Is It OK To Slightly Oversize A Mini Split? What Is ‘Slightly’?

“You can either go with a 2.5-ton or a 3-ton mini split.”

This is a classic HVAC statement when estimating mini split sizes. It prompts the question if it is OK to slightly oversize a mini split heat pump.  We always end up choosing between two mini split capacities, namely:

1. Most optimum size, according to the cooling load calculation. In HVAC, we usually use Manual J calculation to determine the size of mini split we need. For rough estimation, you can also consult this air conditioner BTU calculator. This would be the 2.5-ton mini split.
2. Slightly oversized mini split size, according to the cooling load calculation. This would be the 3-ton mini split.

Now, nobody really loves the complications that come along with oversized mini split systems. The most common ones include:

• “Temperature is low enough but the air is still humid. Why doesn’t my mini split reduce the humidity?” Well, because an oversized mini split can go into short cycling.
• “Temperature is low in the room with air handler but the other room is still warm.” This is a classic ‘hot pockets’ problem we get if the mini split is oversized.
• “I only have 800 sq ft place to cool, why are my AC bills so high in the summer?” Oversized AC can in some cases unnecessarily increase the cooling costs.

As you will clearly know, you must not oversize a mini split (by a lot, that is). But it is OK to slightly oversize a mini split?

In short; it is OK to slightly oversize a mini split. This is even highly recommended. But it comes with a big BUT:

BUT you have to have an idea about what ‘slightly’ oversizing actually means. An extreme example would be if you were to put a 5-ton mini split even though an HVAC professional has calculated that a 2-ton cooling load should be sufficient.

Let’s first explain why exactly is it OK to oversize a mini split to a point. After that, we will look into what that point is; ie. when do you cross the line between slightly oversizing a mini split and actually oversizing a mini split, incurring the stated negative consequences. Further on, you will find a chart with allowable maximum mini split tonnages/BTUs that are still deemed only slightly oversized, not completely oversized.

Why It Is OK To Slightly Oversize A Mini Split?

After most professional cooling load calculations, your HVAC professional will turn to you and say something in line with “Well, we have calculated such and such tonnage/BTUs, so you can choose a mini split with this tonnage/BTU, or slightly bigger one“.

At first, it seems a bit any paradoxical why you would go for a bigger unit. If an HVAC professional who knows his stuff has calculated you need a 30,000 BTU (2.5-ton) unit, why would you go with a 3-ton one that can be $1,000+ more expensive?

In short, 3 reasons:

• Estimating cooling load is not 100% exact science. Even if your HVAC expert has done everything to the book (Manual J book, that is), chances are that the cooling load number is a bit off.
• You can always run a slightly oversized mini split on Low or Medium load. You cannot run an undersized unit on Super High setting.
• Summers are getting hotter. You will probably use the mini split you are buying now in 2040. Summers are getting increasingly hotter, with heatwaves across the US every year, and you can at least give yourself a bit of an edge with a bigger-than-currently needed mini split.

In short, a slight oversized mini split is a safe choice. That’s why the HVAC saying is “if in doubt, always go for a bigger unit”.

If you are wondering if it is better to oversize or undersize a mini split, the answer is clear: Always oversize.

That being said, you should be aware of compromising when oversizing. Not all oversized mini splits are OK. If you oversize too much, you will be left with 1 cold and humid room, and other rooms being both warm and humid.

How much oversizing is too much?

What It Means To Oversize A Mini Split?

As we have pointed out, choosing a 3-ton mini split over 2.5-ton mini split is OK. Choosing a 5 ton mini split when the cooling load calculation tells you to go with a 2-ton mini split in not OK.

Where is that border between advantageous slightly oversized mini split and disadvantageous fully oversized mini split?

Here is how to think about that:

The problem with oversized mini splits is that they will start short cycling. They will turn on and off quite rapidly. This is because they have achieved the set temperature (measured via a sensor in the air handler) in the room the air handler is placed in. They will, for lack of a better word, ignore the temperature in other rooms and humidity levels in all rooms serviced by that mini split air handler.

The most commonly refer number for oversizing is 40% or higher. That means that if your mini split tonnage or BTUs are 40% higher than the calculated cooling load, you will have to face the consequences of an oversized mini split.

When we say that it is OK to slightly oversize a mini split, we mean that the tonnage or BTUs are less than 40% above what the cooling load calculation shows.

Example: If the cooling load calculation shows that you need a 2-ton mini split (that’s a 24,000 BTU mini splits since 1 ton is equal to 12,000 BTU), you will be OK with a slightly oversized 2.5-ton mini split (25% higher) and not OK with an oversized 3-ton mini split (50% higher).

To help you out, we have calculated what size mini splits are still OK (slightly oversized, below 40% over cooling load estimate) and what size mini splits are truly oversized (40% or more over the cooling load estimate):

Here is how to read this chart:

Let’s say you get a 3-ton cooling load estimate. According to the chart, you can go with up to a 4-ton mini split (+33%) but not with a 4.5-ton mini split (+50%). That doesn’t means you should actually go with a 4-ton unit; this is just a maximum allowable AC capacity that won’t send the AC in short spinning.

In this case, you should go with a slightly oversized 3.5-ton unit.

Understanding all this, we hope it illustrates well if is it OK to slightly oversize a mini split. Even more importantly, it gives you some insight into benchmark tonnage values that differentiate a slightly oversized mini split from a truly oversized mini split.