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January 26th, 2023
If I told you that the car you just bought has a 90% chance of being defective, you’d do something about it, right? Take it back to the dealer. Find someone to sue.
You might keep it, hoping that the defect won’t be a big deal. But maybe you discover that the gas mileage is suffering while the engine is working harder than it should. Every time you drive the car, you’re gritting your teeth, knowing that you spent money on a defective product. This scenario should never happen.
Ditto with your home. You may have a defective central air conditioner or furnace and not even realize it. One study showed that 90% of all central air conditioners are installed incorrectly or are sized incorrectly.
The odds are your AC or furnace isn’t operating at peak performance.
One problem in particular in an HVAC system is called “short cycling,” and it’s much more common than it should be. Unfortunately, the average homeowner may not recognize the symptoms. After all, few people spend a lot of time thinking about their air conditioner or furnace - until it breaks and they have to buy a new one.
You may think everything is fine because the system is running and the thermometer is satisfied. But this is a false sense of security. Short cycling is a problem that doesn’t go away, and it’s robbing you of comfort while it’s shortening the lifespan of your heating and cooling equipment.
This article will talk about its causes, why it’s such a problem, and some possible solutions. Fire & Ice technicians and salespeople have seen this issue hundreds of times and can offer some advice.
What Is Short Cycling in HVAC?
Short cycling can happen during heating and cooling.
Your furnace and air conditioner have cycles. When they start up, that’s the beginning of the cycle, and when they shut off, that’s the end.
The question is how long that cycle is, and why it matters. If the shutoff time is too short, that means that the conditioned air doesn’t have the chance to blend fully with the rest of the air in the house.
During a correct cycle, the HVAC system should achieve as much changeover as it can. That’s when the conditioned air from the supply vents mixes in the home and goes into the return vents.
Let’s say the desired temperature for a home is 70 degrees. The furnace could be blowing out 90-degree air from the vents, and when it blends with the air in the home, that’s when the whole home temp reaches 70.
During a shorter-than-ideal cycle, the air conditioner or furnace shuts off before this complete mixture can take place. That leaves hot and cool spots throughout the home. Then the machine kicks back on again, and the process repeats. You never achieve comfort throughout the home.
Now we need to discuss how “stages” affect your HVAC system.
Air Conditioners and Furnaces Have “Stages”
Air conditioners and furnaces can be single-stage, two-stage, or multiple (or “variable”) stages. In a single-stage system, the unit comes on at 100% of its capacity. When the thermostat is happy, it shuts off.
With systems that have more than one stage, your furnace blower can run at 100% when needed, but also at 60%, or it can even run at hundreds of variations. The reasoning behind this is that your home may need just a little bit of added warmth. When the fan is at a lower speed, it runs longer than a single stage. This increases comfort, is less hard on the equipment, and saves you money.
Compare this to driving on a road with cruise control. If you don’t use it, you’ll be accelerating and decelerating all the time. The car has to work harder to get up to the desired speed. With cruise control, the car makes little adjustments as needed. And we know that cruise control increases your mileage per gallon. It’s also easiest on the engine.
HVAC Short Cycling Is More Common With Single-Stage Systems
In a system with a single stage, people tell me that it warms up quickly and it gets really warm. But the air doesn’t mix, so you get those uneven temperatures. One room may be fine, but others not so much.
The way that I explain the feeling of a single stage is if you are relaxing on the couch reading a book or watching TV. In the summertime, you think, “It’s getting kind of warm in here.” You get up and check the thermostat, but it’s at that moment the air conditioner kicks on. So you leave the thermostat alone.
That’s a characteristic of single-stage. It gets a tad too warm, then the AC blasts on. It cools down and shuts off, but the air isn’t mixed enough, so it fires back on.
You wind up in a constant state of being mildly uncomfortable.
Short cycling is a similar concept. In short cycling, your system may be oversized. Say your thermostat is set to 70, and it’s 69 in your house, the furnace starts up with all its might. Your thermostat is happy. It says it’s hot in here. It shuts off. Maybe it only ran for a minute.
Thermostats don’t know the temperature in other rooms and don’t care. They know only what’s in front of them.
The room you want to be warm doesn’t reach the optimal temperature before the heat kicks off again. The air doesn’t mix, so cold rooms stay cold, but the thermostat is happy. Two or three minutes later, the thermostat calls for heat again. So the furnace blasts on again and runs for another minute or two.
How Can You Tell if Your HVAC System Is Short Cycling?
On a moderately hot day, a proper air conditioning system will undergo three cooling cycles per hour, each lasting approximately 10 minutes. This means the compressor will run for 10 minutes, stop for 10 minutes, and repeat the cycle two more times during a single hour.
You can tell a system that’s short cycling when it turns on and off three times in five to ten minutes.
But for the average homeowner, comfort is assumed. They don’t pay attention to their furnace or air conditioner. They know that they can go over and set the temperature, and almost all the time, the system works. The average person wouldn’t think about their air conditioner short cycling.
Unless it starts short cycling suddenly, you’d probably never realize that there’s a problem.
But there are some subtle signs that it might be doing so.
In the summer, if the indoor humidity makes your home feel like a jungle, it could be a sign that the air conditioner isn’t running enough. ACs remove excess humidity while they run. Short cycling reduces this process. You might achieve a cool home, but the humidity will keep things uncomfortable.
