Mini-split, conventional split, RTU for build?

[strutaeng]

Hi guys, I'm planning to do a house addition to my small house. The new part will be 21x62, two stories, but 21x44 will be multi-purpose/family room double volume space (17' high.) The remainder will be 21x18, 2 levels. The long sides are roughly oriented N-S. I'm in USDA region 3. I'm using R19+R5 continuous insulation for walls, R38 for ceiling. Not much glazing.

My AC guys (also my BIL) told me use a another conventional split system for the addition, but I'm considering using a mini split for the family room (21lx44wx17h) and using my existing AC to cool/heat the remainder 2-story part, since my existing conventional split is way oversized for my current house (4 ton for 1350 sq. ft.) This will require some ductwork for the existing system.

Considering the family room is long, I suppose the 2 ceiling cassette system will work fine if I got min-split? I won't have access to the ceiling because of the height. I have gas service, but have not decided if the mini-split will use a heat pump to heat. Frankly, I don't know much about min-splits.

Or should I just go conventional split (I like 2 speed) with gas heating? RTUs are not used in residential DFW, but I'm building from scratch, so anything is possible. My AC guy does split, RTU and also has installed minis.

Thanks in advanced.

 

[Black Oak]

In doing my research on mini's, most pro's told me to avoid the cassettes. You could search the many threads here to learn more. The top tier brands might be OK, I don't know.

 

[strutaeng]

I should add that I can wall-mount head unit along one wall.

 

[eddieK]

Hi guys, I'm planning to do a house addition to my small house. The new part will be 21x62, two stories, but 21x44 will be multi-purpose/family room double volume space (17' high.) The remainder will be 21x18, 2 levels. The long sides are roughly oriented N-S. I'm in USDA region 3. I'm using R19+R5 continuous insulation for walls, R38 for ceiling. Not much glazing.

My AC guys (also my BIL) told me use a another conventional split system for the addition, but I'm considering using a mini split for the family room (21lx44wx17h) and using my existing AC to cool/heat the remainder 2-story part, since my existing conventional split is way oversized for my current house (4 ton for 1350 sq. ft.) This will require some ductwork for the existing system.

Considering the family room is long, I suppose the 2 ceiling cassette system will work fine if I got min-split? I won't have access to the ceiling because of the height. I have gas service, but have not decided if the mini-split will use a heat pump to heat. Frankly, I don't know much about min-splits.

Or should I just go conventional split (I like 2 speed) with gas heating? RTUs are not used in residential DFW, but I'm building from scratch, so anything is possible. My AC guy does split, RTU and also has installed minis.

Thanks in advanced.

Upstairs and downstairs gain and release heat at differing rates - Upstairs has additional heat load from attic/roof. Downstairs is under a conditioned space.

I would avoid just adding ducts, or installing a single system to condition two different level zones, unless there is room to install control dampers (manual or low voltage) and/or installing a zone control system.

In my experience adding the zone control is as expensive as installing 1 min split system.

The advantage to the mini split is it does provide heat and the SEER rating is substantially higher. I also recommend only using either Mitsubishi or Fujitsu.

 

[yeldogt]

I used two mitsubishi cassettes -- one for each large 2nd floor bedroom in a big cape. They don't have the same controls as the wall units. This is a three head hyper heat system -- the other is a wall unit.

How much heat do you need?

On the above project -- I removed the above second floor unit in the attic -- the two ceiling units replaced that unit. The third unit (wall) went into a new single story slab on grade one room 15-22 addition w/ cathedral ceilings. It's open to the house --- the mini is the AC for that space -- the house has radiant heat -- but the mini did the room heat before it was finished.

Also -- I use spray foam. Look into doing a flash coat and batts -- find the money -- cut someplace else. People just don't understand how foam transforms a house. I try and tell everybody - but ... unless you have lived in one you don't get it. With proper foam the HVAC needs drop like a stone -- still need fresh air and humidity control.

I have done zoned systems -- with new equipment -- they are fantastic. But - they require someone to spend some time doing it correctly.

 

[MushCreek]

+1 on foam. Our house is ICF (concrete inside of foam) and our cooling load for 1440 sq ft is 9000 BTU in hot steamy SC. That's 3/4 ton for the whole house! We have two wall-mounted Mits minis- a 12K that does the whole house, and a 9K in the master suite for flexibility.

 

[theoldwizard1]

In doing my research on mini's, most pro's told me to avoid the cassettes.

I have heard the same. Maybe fine for a small room like a bedroom, but certainly not a large room.

 

[LS6 Tommy]

I'd go with two systems, regardless of what type you choose. You'll never be happy with one system serving two zones. I'm also one of the "Pros" that avoids cassettes like the plague.

Tommy

 

[yeldogt]

I'd go with two systems, regardless of what type you choose. You'll never be happy with one system serving two zones. I'm also one of the "Pros" that avoids cassettes like the plague.

