Help Sizing a Mini Split System

NY2024 · April 30

Hi all,

I truly appreciate the help I have received from all of you on the Steam page. I was hoping you might have some advice on sizing a mini split system here on the Main page.

We have a 1924 townhouse in southern Westchester County, NY. The above grade square footage of the townhouse is approximately 1650 sq. ft. spread over three floors. Although several of my neighbors (with near identical townhouses) have traditional ducted central air systems, I would prefer to install mini splits to avoid the complicated installation process and associated cost of creating duct work in a house of that era.

Currently, I have a wall thru AC on the first floor (14.5k BTU) and a wall thru AC in one of the second floor bedrooms (6k BTU, which doesn't keep up with the heat in that bedroom, but I think it might be a problem with the AC itself). The third floor bedroom has a 5k window AC which also doesn't keep up with the heat in the that room (it's the cheapest window AC that GE makes though and should be replaced with something better).

I intend to keep my steam boiler/radiators and do not intend to use the mini split system for heating, only for cooling.

Please let me know what you think of the below system design I jotted down, which contemplates a 42k compressor with five 7k indoor units (perhaps one of which could be a 9k mini floor unit instead) attached to that one compressor.

I would also be amenable to having four 7k heads attached to a 30k compressor and leaving the Third Floor bedroom with a window unit (albeit a better one than what is currently there).

I would also be amenable to breaking up the mini splits over two compressors of 18k each rather than one compressor of 42k (assuming 5 heads) or 30k (assuming 4 heads).

The most important thing to me is being able to use the mini split system successfully to dehumidify the house during the summer. I really don't want to oversize the system because I understand this can kill the dehumidification potential of the mini splits.

The brand that I am looking at the most is Gree, because the price is considerably cheaper than the quotes I have received for Daikin or Mitsubishi.

EdTheHeaterMan · April 30

I counted only 5 indoor units with a total of 35,000 BTUh or 37,000 BTUh. Why do you think you need a 42,000 outdoor unit? I believe that a 35,000 BTUh outdoor unit will be sufficient for your needs.

NY2024 · April 30

Hi Ed, the 42,000 BTUh model (made by Gree) is the only model with the ability to attach 5 heads. The 36,000 BTUh model (the next largest model) can attach 4 heads. The 30,000 BTUh model can also attach 4 heads.

EdTheHeaterMan · April 30

Then that is just fine. It makes sense now. Those mini-split multi-zone condensing units are variable speed units anyway. Once you get to the point of needed more than the minimum BTU output of the compressor, then that compressor will ramp up to or down to the actual load requirements of the building. Only if there is a rare condition of turning on the system when the home is over heated (like returning from vacation or after an extended electrical blackout) that compressor will never actually go to full capacity of 42,000 BTU. On the hottest day the compressor will run at 35,000 BTU if all the rooms require all the capacity of each indoor unit on full. That is unlikely that it will ever operate at anything above 35,000 BTU.

pecmsg · April 30

https://forum.heatinghelp.com/discussion/comment/1854063#Comment_1854063

Limit the indoor units to 1 to 1 or 2 to 1. The problem with multiple head installations is when something goes wrong the whole system is down and it could be down for a while depending on what's wrong!

Big Ed_4 · April 30

Nice layout , it's going to be tight by the condenser .

I would put the 9k up in the attic.

EdTheHeaterMan · May 1

The larger 13 x 11 bedroom on the second floor has no air conditioning ?!? have you thought about using 2 outdoor units and splitting them up so you have 3 and 3 indoor zones. That will mean that @pecmsg's concern about one unit causes all 5 to fail, will have less of an impact on your home. That way you have 2 chances to have some cooling if one of the outdoor units fails. You also have an un-air-conditioned room problem solved.

Just an idea.

If you have budget concerns, then you maybe able to do one system this year and another system in the future. You just need to select which rooms are more important to do first. Like maybe the living room and your bedroom and perhaps the attic or one other bedroom. Then tackle the other 3 later.

Kaos · May 1

There is oversizing and there OVERSIZING. Sqft/ton is a terrible measure but that 3.5 ton unit puts the place at 470sqf/ton which is silly for a detached house never mind for a townhouse (of reference that is the real world cooling load of a 4400sqft 100 year old house with new windows).

The units do modulate, but the multi splits don't have great rundown. If this is actually the unit you are looking at and these numbers are correct, seems to have much better modulations than most but still some limits:

https://ashp.neep.org/#!/product/160319/7/25000/95/7500/0///0

It still means that the heads will be either fully on or off. This creates some comfort and performance issues especially in small rooms. The issue is that runtimes will be very short so you'll get a blast of super freeze followed by long delays where the condensation on the coil is slowly re-evaporated (thus not much humidity removal). Rooms will be cold and clammy.

I would put a single wall mount on the main floor (you don't need the two there for cooling).

