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Why an Oversized Air Conditioner Is a Problem

An air conditioner that has more capacity than your home needs causes comfort issues and is hard on your equipment. We look at the problems it creates, and some possible fixes.

Why an Oversized Air Conditioner Is a Problem

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Bryan Carnahan

Revised:

January 23rd, 2023

Homeowners have to put a fair amount of trust in contractors. You rely on Google reviews, Yelp, word of mouth, brand-name recognition, past experiences, and more to make costly decisions that affect your entire family.

A decision to hire the wrong contractor can result in a lost investment. Adding insult to injury is the stress that goes along with having to live with a slipshod addition or a crummy repair job.

This can happen in HVAC, and it’s too common a problem.

According to Carrier, residential air conditioners usually range from 1.5 to 5 tons. Anything with a larger cooling capacity would be considered light commercial.

Tonnage ratings are typically expressed in increments of .5 tons, so residential ACs are usually rated as 1.5, 2, 2.5, 3, 3.5, 4, or 5-ton units.

It’s a rare thing when I go into a home for a sales call that the homeowner knows the exact tonnage. After all, you don’t spend much time staring at the lettering on your central air conditioner. You mow around it and perhaps cover it in the winter, but it’s not - nor should it be - the focus of your attention.

The problem is that it may not be the right size for your home. If that’s the case, it causes comfort concerns and is rough on your entire system. In this article, we’ll look at what happens if your AC is oversized and moving too much air for your home, and explore possible solutions.

What Happens When Your Air Conditioner Is Oversized?

The AC system has two jobs: cooling and dehumidifying.

If your unit is too big, it will cool quickly and satisfy your thermometer set point. It’s called “short cycling.” A cycle should be long enough to allow the air in the house to mix with the conditioned air coming from the vents. When the cycle is too short, the room that has the thermometer, which is usually near the center of the house, will cool off quickly. Too quickly.

Once the set point is satisfied, the thermostat will shut off the system. If you have rooms farther away from the main trunkline, they’re not going to get the same amount of conditioned airflow as the area where the thermostat is.

Rooms farther away from the furnace will be the last to receive conditioned air and the first to be cut off. Your whole house will feel the effects of short cycling, but those rooms will be the worst. You could have huge temperature inconsistencies. In a small ranch house, it might not be as evident. But if you have a two-three story home or a split-level home where the air has to travel a long way, you’ll have a huge temperature difference.

Meanwhile, you’ll have a few cool rooms, and the warm air will find the thermostat soon. So it turns on and off and on and off.

ACs Also Remove Humidity, Which Affects Comfort

Comfort-wise, humidity is the big deal. A short cycling air conditioner doesn’t stay on long enough to do its second job, which is to dehumidify your house. We’re in Columbus, Ohio, so obviously, dehumidification is a big deal. What you wind up having is a cold jungle. It’s nice and cool, but it’s muggy.

And because we’re not taking away the humidity and warm air, the temperature will rise. By the time you go to bed and want it cooler, it’s 70%-75% humidity upstairs, and ten degrees hotter.

The Mayo Clinic suggests setting your home humidity level at 40% to 50% for maximum comfort. Anything above that and you’ll be sweltering on a humid day.

Why Could Your Air Conditioner Be Oversized?

If your air conditioning system is too big, it’s not your fault. Rather, it’s the fault of an HVAC contractor that didn’t do his job right. Perhaps he looked at your old AC’s size without addressing whether that one was the right size or not.

Or he may have made an estimated guess on tonnage based solely on the square footage of your home, neglecting factors such as the size and number of windows, the quality of the insulation, the number of occupants, and so on.

All this and more is covered when an HVAC-trained technician does a Manual J load evaluation, a procedure recommended by both the Air Conditioning Contractors of America (ACCA) and the US Department of Energy for every HVAC job.

We at Fire & Ice recommend and perform this calculation. It’s common sense: square footage doesn’t account for the myriad variables in homes.

If your AC is 15 years old or more, it’s probably time to invest in a new model. That is the perfect opportunity to have a proper Manual J load calculation done. That will help your entire HVAC system.

How Does AC Short Cycling Affect Indoor Air Quality?

If the inside air doesn’t get humidified, and the relative humidity is at 60% or higher, that’s where you’re going to get mold, bacteria, and dust mites, which love to hang out in your ducts - a cool, dark space. It’s like a petri dish of humidity. Short cycling allows all that stuff to thrive in your ducts, which means the whole house will support it as well.