Even temperatures throughout the home mean that everything is working as it should. If the downstairs seems fine, but the bedrooms upstairs are warm and muggy, it could be that your air conditioner can’t do what it was designed to do.
What Other HVAC Problems Can Cause Short Cycling?
- A clogged air filter means your air conditioner will have limited airflow, which causes hot air to build up inside the system. This causes the system to overheat, which can cause it to cycle off. Replacing or cleaning the filter regularly can prevent this. Read more: How Often Should Furnace Filters Be Changed?
- Frozen evaporator coils can cause short cycling. Air conditioners can freeze up when they’re running while it’s too cold outside. If this happens, you can turn the system off to allow it to thaw. If the coils freeze multiple times, you should have your system examined by an HVAC contractor.
- Electrical problems can cause short cycling. A malfunctioning thermostat is just one example. You will need professional help in order to safely repair this.
Location of the Thermostat Might Be the Cause of Short Cycling
The location of a thermostat can definitely play a part in this. Maybe it’s located in a small room that has a supply vent but no return vent. That room will heat up quickly. The thermostat will reach its temperature quickly, then shut off the furnace. Meanwhile, the rest of the house remains chilly.
Or the thermostat could be on the backside of the wall that has the stove or refrigerator. That stuff puts out heat. If it heats up that wall, the thermostat will sense it and be fooled.
I have been in homes where the thermostat is on a south-facing wall, and the homeowners have a lot of windows and the sun hits it all the time. When the sun hits the thermostat, it’s going to heat up. I know people who have moved their thermostats because of the way the sun was hitting them.
How to Prevent Short Cycling: Why a Manual J Load Calculation Is Essential
As part of every estimate at Fire & Ice, we do a Manual J load calculation, which is recommended by both the Air Conditioning Contractors of America and the US Department of Energy. (We also recommend that homeowners insist on it being done.)
A Manual J Load calculation produces two numbers: the total amount of BTUs (British Thermal Units) per hour gained and the number of BTUs of heat loss per hour. A BTU represents the amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit.
Both heating and cooling calculations in a Manual J factor in the square footage of all exterior windows, doors, walls, ceiling height, amount of insulation, number of occupants, total square footage, and more. The heating side looks at how much heat is lost in an hour, and the cooling equation factors how much heat comes into the home and is retained per hour.
Customers don’t understand it when I do a load calculation and show them that their unit is oversized for their home. We tell them a surprising fact: About half of all air conditioners and furnaces are sized incorrectly. That means approximately one-fourth of units are oversized, meaning that short cycling is pretty common.
How did the HVAC company that installed those oversized units get things so wrong? Probably laziness. They may have seen what size the old system was and used that figure. Or perhaps there are fewer occupants in the home now. Children move out and the empty nesters are stuck with a system that was built for more occupants.
Solutions for Short Cycling
If your system is aging, and you’re thinking about a new one, that would be the perfect time to talk to a seasoned HVAC contractor who knows how to accurately measure the load of your home. If you’re not happy with the sizing recommendation, get a second or third opinion. You could save yourself a major headache if the size is finally right.
If a new system isn’t in your budget, or your system is just a few years old, you can try a zoned system. You can read this article that fully explains how this sort of arrangement could make your home more comfortable.
A simple zoning solution could be the use of portable heaters or air conditioners for often-used rooms. This doesn’t take care of the main problem - your system is still oversized - but at least you can regain some comfort.
A whole-home or portable dehumidifier can take the edge off of hot, muggy days in the summer. You need to make sure you’re emptying the bucket regularly for portable ones. A whole-home unit can be installed so that excess humidity will go down the drain.
A more expensive option is an investment in ductless mini-splits. They give you the ability to customize temperatures in individual spaces. They condition the air in one room, though you have the option of installing up to five heads with each unit.
A head is an individual indoor unit that is connected to the outdoor unit and provides cooling or heating to a room or area. These systems give you temperature control over different rooms: You can cool one room while heating another.
Read more: https://indoortemp.com/resources/cost-of-mini-split-ductless-system
A Smart Home Can Help With Short Cycling
A smart thermostat working in conjunction with automatically controlled dampers in the ducts can assist with humidity control and uneven temperatures. It’s worth mentioning that such a configuration would involve a high-end furnace that has a variable-speed blower. This blower could work at a reduced capacity - say 60% or less - and would run for a longer interval. You’ll need to speak with an HVAC professional to discuss HVAC compatibilities and options.
HVAC Short Cycling in Columbus, Ohio
An HVAC unit that short cycles creates uneven comfort, will wear out sooner, and be the cause of unnecessary repair bills. You may not be aware of the problem, but you’ll spend money, time, and energy trying to make your home a pleasant place to be.
The best way to avoid this is to hire an HVAC contractor who doesn’t cut corners. You’ve paid plenty to have a piece of machinery put in place. It should do what it’s designed to do from day one until it wears out.
If you’re ready to talk about your HVAC system because you think you could do better and you live in Columbus or the Central Ohio area, we’re here and would love to talk with you. Enter your zip code in the graphic below to see if you’re in our service area. We look forward to meeting with you.