For the rest, you want a single ducted unit on the 3rd floor with ducts running down to the rooms bellow. This can be mounted in the ceiling of the closet on the 3rd floor with a return straight to the stairwell and ducts down through the floors to ceiling registers bellow. The important bit is the high mounted return on the 3rd floor. Problem is a lot hvac installers don't want to do ducts, so you might have to do some searching or have the ducting done by someone else and the HVAC installer only installs the equipment.

This gets you down to a 2 zone system so you can now use an outdoor unit that is closer to right sized.

NY2024 · May 1

Thank you very much for your comments!

Kaos - would a better designed system be as follows (assuming that running ductwork is not feasible due to cost concerns as well as finding a proper contractor to do the work).

In the foyer, mount a single zone 12k Gree mini split such as: 4LIV12HP115V1AO (with compressor mounted on the wall underneath the Foyer window) - it seems like from the specs this unit can cycle between 2,388 BTUh up to 13,307 BTUh
Nothing in the dining room
In the 2nd floor bedroom (9' 5" x 14' 8") mount a single zone 12k Gree mini split such as: 4LIV12HP115V1AO (with compressor also mounted underneath the wall outside the Foyer window) - again, it seems like from the specs this unit can cycle between 2,388 BTUh up to 13,307 BTUh. Additionally, the door between bedroom (9' 5" x 14' 8") and bedroom (13' 2" x 10' 10") can be opened to allow circulation between those two rooms
In the 2nd floor bedroom (9' 4" x 11' 10") mount a 7k head and in the 3rd floor bedroom, mount a 9k mini wall console. Connect both of these to a dual zone Gree 18k compressor MULTI18HP230V1EO (with compressor mounted on the ground next to the back patio stairs).

Is there any configuration you think would work better than this? Or be more efficient? Would it be better to use two dual zone Gree 18k compressors (MULTI18HP230V1EO) and connect two heads to each one? It seems like the MULTI18HP230V1EO (Gree's smallest dual zone compressor) can cycle between 7,000 and 19,600 BTUh.

Is there a different brand that would offer compressor sizing more appropriate for my needs? The most important thing to me is to have a system that effectively dehumidifies the house.

Thanks!! Mike

ratio · May 1

The current version of Mitsubishi's design software, Diamond System Builder, does mini & multi splits and is freely available at mylinkdrive.com (or trane.mylinkdrive.com if you prefer). It won't help with the sizing, but it will guarantee that everything's compatible, including over/under committing indoor sizing vs outdoor sizing.

Mitsubishi/Tranes are more expensive, but they make up for it with better install training, tech support, & parts availability.

I'm not a dealer, but I have taken the VRF install courses. Very hands-on.

EBEBRATT-Ed · May 1

42K sounds way to large to me.

You need to do a heat gain calculation.

Kaos · May 2

I'm not a fan of wallmounts in a bedrooms. Living with it right now and will be replaced by a ducted unit in the near future. On a one to one it can be made to work, but you sill have to watch oversizing. Typical bedroom cooling load in a sandwich floor is in the range of 2000 to 3000BTU, so you want a smallest unit you can get.

If you look at taller houses, most of the cooling load is on the top floor since air leaks from stack pressure in the summer bring in the hot air from the roof space into there. Generally if you can cool the top floor, the rest of the house will be comfortable.

If ducts are currently out, what I would do instead is get a ceiling mounted 4 way cassette that can accept jumper ducts. Some of the Mitsubishi and LG ones do ie:

https://ashp.neep.org/#!/product/28903/7/25000/95/7500/0///0

Those square knockouts are for the jumper ducts. These are meant to help distribute flow in larger places but if you use some larger ducts, it can run to a bedroom.

Mount this in the hallway ceiling on the 3rd floor (important, not in the attic, but bellow the ceiling). A handyman can box it in with drywall or some nice wood after the install.

Main floor, stick to the wall mount. I would not install anything on the 2nd floor, or if you really must a 6k or 7k wallmount in the bedroom on its own outdoor unit.

With open doors, this 3rd floor unit will cool the rest of the place reasonably well. If down the road you want better local cooling, you can install the jumper ducts and run in them through a couple of bulkheads to the spaces beside and bellow.

There is now a lot of newer R32 stuff coming, so I would look through those to find something that works as R410 is being phased out.

Instead of the ceiling cassette, you can do exactly the same thing with a slim ducted unit with nothing but an air filter on the intake a grill on the supply. These do more pressure and would be easier to duct down the road. The nice part about the slim unit is you can put a real air filter on them so much lower maintaince.

Now down to two zones and since they are larger, the multi split would work well. Most allow for 130% oversizing on indoor units so something like a 24k outdoor unit with an 18k ceiling cassette or slim ducted and a 9k wall mount would be in the ballpark.

Help Sizing a Mini Split System

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