention 3 outside units would be sort of ugly.

It sounds like a normal ductful system might be cheaper. I can run ducts just fine, but I don't know anything about the units. And it looks like about the smallest unit you can get is 1.5 tons which is too big.

I would want an advanced heat pump that both heats and cools so I don't need to run a gas line, and heats down to single-digit F temperatures like a good mini-split. But it seems like you need HVAC experience to spec out a central system. It's not as easy as just picking a BTU and a SEER like with mini splits. How do I actually shop for a normal central system, and do you think it will be cheaper than mini-splits in this case, or should I stick with what I know and get a 3-head system?

 

[danski0224]

I would want an advanced heat pump that both heats and cools so I don't need to run a gas line, and heats down to single-digit F temperatures like a good mini-split.

And what is the heating capacity at that temperature? Is it enough?

What heating and cooling capacity is needed for the space? You could guess, I suppose.

It's not as easy as just picking a BTU and a SEER like with mini splits.

It doesn't really work that way. heating BTU will vary with outside temperature. Typical spec points are btu capacity 47* and 17* F. Unless the manufacturer publishes XXXX btu at Y*F. There will be equipment matches to attain this- maybe not so much with mini's, but definitely with split systems (even fancy inverter ones).

How do I actually shop for a normal central system, and do you think it will be cheaper than mini-splits in this case, or should I stick with what I know and get a 3-head system?

There are ducted mini splits.

A "regular" split system with an inverter compressor will not be "inexpensive".

A "standard" air source heat pump/AC unit will not be a good choice.

As far as price goes, you are on your own there. I'm sure that there are a few members here that know how much it should cost...

 

[JackOfDiamonds]

Now that I shop around a little more, the smallest available 3-zone mini split systems run about $2700. They will be oversized for the space, but with the inverter compressors, experience suggests they handle being oversized pretty well, and that should help with the heating capacity on cold days. I doubt a ducted system, at least not a modern one with a good heatpump ratio, would be cheaper plus would probably be oversize.

I will look into the ducted mini split though. The total space should be easy to heat/cool with a 12,000 btu mini-split, it's just being in 3 different rooms that complicates it.

 

[Firebrick43]

I heat and cool shoulder season with one 18k mini, 4 rooms and 1200 square feet and it does great. Winter heat is in floor hydronic. The way minis fan back and forth really helps.

Doors are usually open and sometime in the bedroom a fan helps circulate but there has been no complaints of discomfort

The office door and closet doors are louvered for circulation

 

[chinboys]

I am partial to LG heat pumps since I use them for own home and have installed for friends.
A two zone 18k unit driving a 9 and 7k wall evaporator might fit your needs and priced under $3k. You can use the larger capacity evaporator to cool or heat your bathroom.

 

[SBAG]

Mitsubishi has 6K single zones.

 

[Dig Doug]

They have ducted mini splits, I just bought one for my bedroom, it’s so you Don’t have a big Ol cube hanging on the wall

There is a cassette that gets mounted in the attic w/ supply & return ducting

 

[JackOfDiamonds]

If it's in the attic, then it's in non-conditioned space right? I'm sure that has some impact on its efficiency, even if you try to insulate it.

I do like the idea of having a single air handler in the attic or something and then running ducts to each room. Basically making it a miniature central AC system. Because 12,000 BTU should be more than enough for all 3 rooms, but you can't buy normal central systems under 1.5 Tons and those are big. But to do ducts I have to think about how to get return air back to the air handler too, check the pressure drop of the ducts and so I can't decide if that's the best of both worlds or the worst of both worlds.

If it were just me, I'd probably put a single mini-split on one side of the addition and just let the other rooms be conditioned by diffusion or with some ceiling fans. Pretty sure that's the spirit of the whole mini-split thing in the first place, because outside North America people don't think it's required that every room be exactly 72 degrees all the time. But I'm sure as soon as there's a hot day and the bathroom is 5 degrees warmer, the quality control organization (wife) would complain. Actually if we had a separate indoor unit in every room, even if oversize, it would let us deliberately adjust temperature in each room.

 

[Jackfre]

When I represented Fujitsu and Tjernlund I solved this kind of situation with Airshares by Tjernlund. Yu can make fancy controls but easiest is to control it off a wall switch. Position it in a corner away from the door and short cut the door for a return path. The other option is to use the 12k mini with the concealed air handler. If you do this be very careful of the duct lay-out as most of these evaps do not have much static capability. We did thousands of apartments this way.

 

[danski0224]

If it's in the attic, then it's in non-conditioned space right?

Putting any HVAC equipment or ducting in an unconditioned attic is the absolute worst thing you can do.

If it is a permit job, and if your municipality is following the 2012 Energy Code or newer, you might have some surprises ahead with respect to ductwork and leakage testing.

If that's the path, then spray foam the attic roof and make it conditioned space. Or put foam panels on the exterior, like a commercial flat roof- but this brings issues with exterior trim.

 

[428PI]

I have perhaps 1200 sq ft on my main floor and am heating and cooling it with 27k. One unit is a 9k in our bedroom and I have an 18k in dining room. Will keep main floor cool in summer (around 73-75 and way cooler in bedroom. But in winter in normal weather allows hallway to get to 65 but we had a cold snap of below 0 weather and it got to 55 in hallway. I wouldn't be afraid of 3 9k units if you plan to use them for most of your heat. I would have a backup though just in case. Our bedroom is perhaps 175 sq ft.