Tommy

I don't get why you say this ? Building new and properly designed the new VS units w/ zoning are fantastic _ grew up in a house with 3 zones from the 50's. I would much rather have one zoned vs two separate. The only time I did 2 units -- I regretted it.

I have done many houses with zoning -- typically three ...up/ down and great room.

So far I'm happy with the cassettes ... this was the third summer.

 

[yeldogt]

I have heard the same. Maybe fine for a small room like a bedroom, but certainly not a large room.

Some say they are not reliable -- they use them in large offices .. so they can do large spaces.

Mine don't have the same controls -- they don't seem to have the same VS control of the system as the wall units. Mine work very well -- the bedrooms are large. 22x26

 

[strutaeng]

So what size mini-split would you recommend for the (21lx44wx17h) family room?

By the square foot "rule" I'm guessing 2 ton, but volume-wise a 4 ton?

 

[Sjfab]

with high ceilings like that you will have to think about maintenance on the cassettes. Cleaning the filter will be a chore, what about condensation removal?

better off doing a separate system for the addition.

To me, mini splits are designed for retro fit applications.

 

[Derek8819]

I have a Friedrich cassette in my 225sq foot above garage bonus room, been on 2 years straight and not a hiccup yet. The garage below has a regular wall mount head connected to the same outdoor unit. The cassette for sure isn't as powerful and takes longer to cool/heat the space (even though its smaller) when you want to surge it one way or the other when the bonus room is occupied. It looks cleaner though than the wall mount.

 

[MushCreek]

I used HVACCalc for my calculations. Guesses can be way off, which is not good in either direction.

 

[yeldogt]

So what size mini-split would you recommend for the (21lx44wx17h) family room?

By the square foot "rule" I'm guessing 2 ton, but volume-wise a 4 ton?

It's all about insulation -- with full foam and great windows I have done 1000 sf per ton with great results. Also, if you have bleed through from another system -- that will be factored into the mix. With high ceilings -- the key is to AC the area 8' and lower -- don't be blowing the heat off the ceiling w/ a fan.

Also -- the cassettes can't be too far up -- the filters need to be cleaned. Not going to work for you -- they also need to be level.

 

[strutaeng]

It's all about insulation -- with full foam and great windows I have done 1000 sf per ton with great results. Also, if you have bleed through from another system -- that will be factored into the mix. With high ceilings -- the key is to AC the area 8' and lower -- don't be blowing the heat off the ceiling w/ a fan.

The other system will have air flowing through this room, but the temperature of adjacent spaces will be kept constant.



Also -- the cassettes can't be too far up -- the filters need to be cleaned. Not going to work for you -- they also need to be level.

Yes, I realized this. I posted an image of the floorplan: I can mount the head units against the balcony/hallway at the second level. This will locate them higher to the ceiling, but will still be accessible.

Thanks you for your help.

 

[Trey T]

Looking at long-term investment, I would stay away from the popular the mini-split (wall unit or cassette). For your case, the primary purpose of using mini-split is bc you have no access for duct work - plain and simple. If it was my home, I would go with the conventional central air.

 

[yeldogt]

Looking at long-term investment, I would stay away from the popular the mini-split (wall unit or cassette). For your case, the primary purpose of using mini-split is bc you have no access for duct work - plain and simple. If it was my home, I would go with the conventional central air.

Not sure what exactly ... long term investment refers.

In an ideal world my preference would be a properly designed conventional ducted system -- VS equipment with zones. Unfortunately -- this is not always possible. MS now offer a solution that was previously not possible.

Surprised how well they work in conjunction with spray foam. A great benefit w/ducted systems is the ability to have many air outlets -- Conventional construction requires this for comfort as the many leaks create drafts -- this is not required with foam and the single point source works very well.

 

[LS6 Tommy]

I don't get way you say this ? Building new and properly designed the new VS units w/ zoning are fantastic _ grew up in a house with 3 zones from the 50's. I would much rather have one zoned vs two separate. The only time I did 2 units -- I regretted it.

I have done many houses with zoning -- typically three ...up/ down and great room.

So far I'm happy with the cassettes ... this was the third summer.

Every single "zoned" conventional split system I have dealt with has had coil freezeups, capacity issues, and eventually damper actuator issues. I've serviced probably 40 of them.

The cassettes I've dealt with (multiple brands) never move enough air, have condensate drip issues and eventually burn out the blower run caps, which requires almost complete disassembly of the unit to replace the cap. I've serviced 100s of them.

If you've had better luck with a larger number of both typed of units, I have no rebuttal.

Tommy

 

[bobbyjean]

ductless with high seer ratings are all heat pumps with inverter technology,go a little large on sizing,add ceiling fans,tap into system for upstairs...check static pressure of both supply and return with clean filter...dont just add runs without doing that first-return 1'' supply .5 to .8-your ac guy should know ,if he doesnt get a ductolator for yourself,you can make a water tube manometer for near free..sorry not good with adding links here but this info is all online