Your HVAC system consists of heating, ventilation, and air conditioning systems that all work together to keep your home comfortable and livable. When they’re designed and installed properly, they’re a godsend, especially AC on a muggy summer day. The system must be sized precisely for your home and for the climate conditions where you live, though, and bigger is definitely not better. In fact, an air conditioner that’s too big will present you with a few problems that you just shouldn’t have to deal with. You’ll end up frustrated and probably have to replace that system prematurely.
Short cycling: Oversized systems will present this problem early and often. Because they have too much capacity for the volume of the space, they heat and cool the space rapidly and achieve the desired temperature at the thermostat rapidly. That “short cycle” keeps the components in the less-efficient start-up phase rather than in their more efficient normal operating range, so they turn on and off frequently, which leads to temperature swings that you can feel, as well as reduced comfort and, eventually, higher maintenance costs. It’s sort of like driving your cold car for two blocks in the winter and then shutting it off. You’re never letting it warm up to where it’s operating efficiently.
Inadequate dehumidification: Short cycling of your AC unit leads directly to inadequate dehumidification, as the AC unit can achieve a desired temperature quickly, but not necessarily a comfortable level of humidity. It simply takes longer for an AC unit to remove moisture from the air in your home and it can’t do that when it’s short cycling.
Uneven Temperatures: You’re also likely to have hot and cold rooms with an oversized HVAC system, too. Since it’s not operating long enough to fully distribute the conditioned air, you’ll have cooler and warmer areas and decreased comfort.
Higher energy bills: You could also notice higher energy bills with an oversized HVAC system than you should be paying. An oversized AC system’s startup phase is not its most efficient operating time. Because of short cycling’s inadequate dehumidification, you might actually turn the temperature down to try to manage the humidity and end up using even more energy. In short, the AC unit could end up short-cycling even more frequently and using more energy.
The same could happen with a short-cycling furnace. If you just can’t get a room warm enough due to short cycling, you would bump the thermostat and cause the furnace to run longer to get that room warm, using even more energy. It’s a vicious cycle.
Shorter lifespan: Short cycling with all those starts and stops, no surprise, causes more wear and tear on the HVAC components, as well, which could then shorten their lifespan. High-quality HVAC components should last more than a decade when they have proper maintenance, which includes a yearly service.
Increased Costs: Typically, larger HVAC components cost more upfront, which isn’t surprising. They also have higher operating costs due to increased energy consumption and may cost more to maintain.
Noise: You might also find that a larger HVAC system kicks on with a pronounced and distracting noise due to high airflow. If this is happening frequently due to short cycling, you may find it irritating, to say the least.
Environmental Impact: Higher energy consumption also costs more greenhouse gas emissions and strain on the electric grid.
Space Constraints: Finding space inside and outside your home for HVAC equipment is not always easy, so right-sizing is smart for that reason, too. Mini-split heat pumps and their small footprint can provide a big advantage over traditional units.
Warranty Issues: Manufacturers will typically specify proper sizing for their equipment along with warranty terms. Deviating from those terms may void the warranty, which would be quite vexing after you’ve just spent thousands of dollars!
So, with all those potential problems, why would HVAC systems be oversized? There are a few reasons:
Customer expectations: Contractors fear the callback from a customer who says the system does not keep the house warm or cool enough. That alone is a strong incentive to make an accurate calculation for a complete HVAC system and then bump it up a notch “just to be safe.”
Design margins: Your HVAC contractor will use a formula called Manual J to design your HVAC system. This formula is the industry standard. A contractor may build in a little extra margin that goes too far, or perhaps they are simply not very good at using Manual J.
Load estimation errors: Your HVAC system has to operate in a wide range of conditions, and some of those will be extreme, such as at the 5% margin of heat or cold. That provides some strong incentive to oversize the system to a certain degree to ensure that it will be able to deliver during extreme heat, cold, and humidity conditions.
Lack of contractor training: For contractors who haven’t had the proper training or who haven’t developed much HVAC expertise, a little bump in oversizing might seem like a reasonable and safe shortcut to avoid problems in the future.
Quick heating and cooling: A contractor could be faced with a customer who wants a room or a building to heat and cool very quickly, even though the performance will suffer at other times. That may be a conundrum for the contractor who anticipates potential problems, but as long as the customer is made aware ahead of time, that’s all they can do.
Manufacturer incentives: Some manufacturers may prioritize profits over all else and so push their larger systems when they’re not actually the best choice for a customer. Many contractors work extensively with the tech support departments of the major manufacturers and so depend on that advice. Thus, more profitable oversized systems can get installed more often than they should be.
Unless you know how to calculate your heating and cooling load, you are best off in getting 3–5 estimates for your HVAC system and comparing. If they all come back with the same size equipment, that’s a good sign! If not, you can start asking questions about how the contractor has chosen to size the equipment.
And remember that the Inflation Reduction Action provides a 30% tax credit on a host of HVAC equipment through 2032, so you can save some serious money as you update your system.