mini split could solve it, but there are trade offs in many cases.

How big is the house and location of it?

Mini spilts work well with large open spaces. Old houses tend to have small rooms with walls. It may be difficult to achieve uniform temperatures in the winter.

Mini splits work well with open floor plans. But not necessarily large rooms.

The heads sit on one side of the room typically hung on an exterior wall of structure, the return of the heads draws air from near where the head sits on the wall.

So if the room is large, the other side of the room will be hotter / colder depending on the load of the structure. (Load: how hot or cold it is outside, plus how many occupants, appliances etc.)

In larger rooms it's better to go with cassette style head that is located in ceiling as this can be installed centrally in room for reduced hot or cold spot problems in that heads mounted on exterior walls often cause. (This problem is application specific no two jobs are alike in this regard.)

An additional problem is cassette style heads need a good amount of room to be installed in ceiling. Due to architecture of structure it may require a good amount of carpentry work to pull it off. In some cases it may not work.

You should also not forget about 'proper' maintenance of these units.

Franco, air conditioning doesn't travel thru walls... you can't just size a mini split for the whole house and expect it to work.

Each room would have to be sized individually with separate units. That equipment costs money AND then it cost more money to maintain it and repair it as it breaks.

Not saying you can't do mini splits for a home, but typically it's quite expensive from that of a normal central ducted HVAC system... unless the house is very small, with only a few rooms.

These systems more or less originated in Japan, here is a chart by comparison that shows sizing of typical living quarters in Japan. (shock and awe)

This is why mini splits don't work as well as you think here. They can work, but the costs almost always outweigh the benefits. Average living space in Japan

My perception about uneven heat or cool from mini splits is that the former is more of a problem. I live, however, in a raised house with cold floors and it might not be so bad in other situations. Part of the issue is that as the set point is reached, the blower slows down considerably. I am looking at data that indicates it drops to about 1/3 of the highest air flow in some Mitsubishi units. Under conditions near the set point, there is considerably less mixing of air in the room and lots of opportunity for thermal stratification. They make good ceiling heaters. In heating mode, or in dehumidification mode, I always have ceiling paddle fans going in the rooms that I have them. It fixes the problem.

Air conditioning does travel through walls to some extent. Structures vary a great deal in this respect. If you have a well insulated and air sealed home, the heat and moisture transfer rate between rooms is much higher than through the exterior walls so more even conditions will occur. In a poorer structure you will see a larger temperature differential between rooms with heating and cooling sources and those that do not. That is why zoning does not save you much energy in good structures. Accepting all that, no, you can't get the performance in evenly cooling a whole home with the most common type of mini splits that you can with higher static pressure ducted systems.

That said, I like the way mini splits work in a home. A typical summer weekend day with both of us around goes like this in my house. Wake with one bedroom unit going powered by an outdoor unit supporting two. The door is open so the rest of the house is getting some cooling. It is not uncomfortable anywhere unless you start doing push-ups. The high wall unit is running at low capacity and mostly dehumidifying. Hit the kitchen for breakfast. If a lot of cooking is to be done, turn that one on. Outside to do work in the cooler part of the day lots of times for me, but come in for exercise sometime in the AM and turn on another unit in that room. The remainder of the house is still pretty comfortable. As we move around the house and heat load keeps increasing, we may have 4 of 7 going, but probably more like 3 at a time. We have a mix of 6000 and 9000 btu high wall units. During the course of the day we have gone from running less than 6000 btu to a theoretical max of maybe 30,000 btu, but those 3 or 4 units are not running anywhere near full capacity. They are really sucking the water out of the air so the indoor temp is probably above the design temp and comfortable With humidity in the low 40s. We have a total of 51,000 btu. The only time I've ever run it all is for fun, coming back from a trip, or If I forgot to set them to come on in the afternoon. It has been a long time since I reviewed the data. Total heat loads for the house look like about 31,000 btu and install "recommended" 38,500. I guess the difference is a safety margin. In hindsight, I should of convinced the contractor to skip the 9000 btu units. That would have dropped us back to 42000 btu and would have been adequate. What we have works due to variable speed nature of the systems. They run at highest efficiency at part load and that is notable.

I often tell people that if you are happy with the cooling you get from window units, mini splits will work for you. For similar reasons, I think that they are a great solution for a home with nice hydronic radiant heat in an area where not much cooling is needed and the homeowners don't want to lose the hydronic.

