Experiences with mini split HVAC systems?

I’m going to either have to replace a motor on my pellet stove or go with another heat source. My house is 160 years old and has very little crawlspace area. Current;y I’m heating with wood pellets and have a window A/C unit for cooling. I’m thinking of installing a mini split system. Does anyone have any experiences with them?


No idea, but we are considering one of the mix n match sort [we will need something like a 12000 and a 30000 split, as we have 2 spaces in our house, a 12x20 bedroom and a 20x30 main house]

I have heard good things about mitsubishi, but we are planning on taking a fair amount of time researching when it is time to buy. I am seriously tired of the whacked hot water baseboard piece of crap we have in the house - classic problem. Of you are in the room you have to pretty much be sitting next to the damned thing to get warm, and the side towards it roasts while the side away freezes and if you have any furniture in the room, it can not go near the wall or you block the damned heat.

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I’ve installed a couple of them, and had good luck with them. Both were heat pumps, which work great down to freezing, but cannot work below freezing. I have seen one that combined electric AC and gas heating that would work to sub-zero temperatures, but I didn’t get the manufacturer.

Mitsubishi’s “Mr. Slim” line is popular but expensive. Samsung is of equal quality, but a lot less expensive. Carrier and Goodman appear to be the same Chinese OEM unit, and you’ll get (IMO) lower quality without saving much. Not a big fan of Goodman in general, which is a notorious “Builder’s Grade” brand of AC.

The two I’ve installed were for converted spaces - one a very large porch that we converted to an accessible master bedroom suite when my brother-in-law developed MS. It heated and cooled a space 15 foot by 24 foot. The other was for a friend’s art studio, which had formerly been a two car garage.

The main reason to go with a higher end unit is air handling. The Samsung and Mitsubishi units do very sophisticated air circulation, with motorized louvers that do a pretty good job of moving the air in a good sized room.

**Thanks, Pat and Aru

Gaffa** - Heat pumps are pretty common in Tennessee, where it might get as cold as 10 degrees, but not frequently and not for long. The area I’m looking to cover will be about 600 sq ft. My step-father keeps sort of pushing those free-standing A/C heat units that vent through the window. Something like this, I guess. I just figure a permanent fixture is going to be more efficient and will keep the area more comfortable.


Not a fan of those at all. They are very inefficient compared to a split ductless. Hell, they’re inefficient compared to a window unit! That, and you want to have the cooling up high, which is pretty much impossible with those.

gaffa - Doesn’t the heat stay way up high with the split units? Is the fan strong enough to blow i t down to where I actually live? I have 10’ ceilings in this part of the house. I know there’s nothing requiring you to have them all the way near the ceiling, but out of the way would be nice.

Also, how much is the installation, approximately?


I installed both of them just above head height, and both rooms are very comfortable with good air flow. Heat pumps in general operate at a much lower temperature than gas furnace, so some people think they are not as warm, but they will get the space to the same temperature, they’ll just move a lot more air to do it, rather than rely upon the radiant heat of a mass of fairly hot air.

I honestly couldn’t say, as I was doing this with my brother. But here’s what’s involved:

The exterior unit either hangs on a wall, or sits on a pad like a regular condenser. It will require either 110 or 220 electrical service, depending on size. It will also require a smaller cable to the interior unit that powers the fan and controls the operation of the exterior unit. The main connection between the pair is the two copper lines, one larger, one smaller, known as a line set.

Most of the interior units are designed so that only one hole has to be drilled for the line set, the small electrical cable and a condensation drain to the outside. Usually you only have to drill a 2" hole (plus the holes necessary to mount the interior unit on the wall).

You’ll have to add either a single or a double breaker to your electrical panel for the unit, plus wiring between the panel and the exterior unit.

You save over a forced air furnace by not having to install any duct work, or a separate thermostat, or build a space to hold the forced air unit.

There are too many variables to give even a ballpark. But they go in a hell of a lot quicker than any other type of unit, which means that a lot of people will try to talk you out of it - less equipment + less labor = less profit.

gaffa - Did you have to charge or fill the system with coolant?


Depends on the length of the line set. Under 25’, the units we installed had enough R410a to pressurize the line set and the interior unit. But you will need a set of gauges and to be prepared with some additional coolant if, for instance, one of your connections is not perfect the first time.

While installing a duct-less system and it’s wiring can be a DIY project, releasing the refrigerant and starting up the unit is not DIY-able.

Having installed units from different manufacturers, I am most impressed with Fujitsu.

A couple other thoughts…

Most mini-splits— at least the good ones----will produce more than 80% of its capacity to well below 0°. I’ve installed units that will work to -13°.

They’re great for open, local control. If you’re wanting to cool separate rooms they’re not very good stand alone. The good news is that you can install multiple evaporators with a single out door unit. That way you can control separate rooms independently.

They have SEER ratings as high as 27. (The average 3 ton standard tops out at 18 SEER)

raindog - How many BTUs should I need for about 600 sq ft? And approximately how much should it cost installed? I’ve described my situation above - old house, no duct-work and it would be difficult to install given the lack of crawlspace under the house. I’m tired of not being cool enough in the summer or warm enough in winter. I"m tired of hauling in wood pellets for the pellet stove. I want to join at least the 20th century. Mind you, that 600 sq ft is only the area that I most live in -I can handle the rest of the house not being comfortable. If I want to install it in the rest of the house, I could get a 2nd unit.


Generally you’ll get 500-700 ft² per ton of cooling. The better the envelope the more ft² you’ll get.

With an old house like that, you’re certainly on the low end. Most of the mini-splits start at 1.5 tons.

So…for 600 ft² I would guess a 1.5 ton unit will work just fine.

I sell/install lots of standard HVAC systems but only a handful of mini-splits.

Let me check with my vendor tomorrow and I’ll get you a thumbnail sketch as to what your costs might be.


12,000 Btus equals 1 ton

so…1.5 tons would be 18,000 Btus…

While I’m thinking about it…

An A/C system removes sensible heat, and latent heat. (humidity)

Sensible heat is measured with a thermometer. Humidity is measured with a hygrometer.

But A/C systems are controlled with a thermostat. What that means is that is that the latent heat/ humidity is not directly controlled.

Only sensible heat is controlled via the thermostat.

That makes it critical that you not oversize an A/C system. If it is over sized it reduces the sensible heat rapidly enough that it satisfies the thermostat and didn’t run long enough to fully dehumidify.

Ergo…you’re nice and cool. And clammy.

Bigger isn’t always better. :wink:

What is the delta T used in that calculation? I am curious because DT is, I believe 20 degrees in Arkansas, but has been 35 degrees much of this week.

16-22°; 15-20° as a practical matter.

Humidity is a form of heat, and the A/C system has to do that “work” as well, so if there is really high humidity I might expect the delta to be a bit lower.

In that instance the A/C is working just as hard but a larger share of its work is being devoted to removing latent heat----which can’t be read with a thermometer.

So…high humidity maybe closer to 15°. regular humidity maybe closer to 20°.

But 35°—while it sounds great!, is not a good number. With no other information, I would guess there is an airflow issue; the same amount of cooling is being done in a smaller volume of air.

So, I’d look at:

Dirty filter
Dirty coil
Too many supply registers closed.
Carpet etc obstructing the return air.

The “ideal” is something like 75° going to your furnace (return air) and 55-60° coming out (supply registers); a 15-20° delta.

I was speaking of heat gain through exterior walls that the AC has to overcome.
Q=UADT. The outside temperature has been 107 or 105 and is expected to continue for some time.