There could be some static pressure issues. If the system is designed for a 3-ton load, but you have a 4-ton coil, the ductwork is not going to be able to satisfy the amount of airflow necessary to put that through.

The static pressure could be too much on the supply side: too much air being forced into too small a duct. And it could be too little on the return side. Not enough air is making its way back into the furnace.

Why Is Air Conditioner Short Cycling Hard on the HVAC Equipment?

The AC will be inefficient. Like any machine, the most power is being used when it turns on, not when it’s running. So if the thermostat is turning the system on and off all the time, your energy bill will actually be higher than if you had a properly-sized smaller unit that runs its full cooling cycle.

The AC’s compressor will age quickly. You can hear older air conditioners make that awful noise when they start up. It can sound like it’s a 15-year-old air conditioner in eight years or so.

When the AC is turning on and off, so is the furnace. The cool air coming from the air conditioning unit has to wind up in the furnace, where the blower will send the air through the ductwork.

If your system is set to “auto,” every time the AC kicks on, so does the fan. That means that the motor-bearing unit and the bearings on the spinning motor are turning on and off all of the time also.

That means your furnace will have the same longevity issues as your AC will have. The heating elements of the furnace will be unaffected, but the blower motor will work harder than it should.

Coping with an Oversized Air Conditioner

One partial solution is to run portable dehumidifiers in your basement, particularly during the most humid months. It costs money to buy and run those, and they are not efficient. Your energy bill will go up because you’re trying to augment what the AC should have done by itself. You have multiple machines doing what one machine should have been doing.

One unit that might make a substantial difference and not run up your energy bill is a whole-home dehumidifier. One such example is the Aprilaire E100 whole-home dehumidifier, the newest of its type in the Aprilaire line of indoor air quality products.

It is capable of removing up to 100 pounds of water per day (12+ gallons) and can be installed in the ducts or can remain free-standing. Either option will contribute to dehumidifying your whole home.

Getting rid of excess humidity doesn’t affect the temperature. But reducing humidity makes it feel two to three degrees cooler even if the thermostat is set to exactly the same temperature. Not only do you feel more comfortable, but you also save money if your thermostat is set a little higher.

Another solution would be to have your ducts cleaned regularly. This will at least get rid of the junk in your ventilation system. It will also keep your furnace’s blower cleaner. It won’t do anything about the temperature or humidity, but having clear air means less mold and fewer dust mites in the air, which benefits your entire family.

And if you want to make the air even cleaner, we recommend cleaning your air filter per the manufacturer’s recommendations. Some filters should be replaced monthly. Others are good for a year. A clogged air filter stops doing what it was designed to do. Particulates will find their way around a clogged filter and wind up back in the ducts.

If your thermostat has a “fan only” setting, you can put that to good use by letting your HVAC system run without using the air conditioner. That will help circulate the air better, and it will save wear and tear on your AC.

Another option is investing in a ductless mini-split. They are the most efficient heating and cooling system you can buy, and work as heat pumps. They are designed to be zoned; one outdoor unit can have up to five heads, meaning it can heat or cool five rooms at the same time. They are flexible enough to monitor and adjust temperatures in each room. One room can be set to 70 and another can be 68. They also tackle humidity issues.

Yet another option is a smart home run by a smart thermostat. It can manipulate automatic dampers that reside in the ducts. That can help even out the temperature differences.

How to Prevent a Short Cycling AC in Columbus, Ohio

We wish we didn’t see AC systems short cycling, with units that weren’t designed to cool the space properly. Alas, it isn’t so. It’s also common that we meet homeowners who don’t know that their HVAC system isn’t up to snuff. They have equipment that is wearing out too soon using too much energy, and they are living in uncomfortable homes.

You deserve better than that.

If you suspect your HVAC is underperforming, please give us a call for a no-hassle consultation. Together, we can come up with a plan that makes sense for your unique circumstances.

In most cases, we offer next-day installation. Fill in your zip code in the graphic below to see if you’re in our service area. We look forward to hearing from you.

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A heat pump that runs constantly can cause your system to overwork, causing it to have a shorter lifespan. In this article, we look at common heat pump problems, why they are happening, and how to fix them.

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