What Size Mini Split Do I Need?

Almost every customer arrives with a number they found online. Twelve thousand BTUs for a room this big, eighteen for a room that big. Those charts are not useless, but they are a starting point for a conversation, not an answer.

Here is what actually goes into the number.

Square footage is one input out of eight

Two rooms with identical floor area can need very different equipment. What changes it:

• Insulation. An uninsulated 1940s wall assembly loses heat several times faster than a modern one.
• Windows. How many, which direction they face, single or double pane, and whether they leak air around the frames.
• Ceiling height. You are conditioning volume, not floor area, and a cathedral ceiling holds a lot more air than a flat eight foot one.
• Sun exposure. A west facing room with big windows picks up serious heat in the afternoon. A north facing room never does.
• What is above and below. A room over an unheated garage behaves nothing like a room over a heated basement.
• Air leakage. Older houses in this state can turn over their entire air volume several times an hour without anybody opening a door.
• What happens in the room. A kitchen with a range and a dishwasher gains heat. A bedroom with two people sleeping in it gains a little. A home office with three monitors gains more than you would think.
• What you want it to do. Cooling only, heating only, or both. Heating in January is the harder job here, and it usually sets the size.

Why bigger is not safer

This is the part that surprises people. An oversized system is worse than a correctly sized one, not better.

A system that is too large satisfies the thermostat quickly and shuts off. Then the room drifts, and it fires again. That is short cycling, and it causes three problems.

First, comfort. You get temperature swings instead of a steady room, and the strongest airflow happens right when the room already reached setpoint.

Second, humidity. In cooling season, pulling moisture out of the air takes run time. A unit that runs in short bursts cools the air without ever really drying it, and you end up with a room that is 70 degrees and clammy. Undersized-feeling comfort from an oversized machine.

Third, wear. Starts and stops are the hard part of a compressor’s life, not steady running.

Modern inverter driven mini splits handle this better than old fixed speed equipment, because they can throttle down instead of cycling off. But they still have a minimum output, and if your load is below it most of the year, you are outside the range the machine is good at.

What a real sizing looks like

We do a load calculation. It means walking the space, measuring, noting construction and insulation where we can see it, counting and orienting windows, and looking at what is above and below. Then the numbers go into a calculation that produces a heating load and a cooling load for the actual building.

It takes longer than reading a chart. It is also the reason the system works properly in February and does not leave the room damp in July.

A few patterns we see a lot

One head for the whole first floor. Sometimes this works, if the floor plan is genuinely open. More often the head heats the room it is in beautifully and the rooms around the corner stay cold. Air does not turn corners on its own.

Sizing for the worst day of the decade. If you size for minus twenty, you are oversized for the other three hundred and sixty days. Better to size for a normal design temperature and have a plan for the rare extremes.

Forgetting the shoulder seasons. A lot of comfort complaints happen in October and April, not in January, because that is when the load is smallest and an oversized unit has the least to do.

If you want a real number for your house, call us. We will come measure it, and the estimate is free whether you buy from us or not.