Is It Okay To Slightly Oversize A Mini Split?

No. If you can avoid it, you should never oversize a mini-split – even slightly. It might not seem like a big problem. But that simple mistake can cost you a lot of money in the end, not to mention the trouble of frequent repairs.

“But a bigger mini-split means I can cool my home better and warm it more. So why is that a bad thing?” some people might argue.

Unfortunately, it doesn’t always work that way. Although you’d expect a bigger AC to cool better and a larger heat pump to keep you warmer, it’s slightly different in practice. Let’s find out.

How Inverter ACs Work

Although some models still use single-stage and two-stage compressors, most mini splits in use today use unique compressors powered by specially designed inverter technology motors. Maybe we should explain the first two before discussing inverter compressors.

Single-stage compressors only operate at one speed, i.e., 100%. Either the AC is off, or it’s running at full blast. When you turn on the AC, it will kick off at 100% capacity and continue running until the thermostat setting is reached, then cycle off until temperature rise above the thermostat setting (during the summer when you’re cooling).

Two-stage models are slightly different, equipped with two operating speeds, typically 65% and 100%. So, you can have the two-speed AC running at 100% during the hottest days and lower it to 65% on less hot days.

Some switch between the two-speed settings automatically. Once the thermostat setting is reached, the unit cycles off until ambient temperatures rise beyond the thermostat setting. At this point, the AC resumes operation.

You can see why the two technologies aren’t very efficient. First off, the fixed operating speeds mean that you’ll draw a lot of energy even when your home is relatively cool.

More importantly, single-stage and two-stage motors start far too often. Single-stage units often restart up to 18 times per day, while two-stage models restart about 8-12 times per day. This can result in energy wastage.

Inverter technology, designed in the 1970s and popularized in the 1980s, solves this problem by allowing the motor to run at any speed between 40% and 100%.

Unlike in single-stage and two-stage units, where power is sent directly to the compressor, inverter compressors send electric current to the inverter first. The inverter then regulates how much power reaches the compressor. This way, it’s possible to regulate the speed of refrigerant flow.  

Why the Inverter AC Won’t Keep You “Cooler”

Accurate regulation refrigerant flow means that a bigger inverter air conditioner won’t necessarily keep you cooler in the summer and warmer in winter. Here’s why;

Air Conditioners maintain OPTIMAL comfort – no matter the size

The AC consistently reads ambient temperatures and automatically self-adjust to produce just enough cool air or warm air to keep you comfortable. If it detects that the home is too hot, the inverter will pump refrigerant faster, thus releasing cool air into your home faster and while also extracting heat fast. However, once the home is at a favorable temperature, the refrigerant flow slows down, and the unit may even cycle off.

This happens no matter the size of the AC. Granted, a 20,000 BTU AC can remove more air running at 65% compared to a 17,000 BTU AC. However, if the inverter technology determines that you need increased cooling capacity, it would simply raise the operational speed of the 17,000 BTU air conditioner to say 75%. So, in the end, both people can enjoy the same level of cooling.  

Air leaves the supply vent at the same temperature for all ACs

Another thing to keep in mind is that the temperature of the air leaving the supply vent is the same for all mini-splits. It doesn’t matter if it’s a 30,000 BTU AC or a 15,000 BTU unit. The temperature at the supply vent is usually 85˚F to 92˚F.

The temperature difference between the supply and return vents is within the same range, typically 14˚F to 20˚F. If your supply air temperature is lower, it’s most likely an appliance malfunction rather than a size issue.

Therefore, you should never oversize an air conditioner intentionally, thinking that it will give you “cooler” or “hotter” air. That’s not how air conditioners work.

Other Dangers of (Even Slight) AC Oversizing

Aside from understanding that choosing an oversized AC doesn’t necessarily guarantee higher cooling (or heating), it’s also important to learn the dangers of an oversized AC. These include;

Short cycling

Short cycling refers to short and potentially dangerous AC cooling cycles. Naturally, the air conditioner runs for about 10 minutes, cycles off for the next 10 minutes, then resumes cooling for another 10 minutes, and so on. The cycle keeps your home well cooled while ensuring that the AC is well rested for long appliance life.

However, the 10-minute cycles are also guided by ambient temperatures. The AC will go off as soon as the room is cool enough, often in line with thermostat settings. An oversize AC is likely to reach the thermostat setting faster, resulting in shorter cooling cycles. Rather than take 10 minutes to reach the thermostat setting, it may only take 5-7 minutes. This is known as short cycling.

Short cycling can be costly and even dangerous. First, it causes the AC to use a lot of power. The frequent ON/OFF cycles can draw a lot of electric currents. Additionally, short cycling accelerates appliance wear and tear. You might find yourself replacing the AC sooner than you imagined.

Hot spots in the room

Another issue closely tied to AC short cycling is hot spots (and cold sport during cooling). As we’ve seen, short cycling often means that the AC only cycles for about five minutes before going off. It may then stay off for another five minutes before roaring back to life. Sometimes the cycles are shorter.

Unfortunately, this may not allow enough time for thorough distribution of air throughout the room. Given that the fan also goes off when the AC cycles off, you may notice hot spots, especially in areas far from the air conditioner.

Often, you need additional air conditioners to cool the hot spots. Or you might be tempted to raise the thermostat settings – both of which are costly and unnecessary.

Moisture issues

Most consumers don’t always think about it, but your choice of AC also directly impacts moisture levels in your home. That’s because the air conditioner is the primary dehumidifier in most homes. In the process of extracting excess heat, it also removes excess moisture from your home.

This process is significantly compromised if the AC is oversized. Why? Because, when working, larger air conditioners remove too much moisture, often leaving the area dry. However, the short cycling can quickly reverse the dehumidification gains, leaving the room sweating with moisture.

In the end, you’ll likely have bouts of extremely dry air and highly humid air. It creates a very uncomfortable situation.

Stop Overthinking; Choose the Right Size AC

All these problems are often caused by overthinking. Perhaps you’re wondering what would happen if the AC becomes less efficient or worrying about sweltering summers. The right-size air conditioner will address those issues adequately and without fail.