If a person in Japan has 300 sq ft, that is 1200 square feet for a family of 4. That is not shocking to me. I am looking at one source that says 300 per person is what the US had in the 1950s. Below is another source that is fun to look at.. What is driving the increase, larger homes or smaller families. The answer is both and the magnitude of the effects is different in different eras.

The biggest problem that I see with mini splits is not big rooms because you can always make a bigger one or add a second unit. There are even good controls for two in one area. The biggest problem is installing low enough capacity since the smallest are 5000-7000 btu depending on manufacturer. Yes ,the variable units throttle back to about 30% of that, but that is the compressor so with more indoor units ganged on a single outdoor unit means not as much modulation and more bang-bang operation. That means in a place with smaller rooms (Japan), mini splits should be a bigger problem than in places with big rooms (USA). The inability to hit low enough capacity with a unit in each room is part of the reason for the higher cost, you will be over capacity no matter what you do unless you have very large rooms.

https://www.propertyshark.com/Real-Estate-Reports/2016/09/08/the-growth-of-urban-american-homes-in-the-last-100-years/

If a mini split system was sold and installed with heads in only a few rooms of the structure with the promise that it would cool rooms in which didn't have any indoor heads installed, there would be complaints at some point. Especially if said home occupant was accustomed to ducted HVAC system.

These complaints would come in the dead heat of summer, because that is when cooling loads are at their greatest.

Mini splits are more or less designed for spot cooling. If the spot is large or multi room structure a mini split can work but you will need much more equipment than a single unit to pull this off adequately under all heat loads that you plan to use the structure for.

Heat moves from hot to cold. There is no such thing as cold, just less heat or absence of heat if you prefer.

An air conditioner works by removing heat from a structure. It doesn't provide any 'cooling'. The amount of heat an air conditioner removes is dependent upon it's capacity.

The underlying problem is the structure is continually taking on new heat in the form of radiation. Radiant heat infiltrating the structure thru windows, thru walls, thru ceiling etc.

People core temperature is a shade over 98 degrees. The more people, the more the heat load. This all comes into play about whether you will or won't be comfortable in a room.

I don't disagree with the fact that using a single mini split to cool more than one room can frequently fail. It might work sometimes. IMHO, having been exposed to lots of situations like this, mini splits can be a very good solution for people in Northern climes who have baseboard heat in older homes with no cooling or put in one or two window units for a few weeks. That is a very common scene. If someone is happy with the heat withdrawal provided by window units, the mini splits are probably a good solution if they would like a permanently installed system with a little better performance.

As a heat source, a couple of mini splits heating a home would be kind of like going back to a wood or coal stove in the kitchen and maybe another in the living room. Apparently it kept people from freezing to death. How comfortable was it? It could lead to better family communication with everyone gathered around the "fire" on cold evenings. Do you like camping?

Yes, you can think of heat running downhill. Hot things are a state of higher energy than the same things that are cool. To move it in the other direction, imposing your own order on the system fighting the tendency to disorder, you need to apply more energy.

Just call me the entropy manager.

Not too long ago, there was a guy with a small home in central Penna that was heating and cooling his small home with mini splits. IIRC, he first had one and then two. His detailed reports here provide a good illustration of the ups and downs of that. Does anyone remember his handle?

In addition, here are some studies that discusses some of the issues:

Long-Term Monitoring of

Mini-Split Ductless Heat

Pumps in the Northeast

Building America Report - 1407

December 2014

Kohta Ueno and Honorata Loomis

and

Building Energy 15

Minisplit Heat Pumps: Lessons from the Field

Marc Rosenbaum and Kohta Ueno

`

The good news is that you have all summer to figure out an economical solution if you are moving into this home.

You have everything in place but a boiler, radiators, circulator pump and controls (thermostats etc.). How much is a new boiler going to cost compared to a couple of MSHPs? What is your heat load? How are you making hot water?

Do you have nat gas? I hope so. If your heat load is low enough, you can put in a Westinghouse high efficiency water heater for a couple of kilobucks. Post back if you have natural gas and can get away with 76,000 BTU and we can discuss more. The appliance I am thinking of is one of the WGR060NG076 series. Any water heater can heat the home, but this one is a very cool design. I think that there are three tank sizes, but they all make 76 kBTU. It should be a good PNP replacement for your high mass boiler and if you have moderate DIY skills, you can probably install it yourself.